MIDDLE EAR
MECHANICS
STUDIED BY
LASER DOPPLER
INTERFEROMETRY
M.S.M.G. Vlaming
TR diss
1596
if
MIDDLE EAR MECHANICS STUDIED
BY LASER DOPPLER INTERFEROMETRY
MIDDLE EAR MECHANICS
STUDIED BY
LASER DOPPLER INTERFEROMETRY
PROEFSCHRIFT ^
ter verkrijging van de graad van doc
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus,
prof. dr. J.M. Dirken,
in het openbaar te verdedigen
ten overstaan van een commissie,
aangewezen door het College van Dekanen
op dinsdag 15 december 1987 te 14.00 uur door
MARCELLUS SIMON MARIA GEMMA VLAMING
geboren te Grootebroek
doctorandus in de wiskunde en natuurwetenschappen
van Kralingen's drukkerij & uitgeverij - Loenen a/d Vecht « * - » ^ ■ ■
TR diss
1596
Dit proefschrift is goedgekeurd door de promotoren: Prof.dr.ir. F.A. Bilsen en Prof.dr. L. Feenstra
This study was supported by the Netherlands Organization for the Advancement of Pure Research (ZWO).
Aan Ellen, Marie-Louise en Margriet Aan mijn ouders
CONTENTS
CHAPTER 1. GENERAL INTRODUCTION 11
1.1 Scope of the present study
1.2 Measurement of small vibrations in the hearing organ 1.3 The peripheral auditory system in man and cat 1.4 References 12 13 15 17 CHAPTER 2. METHODS 2.1 Laser interferometry
2.2 The Laser Doppler Velocity (LDV) meter 2.3 Performance of the LDV instrument 2.4 Calibration 2.5 Sound stimulation 2.6 Signal processing 2.7 Concluding remarks 2.8 References 19 19 20 24 30 31 33 34 35
CHAPTER 3. MECHANICS OF THE NORMAL HUMAN MIDDLE EAR Method
Results
Discussion and conclusions References 37 39 41 46 48
CHAPTER 4. MECHANICS OF THE RECONSTRUCTED HUMAN MIDDLE EAR Experimental procedures
Malleus to stapes superstructure reconstruction Malleus to footplate reconstruction
Discussion and conclusions References 49 51 51 56 60 61
CHAPTER 5. MECHANICS OF THE CAT'S MIDDLE EAR 63
5.1 Methods 63 5.2 Tympanic membrane vibration 65
5.2.1 Transfer function of the normal intact ear 66
5.2.2 Effect of middle ear pressure 68 5.2.3 Effect of middle ear cavities 70 5.3 Vibration of malleus with fully opened middle ear 72
5.3.1 Vibration of malleus 72 5.3.2 Vibration of malleus after interruption of the
incudo-stapedial joint 73 5.4 Vibration of middle ear ossicles 74
5.4.1 Vibration of stapes head 74 5.4.2 Slipping of the ossicular joints 75
5.5 Discussion and conclusions 78
5.5 References 79
CHAPTER 6. NETWORK MODELS FOR THE MIDDLE EAR 81
6.1 Analogons 81 6.1.1 Electrical analogons of a mechanical system 82
6.1.2 Electrical analogons of an acoustical system 83
6.2 Structure of middle ear models 85 6.2.1 The Zwislocki model 85 6.2.2 Model according to Shaw 89 6.2.3 Comparison of existing models to mechanical
measurements 90 6.3 Acousto-mechanical middle ear models 93
6.3.1 General 93 6.3.2 General basis for an acousto-mechanical model 94
6.4 The acousto-mechanical 2-zone middle ear model 95
6.4.1 2-zone tympanic membrane 95 6.4.2 Detailing the 2-zone model 98 6.5 Generalization to 3-zone and N-zone models 100
6.6 Discussion 101 6.7 References 103
CHAPTER 7. ESTIMATION OF MODEL PARAMETERS
7.1 Tympanic membrane: 1-zone model
7.1.1 Compliance of the membrane
7.1.2 Mass of the membrane
7.1.3 Damping of the membrane
7.2 Tympanic membrane: 2-zone model
7.2.1 Mass of membrane zones and malleus
7.2.2 Compliance of membrane zones
7.2.3 Damping of membrane zones
7.2.4 Coupling impedance Z
7.3 Tympanic membrane: 3-zone model
7.4 Non-membrane impedances
7.4.1 Cochlea, stapes and incus
7.4.2 Incudo-mallear joint
7.4.3 Complete model: adding the middle ear cavities
.7.5 2-zone model for the human middle ear
7.6 Simulated acoustic impedance
7.7 Discussion
7.8 References
7.A Appendix: Second resonance for the 2-zone model
105
105
106
107
107
108
109
110
110
110
111
112
112
113
114
116
118
120
122
123
CHAPTER 8. CONCLUSIONS AND SUMMARY
125
SAMENVATTING
131
CHAPTER 1
GENERAL INTRODUCTION
Ever since the pioneering work of v. Bekesy (1960) it has been known that an acoustic wave impinging on the tympanic membrane causes extremely small vi bration amplitudes in the mechanical structures of the ear. These structures transfer the pressure of the acoustic wave in air into a pressure wave of the fluid within the cochlea. Linear interpolation from the vibration amplitudes measured by v. Bekesy (1960) at 140 dB SPL suggests that amplitudes smaller than 0.001 nm carry the sound information at the threshold of hearing.
The interest in measuring these small vibrations has resulted from the wish to get a better understanding of the role of mechanical vibrations in the process of hearing. The primary considerations concern the frequency depend ence of the conversion of sound waves in air into mechanical vibration of the tympanic membrane, middle ear ossicles and basilar membrane. In this thesis we will concentrate on the measurement and modeling of vibrations of tympanic membrane and middle ear ossicles.
The middle ear acts as an impedance matching transformer of acoustic sound pressure in front of the tympanic membrane into perilymph fluid pressure in the cochlea. Direct stimulation of the cochlea by sound waves would be very ineffective because of the high impedance of perilymph fluid compared to air. The tympanic membrane and middle ear ossicles constitute an impedance trans
former consisting mainly of hydraulic pressure conversion, as a result of the ratio in area of the tympanic membrane and oval window at the entrance of the cochlea. The lever ratio of the ossicular chain is involved to a much lesser extent. The frequency dependence of this impedance transformer is determined
by the complex interaction of acoustical quantities (compliance, inertance, resistance) and mechanical quantities (stiffness, mass, damping) in the middle ear.
Understanding and measurement of this frequency dependence is of impor tance to such areas of acoustical science as directional hearing, earphone design, artificial heads, hearing aids, diagnosis of middle ear pathologies and middle ear surgery.
1.1 Scope of the present study
This study is devoted to the development and application of a sensitive measurement apparatus for the detection of extremely small vibrations down to 0.01 nm and less in living preparations. It will be applied to the measurement of auditory acousto-mechanical transfer characteristics.
The first application is aimed at the testing of prostheses in the middle ear. Prostheses are used to reconstruct the sound conductive function of the ossicular chain after disruption of the chain. Such a disruption can result from, for instance, chronic otitis media. Vibration characteristics are meas ured after reconstruction and are compared to the characteristics prior to reconstruction. In this way the influence of different positions of the pros thesis can be examined. In this study two types of prostheses will be investi gated. Results are applied in clinical practice.
The second application involves the middle ear of both cat and man. In this thesis we will measure transfer functions that will add to data from for mer investigations reported in literature. In particular, former measurements are extended by including phase measurements and by increasing the number of measurement locations on the intact or modified middle ear system. Moreover, the reliability of measurements is improved compared to former results by using lower levels of sound stimulation and by employing minimal invasive preparation procedures for gaining access to the middle ear structures. The data obtained are used to derive a comprehensive model of the functioning of the middle ear, extending its range to frequencies which are higher than those which can be obtained by existing models. In general, these models were pre viously derived from data obtained from acoustic impedance measurements. These impedance measurements are of limited reliability at frequencies above a few kHz. Besides, these measurements do not reveal in detail the transfer of sound into vibration of the middle ear structures. The model derived in this inves tigation is more generally applicable and is able to describe the vibration of
each middle ear ossicle as well as the acoustic impedance of the tympanic mem brane.
A third application is not discussed in this thesis, as it has been re ported elsewhere (Vlaming et al., 1984 and Aertsen et al., 1986). It deals with the mechanism of directional hearing in frogs and toads (anurans). Vi bration of the tympanic membrane is measured for several methods of sound stimulation. From these mechanical measurements and supplementary acoustical measurements a model for the frog's acoustic periphery was developed. On the basis of this model, it was possible to develop a mechanism to better under stand the monaural directional sensitivity of the frog under free field stimu lation.
In the next section the measurement of small vibrations in the auditory system is introduced. In the subsequent sections the peripheral auditory sys tem in man and cat will be reviewed briefly. In Chapter 2 the theory of laser interferometry will be described. This results in the development of the Laser Doppler Velocity meter (LDV) for the measurement of vibrations within the ear. The chapter is concluded by a discussion of the techniques for sound stimula
tion and signal processing. In Chapters 3 and 5 the instrument will be applied to the measurement of vibration characteristics of the tympanic membrane and middle ear ossicles. Chapter 3 will deal with the experiments on human tem poral bone specimens, and Chapter 5 with those on living anesthetized cats. Chapter 4 describes how the instrument can be applied to evaluate the use of two techniques in reconstructive surgery of the ossicular chain. Chapter 6 deals with the modeling of middle ear mechanics. After a discussion on exis ting models from literature, a new model that goes even further is proposed. In Chapter 7, data obtained from Chapters 3 and 5 will be used to estimate the parameters for the proposed middle ear model. Results will be discussed in relation to the mechanical measurements as well as to acoustical impedance measurements taken from literature.
1.2 Measurement of small vibrations in the hearing organ
For the measurement of amplitudes down to 0.1 nm and less, several very ingenious instruments have been designed in the past. In order to apply these instruments to the auditory system, at least two main requirements should be satisfied. First, no interference with the delicate vibrating structures may occur: the method should be contactless. Second, the application should not
require any substantial change in the normal anatomy of the hearing organ. Moreover, the instrument should be applicable for in vivo measurements on (an esthetized) preparations where background movements caused from within the preparation can be several orders of magnitude larger.
V. Bekesy (1960) applied two different techniques, i.e. microscopic obser vations under stroboscopic illumination and the capacitive probe. The former method provided the measurement of vibration amplitudes down to some 100 nm, whereas with the latter, more sensitive, method he was able to measure down to some 5 nm. The latter method has been improved in many ways, e.g. by Moller (1963) and Wilson (1974), obtaining a sensitivity in the order of 0.05 nm.
Another method was developed by Hillman et al., (1964) who used the Möss-bauer effect of a radioactive source placed on the object of investigation. The sensitivity was in the order of 0.1 nm. This method was also applied to the measurement of basilar membrane vibrations, e.g. Johnstone and Boyle (1967), Rhode (1971) and Helfenstein (1973). The last of these, however, glued the source on the object to be sure of a firm contact. The load imposed by the source on the vibrating structure is minimized by the use of an extremely small particle.
Another method in which a source is placed on the object of investigation is the SQUID magnetometer technique. In this technique the source consists of a small magnet. Under mechanical vibration this magnet causes a magnetic flux variation which is detected by the very sensitive SQUID magnetometer. With this method vibrations can be measured down to 1 nm. This method is applied to the mechanics of the human middle ear (post-mortem preparations) by Rutten et al. (1982).
The application of an interferometric technique was introduced by Khanna et al. (1968). They employed the classical Michelson interferometer, using a laser as a coherent light source. The instrument was successfully applied to the measurement of tympanic membrane vibrations. In order to increase the reflectivity they had to glue a miniature mirror on the membrane. A basic disadvantage of the method is that background disturbances can show up as a multiplicative noise in the output signal. This problem was partially overcome by several improvements; in a later version of the instrument Khanna et al. (1986) used a very sturdy construction which enabled measurements down to 0.0001 nm. Another approach was taken by Dragsten et al. (1976), who used a known reference signal for calibration, which moreover facilitated the use of diffusively scattered light.
A more intrinsic improvement, avoiding the multiplicative noise problem, can be obtained by applying a heterodyne type of interferometer, in which the reference and target beam are given a different frequency. This technique was first applied in biological experiments by Hill et al. (1977). With the aid of a gold covering, which is used to increase the reflectivity, they measured the change in axon diameter upon electrical stimulation with a sensitivity in the order of 0.05 nm. The Laser Doppler Velocity meter (LDV) whose development and application is described in this thesis is an interferometer of the heterodyne type. In order to avoid the necessity of glueing a mirror to the structure being tested, only light diffusively scattered by the object will be used (Buunen and Vlaming, 1981).
1.3 The peripheral auditory system in man and cat
The overall anatomical structure of the human ear is visualized in Figure 1.1. The peripheral auditory system can be partitioned into three subsystems: the external ear, the middle ear and the inner ear. The external ear conducts the sound pressure waves in open air to a sound pressure in front of the tym panic membrane. The external ear consists of the auricle (pinna) and the audi tory canal (external auditory meatus). The pinna is the visible flaplike por tion of the ear which receives the sound waves. The pinna leads to the exter nal auditory meatus, which is an irregularly shaped canal ending at the tympa nic membrane.
Fig. 1.1: Schematic representation of the human peripheral hearing organ. From Lindsay and Norman (1977).
The middle ear is an air-filled cavity which contains the middle ear vol-umes and middle ear ossicles. The middle ear is separated from the external ear by the tympanic membrane and from the inner ear by the oval and round win-dows of the cochlea.
The tympanic membrane (eardrum) is a cone-shaped structure of about 0.1 mm in thickness attached to the annulus at the bony wall of the meatus. From the middle upwards it is attached to the handle (manubrium) of the malleus (ham-mer). Sound waves bring the tympanic membrane into vibration. Via the ossi-cular chain, consisting of malleus (hammer), incus (anvil) and stapes (stir-rup), this vibration is conducted to the oval window of the cochlea. The tip of the malleus handle (manubrium) is called the umbo. The ossicular chain is suspended in the middle ear cavity by ligaments and muscles. In most mammals there are two muscles: the tensor tympani attached at the manubrium and the stapedius muscle attached at the neck of the stapes head. These muscles are functional to the middle ear reflex, which is thought to serve as a protection mechanism for excessive sound levels.
The middle ear cavities differ among most species. In man (Fig. 1.2A) they consist of the tympanic cavity (including the attic) containing the ossicular chain and a second cavity called the antrum, with the mastoid pneumatic cells which form a labyrinth of small cavities. The antrum is connected to the attic via a passage called the aditus ad antrum. The attic is the upper part of the tympanic cavity. In most animals, such as the cat (Fig. 1.2B) and the guinea pig, the second cavity is just a simple cavity called the auditory bulla, lacking pneumatic cells. The bulla is connected to the tympanic cavity via a
pneumatic cells
auditory
canpL
tympanic
membrane
^-passage
tympanic
cavity
\ Eustachian
tube
tympanic
cavity
tympanic
membrane
septum
bulla
cavity
'passage
Fig. 1.2: Schematic drawing of the middle ear cavities for man (A) and for cat (B).
small hole in the bony septum separating both cavities. The cavities are in communication with the outside world via the Eustachian tube. Its function is to equalize static pressure differences across the tympanic membrane. This tube is acoustically closed above a few Hertz.
The inner ear consists of the cochlea and the vestibular organs. The co chlea is of direct importance to the auditory system. It is a snail-shaped canal containing fluid and having over its length the basilar membrane, which supports the organ of Corti. The basilar membrane vibrates due to the pressure fluid waves caused by the excursions of the oval window. A gradual variation in the mechanical properties of the membrane causes frequency selection to occur in its vibration. High frequencies will generate a mechanical resonance near the basal turn close to the windows. Low frequencies generate resonances hear the outer end or apex of the cochlea. The organ of Corti transduces this mechanical vibration into nervous action potentials that are sent to the brain by the auditory nerve. The measurement and modeling of basilar membrane vi bration is the subject of study by several investigators, with the primary aim of clarifying the origin and existence of a sharp tuning and non-linearity as found in auditory nerve recordings. Recent measurements on basilar membrane vibrations support the concept of a mechanical origin of sharp tuning and non-linearity (Khanna and Leonard, 1986). A survey of mathematical modeling is given, for instance, by Viergever (1980).
The LDV instrument applied in this investigation was also used for the detection of vibration of the basilar membrane in cat and guinea pig. The re sults of pilot experiments confirmed in a large extent the measurements of Wilson and Johnstone (1975). Their measurements and ours (as well as those of many others) failed, however, to show a substantial non-linearity. Most likely this absence of non-linearity is caused by the high vulnerability of the co-chlear condition, as demonstrated in the study by Khanna and Leonard.
1.4 References
Aertsen, A.M.H.J., Vlaming, M.S.M.G, Eggermont, J.J. and Johannesma, P.I.M. (1986): Directional hearing in the grass frog (Rana temporaria L . ) : II. Acoustics and modelling of the auditory periphery. Hearing Research 21, 17-40.
Bekesy, G. von (1960): Experiments in hearing. McGraw-Hill, New York. Buunen, T.F.J. and Vlaming, M.S.M.G. (1981): Laser-Doppler velocity meter ap
plied to tympanic membrane vibrations in cat. J. Acoust. Soc. Am. 69, 744-750.
Light-scattering heterodyne interferometer for vibration measurements in audi tory organs. J. Acoust. Soc. Am. 60, 665-671.
Helfenstein, W.M. (1973): Beitrag zur messung der akustisch bedingten Bewe-gungen und Identifikation des mechanischen Teils des Innenohrs der Katze. Ph.D. Thesis. Eidgenossischen Technischen Hochschule. Zurich, Switzerland. Hill, B.C., Schubert, E.D., Nok.es, M.A. and Michelson, R.P. (1977): Laser in
terferometer measurement of changes in crayfish axon diameter concurrent with action potential. Science 196, 426-428.
Hillman, P., Schechter, H. and Rubinstein, H. (1964): Application of the Möss-bauer technique to the measurement of small vibrations in the ear. Rev. Mod. Phys. 36, 360.
Johnstone, B.M. and Boyle, A.J.F. (1967): Basilar membrane vibrations examined with the Mössbauer technique. Science 158, 389-390.
Khanna, S.M., Tonndorf, J. and Walcott, W.W. (1968): Laser interferometry for the measurement of submicroscopic displacement amplitudes and their phases in small biological structures. J. Acoust. Soc. Am. 44, 1555-1565.
Khanna, S.M., Johnson, G.W. and Jacobs, J. (1986): Homodyne interferometer for basilar membrane vibrations. II. Hardware and techniques. Hearing Research 23, 27-36.
Khanna, S.M. and Leonard, D.G.B. (1986): Measurement of basilar membrane vi brations and evaluation of the cochlear condition. Hearing Research 23, 37-53.
Lindsay, P.H. and Norman, D.A. (1977): Human information processing. Academic, New York.
Moller, A.R. (1963): Transfer function of the middle ear. J. Acoust. Soc. Am. 35, 1526-1534.
Rhode, U.S. (1971): Observations of the vibration of the basilar membrane in squirrel monkey using the Mössbauer technique. J. Acoust. Soc. Am. 49, 1218-1231.
Rutten, W.L.C., Peters, M.J., Brenkman, C.J., Mol, H., Grote, J.J. and van der Marel, L.C. (1982): The use of a SQUID magnetometer for middle-ear re search. Cryogenics 22, 457-460.
Viergever, M.A. (1980): Mechanics of the inner ear. Ph.D. Thesis. Delft Uni versity Press.
Vlaming, M.S.M.G., Aertsen, A.M.H.J. and Epping, W.J.M. (1984): Directional hearing in the grass frog (Rana temporaria L . ) : I. Mechanical vibrations of tympanic membrane. Hearing Research 14, 191-201.
Wilson, J.P. (1974): A sub-miniature capacitive probe for vibration measure ments of the basilar membrane. J. Sound Vib. 30, 483-493.
Wilson, J.P. and Johnstone, J.R. (1975): Basilar membrane and middle-ear vi bration in guinea pig measured by capacitive probe. J. Acoust. Soc. Am. 57, 705-723.
CHAPTER 2
METHODS
This chapter describes the Laser Doppler Velocity (LDV) meter and associ ated procedures for the measurement of the transfer function relating sound pressure and middle ear vibration.
2.1 Laser interferometry
The method of laser interferometry for the measurement of small vibrations in biological preparations has been used by several investigators. In general one of two basic techniques are used, i.e. optical homodyning and optical heterodyning.
The basic principle of the homodyne interferometer is that of a Hichelson interferometer, in which a coherent beam of laser light illuminates the object of interest. The reflected light interferes with the reference beam and the resulting fringe pattern is measured by a photodetector. The vibration of the illuminated object results in an amplitude modulation of the photodiode cur rent and the displacement amplitude is detected by the measurement of this modulation.
This homodyne technique has been introduced for the measurement in biolog ical preparations by Khanna et al. (1968). It was applied to middle ear meas urements by Tonndorf and Khanna (1968). They glued a miniature mirror at the tympanic membrane (i.e. malleus) to increase its reflectivity. In a test set up, vibration amplitudes of an earphone diaphragm could be measured down to 0.003 nm (10 dB S/N, 0.008 Hz noise band width). An improved version of this
homodyne technique (Khanna, 1986) enabled the measurement of vibration ampli tudes in a test setup down to 0.0003 nm (20 dB S/N, 0.008 Hz noise band width). A modified version of the homodyne technique, also applicable for diffusively scattered light, was developed by Dragsten et al. (1976).
Comparing all the merits of these homodyne techniques in detail lies bey ond the scope of this chapter. The disadvantage of the homodyne technique is its sensitivity to background disturbances. Its theory of operation shows that these show up as a multiplicative noise in' the electrical signal. This type of noise makes it very hard to recover the original signal by means of conven tional signal detection techniques. Part of the disturbances can be minimized by using a very sturdy mechanical construction (Tonndorf and Khanna, 1968; Khanna et al., 1986). They can also partly be compensated electronically in a closedloop feedback system. However, the effect of a feedback loop works for a limited class of background movements, i.e. only those that are not too big and not too fast. The effect of disturbances can also be accounted for by using a well-known reference signal as a calibration (Dragsten et al., 1976). These two techniques have the advantage that they also take into account back ground movements due to the heartbeat and breathing of the animal.
An essential improvement can be obtained by using an optical heterodyne interferometer. Instead of using two laser beams at the same frequency of light as in the homodyne technique, two beams of different frequencies are used. Such a heterodyne interferometer is the Laser Doppler Velocity meter as applied here. With this instrument the velocity of the object is measured from the frequency Doppler shift of the light that is diffusively scattered by the vibrating object. This Doppler shift will result in a frequency modulation of the photodetector signal. All background movements appear as additive "noisy" shifts. This additive noise can be electronically tackled in many different ways such as filtering, averaging or phase-sensitive detection. The advantages of the heterodyne technique have been recognized and used by Eberhardt and Andrews (1970), Hill (1977), and was applied by Michelsen and Larsen (1978) to investigate the hearing organ of crickets.
2.2 The Laser Doppler Velocity (LDV) meter
The Laser Doppler Velocity meter is an interferometer of the heterodyne type. The basic idea of its operation is that light scattered by a moving object undergoes a frequency shift f, given by:
(2v/X) sin(9/2) (2-la)
with 9 the angle ,between the incident and scattered beam, v the velocity as projected on a line in the plane of the two beams making an angle of (180-6)/2 with the scattered beam and X the wavelength of the laser light. (HeNe laser :
p
X=632.8 nm; corresponding to a frequency of A.741 10 MHz). For backseat tering as used in our instrument this relation becomes simply:
2v/X. (2-lb)
The layout of the interferometer is given in Fig. 2.1. Coherent light from a 5 mU HeNe laser (Spectra Physics) is split into a target beam and a refer ence beam. Each beam will pass through an Isomet 1205C Bragg cell (acousto-optical modulator). This device uses an internal ultrasonic traveling wave to
79.6 MHz
acoustic stimulator
0.4 MHz
stimulus generator
scope averager lock-in amp. (2«)
Fig. 2.1: Layout of the Laser Doppler Velocity meter (LDV). The laser beam is split for a fraction 1-T1 in a reference beam and a fraction T. in a target beam. The frequency of light for the target beam is shifted by 80.0 MHz and that for the reference beam is shifted by 79.6 MHz. After passing a beam splitter/reflector at T2=50%, the target beam is focused by lens L. at the object. Light (diffusively) scat tered from the object interferes with the reference beam at the photo-diode surface. The output of the photophoto-diode detector is band-filtered and FM-demodulated by the frequency tracker, yielding a signal propor tional to the velocity of the moving object. Small velocity signals are extracted from noise by prefiltering and using two lock-in ampli fiers. An alternative is to use an averager. Lock-in amplifiers or averagers are synchronized to the sound stimulus generator.
produce a frequency shifted outgoing laser beam (efficiency in the order of 90X). The target beam is shifted by 80 MHz and by 79.6 MHz for the reference beam. The target beam is focused by a lens L. on the object. The scattered light from the object is collected by this same lens L. into a parallel re turning beam. By means of a beamsplitter this returning beam is made to interfere with the reference beam at the surface of the photodiode detector (TPD TNO-TH type 731).
The total electric field of the light at the surface of the photodiode is given by:
Et(t) = Es c( t ) + Er e f( t ) , (2-2)
where
E cos(2n(fn + f,)t - *.) the scattered beam and
sc v v 1 d 1
E ,cos(2nf~t - ♦„) the reference beam
in which
optical frequency of the target beam 4.741 1 08 + 80.0 MHz;
optical frequency of the reference beam 4.741 1 08 + 79.6 MHz ;
Doppler shift 2v/X.
Phases t- and *„ are due to the optical path length of the target and refer
ence beams relative to some fixed point at the unsplit laser beam.
The electrical output current I from the photodiode is proportional to the incident light power given by:
I(t) « Et(t).Et(t) = (Es c(t) + Er e f(t)).(Es c(t) + Er e f( t ) ) . (2-3) Since the photodiode is limited in band width, the terms corresponding to optical frequencies and their multiplies are omitted, resulting in a current given by: ,(t) = E f( t ) ref £1 f2 fd
I(t) « E
2+ E
2+
' se ref
2E
sc.E
re£cos(2n(f
1-f
2)t + 2nf
dt + ( t ^ ) ) . (2-4)
This can also be represented by:
I(t) = K[I + I , +
sc rst
2e(I
s cI
r e £)
1 / 2cos(2n(f
1-f
2)t + 2nf
dt + (*
2-*
1))] (2-5)
where I is the intensity from the scattered light and I
r that from the
reference beam collected by the photodetector. K is a factor determined by the
photodector. The symbol c is the mixing efficiency (0<|e|<l) which takes into
account any reduction from optimal interference caused by effects such as
mis-alignment, lack of spatial coherence and dissimilar polarization between the
reference and scattered beams. Henceforth, f-i-fn will be denoted by f_ and
equals the difference in frequency between both Bragg cells: f_=0.4 MHz.
D
It is also possible to shift only one of the laser beams. This was applied
in a former version of the instrument (Buunen and Vlaming, 1981), where the
reference beam was shifted by 0.4 MHz using a rotating diffraction grating
(Oldengarm et al., 1975). Part of the measurements in Chapter 5 were performed
by this shifting method. The performance of this method is, however, some 10
dB less than when two Bragg cells are used.
The current in the photodiode is converted into a voltage and filtered by
a band filter (center frequency = f„) with a Q-factor which can be selected at
3, 10 and 25. The photodiode output is then given by:
V(t) = 2KRe(I
s cI
r e f)
1 / 2cos(2nf
Bt + 2n£
dt + (*
2-«
1)). (2-6)
R is the constant for converting current into voltage. The movement of the
object results in a phase modulation of the signal with carrier frequency f_.
Any disturbances due to background noise affect ♦~-*
1=t„(t)-*
1(t), causing an
additive noisy phase modulation. Any change in the reflectivity of the object
affects the amplitude of the signal only, and has no effect on the instan
taneous frequency which contains the information about the movement. A
so-called frequency tracker (comparable to an FM demodulator) produces a signal
proportional to the instantaneous frequency (minus carrier frequency f
R)
:where C is a constant determined solely by the frequency tracker. For a noise-free situation (last term is zero) and using relation (2-lb) the tracker out put becomes:
U = 4Cnv/X = C v (2-8)
with C =4Cil/X. The signal is therefore proportional to the velocity of the moving object. When the object is vibrating at angular frequency co, with am plitude d, the tracker output amplitude becomes:
U = Ctv(t) = Ctwd. (2-9)
It can be used to calculate the displacement amplitude d directly.
In normal operation the tracker output signal corresponding to the desired velocity signal will be accompanied by noise originating from several sources. For large velocities it is sufficient to have an rms meter giving the rms velocity directly. For small velocity signals with signal levels below, or in the order of the noise contribution, it is necessary to use a technique for signal recovery such as narrow band filtering, signal averaging or phase locked detection using a lock-in amplifier. The method of band filtering is less favorable since it makes phase measurement impossible, whereas both other (synchronous) signal detection techniques do provide the phase information. The techniques for signal processing will be described in Section 2.6.
2.3 Performance of the LDV instrument
The sensitivity of the instrument is given by the lowest level of vibra tion or displacement that can be detected. This sensitivity is determined by the noise level of the instrument, which originates from several sources. The most important sources are:
1) electronic noise from the photodiode and amplifiers 2) laser and Bragg cell noise
3) background vibration noise 4) noise in the frequency tracker.
1) Electronic noise
The photodiode, pre-amplifier and filter contribute electronic noise
the signal. The electronic noise from the photodiode is produced by shot noise originating from statistical fluctuations in photon activity. The instrument will be optimal with respect to its noise figures when the instrument noise is limited by photodiode shot noise as the physical limit. Amplifiers and filters can be designed in principle, irrespective of costs, such that their noise contribution is below that from photodiode shot noise.
The shot noise power is proportional to the total intensity of the incom ing light at the photodiode surface:
P . « I + I ,. (2-10) noise sc ref
For a reference beam which is much stronger than the scattered laser beam, the shot noise will be independent of the strength of the scattered beam.
The photodiode signal power is, according to (2-6), proportional to:
P . , ■= e2I ,1 . (2-11)
signal ref sc
This relation shows that signal power is not only proportional to the inten sity of the scattered beam but also to the intensity of the reference beam.
The sensitivity of the instrument is determined by the signal-to-noise ratio as given by:
P . , E2I ,1
signal ref sc
S/N = 5 - . (2-12) P . I + I ,
noise sc ref
The terms I , and I are determined by the amount of light entering the pho todiode. This is a function of the reflection coefficient a of the object, the
amount of light transmitted by beam splitters S. and S„, and the reflection of the returning scattered laser beam at S~. Beam splitter S. will transmit a fraction T. to the target beam and will reflect a fraction (1-T,) to the ref erence beam. Beam splitter S~ has a transmission of T_ for both beams and a reflectivity of (1-T„) for the returning scattered beam. With I as the laser intensity we have:
Ir e £ = d-T^TjjI (2-13a)
Is c = T ^ d - T ^ I a (2-13b)
S/N « I
T j d - T ^ a - T ^ f f
(1-Tj) + TjCl-T^a
(2-14)
In Fig. 2.2 the signal-to-noise ratio is plotted in arbitrary units as a
function of transmission T.. at T„=50% and o=0.01. In practice, the scattered
beam will be much less intense than the reference beam, so that the second
term in the denominator (determined by I ) can be neglected, resulting in:
S/N « I T J T J U - T ^ C .
(2-15)
0.8 z X 1/1 0.6 f 0.1 c 02 0 J1
1!
0.21
i y
' \
i
1
1
1 04 06 Transmission Ty 08Fig. 2.2: Signal-to-noise ratio as a function of transmission fraction
T
1of the laser intensity that is split into the target beam. Coeffi
cient of object's reflectivity
a is 0.01 and splitter transmission T„
is 50%. Plotted S/N value is relative the maximum S/N value for the
This relation corresponds to the straight-line portion of Fig. 2.2. At this
line, the S/N is optimal at T„=50X. An optimal T. is read from this figure at
1^=95%. The optimum T.. increases towards 100% for a further reduction of o.
However, taking T
1too large will give a drastic reduction of S/N. It is
therefore advisable to choose T. between 70 and 95%, which gives an almost
optimal S/N ratio for values of o in the range between 0 and 0.2. A value of
T
1between 70 and 95% has the additional advantage that the reference beam,
which is proportional to l-Tj. is quite strong, boosting the signal level. An
increase of T
1towards 100% will reduce the signal level, reaching a level
under which the other types of electronic noise (amplifiers and filters) are
dominant. In our optical arrangement we have chosen T„=50% and T.=90%. In
practical situations, while measuring middle ear vibrations, it was experi enced that noise within the system was always dominated by shot noise.
2) Laser and Bragg cell noise
Each Bragg cell is driven by a stabilized oven-controlled crystal oscil lator. Instability in frequency, in particular short term instability, can produce a phase modulation between both laser beams, showing up as a spurious Doppler signal output noise.
The laser source itself exhibits various forms of noise which can contri bute to a noisy signal output from the instrument. The most important noise sources are: 1) intensity variations originating from fluctuations in laser plasma current and 2) laser intermode modulation. The latter is caused by mixing between laser modes in the non-linear laser medium. In practice, their effects showed up only occasionally. Including Bragg cell instability, these noise sources are of minor importance compared to photodiode shot noise and frequency tracker noise.
3) Background noise
Background noise is defined as all vibration measured by the instrument that does not directly result from a stimulus to the object. This can be ambi ent vibration as well as motions from the living preparation such as breath ing, blood pulsation etcetera. Background motion can be reduced by applying vibration isolation and damping of the instrument as well as by the use of a sturdy construction for attaching the animal to the interferometer.
Both ambient vibration and vibration induced from inside the living prepa ration can be reduced to some extent by applying a reference mirror attached to the skull of the animal. Such a construction is described and applied by Khanna et al. (1986), who were forced to pay considerable attention to the elimination of background noises. Such noises are multiplicative in nature in their homodyne type of interferometer.
In our setup no extensive attention was paid to making a very sturdy and/or vibrational isolated setup. Most background noise turned out to origi nate from inside the animal. Using a reference mirror attached to the skull turned out to be too complex for such small gain. Since background noise is additive in nature for our heterodyne type of interferometer, its effect can be handled sufficiently adequately by filtering or applying a synchronous
detection technique.
4) Frequency-tracker noise
The operation of the frequency tracker instrument (TPD TNO-TH type 1077) will be described here briefly. The frequency tracker consists of a frequency discriminator whose output voltage is proportional to the instantaneous fre-quency at its input. This discriminator operates at a mid frefre-quency of 800 kHz. The photodiode output signal with a carrier frequency of 400 kHz is mixed with the output of a voltage controlled oscillator (VC0) to produce this mid frequency. The output of the discriminator is supplied to the VC0 via an integrator which keeps the mixed input of the discriminator at its mid fre-quency. The integrator output is used as output for the instrument. This principle is known as FM demodulation using feedback. An analysis on its operation is given, for one, by Taub and Schilling (1971).
Noise is contributed in several stages of the instrument. With a noise-free input signal, the output noise is contributed mainly by the VC0. Fig. 2.3 presents output noise measured as a function of the added input white noise level relative to the signal carrier strength. This is a simulation of a car-rier signal corrupted by, for instance, shot noise as given in (2-10). When the carrier band filter is set at Q=25, which is typical in normal operation, the flat region corresponds to the VC0 contributed noise, yielding a level of 19 mV RMS (band width 40 kHz). It is seen that below an S/N of -25 dB the output noise increases very rapidly. This is attributed to the threshold ef-fect in FM detection (Taub and Schilling, 1971).
too so
>
£ ö1 ' °
3 O 20 - t O -30 - 2 0 -K> 0 10 Input S / N in dBFig. 2.3: Noise rejection of frequency tracker. Frequency tracker out-put noise (100 Hz - 40 kHz) as a function of the added inout-put white noise signal relative to the input carrier signal.
Fig. 2.4 presents measured noise levels at a 3 Hz band vidth. The noise levels are converted into equivalent RMS displacement amplitudes. The solid line represents the noise level for the complete instrument as measured by focusing the instrument on a heavy metal block (10 kg) with a diffusively scattering surface. This heavy block eliminates most of the background noise sources. It is seen that noise level is in the order of 0.1 nm to 0.03 nm for frequencies under 500 Hz and reduces to 0.005 nm for the higher frequencies. In practical measurements a noise band width of 0.025 Hz is applied using lock-in amplifiers, so that the noise level is further reduced by a factor of 10. nm „is1
!
ʻ2 us3N
01>
1
03^
— ^ S ^ j s - "":^ ^ ^ _
1,'"""-1 i
1 3 Frequency (kHz) 1 ■. A \ t\'"!'"-10 dB
/
,'
-/
.-•""'
-20 -10 -60 30Fig. 2.4: Measured noise level at a 3 Hz band width expressed in equi valent displacement amplitudes as a function of frequency. The solid line represents the noise level for the complete instrument as meas ured from the remaining noisy vibration of a 10 kg weighting block. The broken line corresponds to the noise level measured at the fre quency tracker with an artificial electronic signal. The dotted line represents the total noise level for a former version of the instrument in which the carrier signal was generated by a single rotating grating instead of two Bragg cells.
In the same figure the dotted curve represents the same measurement in which a rotating grating is used for producing a shifted reference beam. It is seen that this grating gives an average increase in noise of 10 dB compared to the instrument using the Bragg cells. This increase is attributed for the most part to noisy fluctuations in the carrier frequency, caused by irregularities in the rotating grating.
The broken line corresponds to the noise level measured at the frequency tracker using an electronically produced FM signal with negligible noise. It is seen that at low frequencies up to 5 kHz the instrument noise level (solid line) equals this tracker noise level. This means that under 5 kHz tracker
noise is dominant. At higher frequencies, the other noise sources, in particu lar shot noise become more important. It is concluded that tracker noise and shot noise are of the same order of magnitude.
For highly reflecting objects (mirror reflection) the sensitivity will improve only slightly, since it will be limited by the noise originating from the frequency tracker. For low reflecting objects the sensitivity is deter mined by photodiode shot noise and thus sensitivity will decrease depending on
the reflection coefficient (a) and frequency tracker noise rejection. In practical measurements at the middle ear ossicles, the reflection coef ficient is in the same order as for Fig. 2.4. This means that noise contribu tions from the frequency tracker are in the same order as that from shot noise. This indicates that, given its application, the total LDV meter is quite balanced when considering its noise figures.
2.4 Calibration
The calibration of the interferometer is straightforward and consists pri marily of the calibration of the frequency tracker. This is performed by ap plying an artificial FM modulated signal with known modulation depth. Such a signal can be constructed quite easily by modern function generators. Accuracy in respect to the depth of modulation depends on the calibration supplied by the manufacturer.
As a check on modulation depth, a second method is used in which the car rier frequency is changed stepwise. Frequency is measured before and after the step using a frequency counter. Inaccuracy is determined mainly by slow drift ing. Both methods gave identical results to within 1%. The calibration factor for the LDV frequency tracker was determined at 0.237 mV/Hz ±1%. A Doppler shift of 1 Hz corresponds to a velocity of 316 nm/s, so that the LDV
calibra--12
tion is determined at 1340 10 Vm/s.
A further check on calibration is made for the complete instrument by using an accelerometer to carry out a comparative measurement. To this end the LDV target beam is focused on the housing of a B&K 4368 accelerometer mounted on an exciter (MB model PM25). Upon stimulation at a frequency of 400 Hz, the accelerometer gave an output 4% higher on the average, at several levels of stimulation. This discrepancy, however, falls mainly within the inaccuracies of the accelerometer (±5%). The agreement is considered satisfactory.
Other aspects relevant to calibration are linearity and frequency re sponse. The linearity of the frequency tracker is verified for Doppler shifts
ranging from 0.5 Hz to 900 Hz, giving a linearity within 2X. Chapter 5 gives
linearity measurements for actual measurements of displacement levels from 0.01 nm to 1000 nm. Linearity can be affected by a spurious carrier signal. A spurious carrier signal is a carrier which is not generated from the interac tion between the reference and the scattered beam, but is generated in some other way, for instance, from electrical crosstalk between both Bragg cell systems and from unwanted reflections within the optical system. It results in an amplitude error which depends on the relative strength of the spurious sig nal (Hill, 1977). The existence of spurious signals can be easily detected by blocking the scattered beam. This check is performed as a regular routine after each alignment of the interferometer.
The frequency response (amplitude and phase) of the LDV meter is measured and presented in Fig. 2.5. The frequency response was also checked with accel-erometer measurements, as denoted by crosses in the figure. The frequency re sponse curves are used for correction in all subsequent applications of the instrument. to m « o 1-10 3 -20 cr -30 Q, U ft Ul S -90 | - 1 8 0
«
| -270 | 01 0 3 1 3 10 30 a Frequency (kHz)Fig. 2.5: Amplitude and phase response of the Laser Doppler Velocity meter. The curve represents the frequency response as measured elec tronically from the frequency tracker. The crosses represent measure ments for the complete instrument as measured by an accelerometer.
2.5 Sound stimulation
For stimulation of the ear, closed sound systems were applied. For meas urements at the tympanic membrane, an otoscope was adapted to stimulate the
tympanic membrane in a closed acoustic system and at the same time to focus the laser beam on the membrane. A sketch of this adapted otoscope is given in Fig. 2.6. The lens in the otoscope corresponds to lens L. of the interferome ter and could be adjusted for focussing over a range of 25 mm. The view field through this stimulator with a laser spot focused on the tympanic membrane is shown in Fig. 5.1. The level of the sound pressure generated by a Beyer DT 48 headphone was measured directly in front of the membrane. A 2 mm probe tube was mounted for this purpose through a small hole in the otoscope's speculum. A 1/2-inch B&K microphone was connected to the probe tube. The probe tube was calibrated by connecting the otoscope with a short rubber tube to a reference microphone. The distance between the probe tip and the diaphragm is 3 mm and is comparable to the distance between the tip and the tympanic membrane during the actual measurements.
heodphone
microphone
Fig. 2.6: Sketch of the otoscope acoustical stimulator with probe mi crophone. The position of lens L. is adjustable over 25 mm in order to focus the laser beam on the tympanic membrane. The membrane is illumi nated by means of a lamp and light conductor.
The otoscope stimulator was not used for measurements at the piddle ear ossicles. In these measurements a simply, small standard stimulator was ap plied, giving more accurate sound levels at higher frequencies. The standard stimulator consists of a tube of 5 cm in length and having an inner diameter of 5 mm with a built-in fixed probe tube. A Beyer DT 48 headphone was again used for stimulation, and the probe is connected to a 1/2-inch microphone. The calibration procedure is the same as before.
The measurement of the sound pressure level is rather sensitive to small changes in the position of the probe tip relative to the tympanic membrane, in
particular for the higher frequencies. For the otoscope stimulator the error in the reproducibility of the measured sound pressure level turned out to be less than 1.5 dB for frequencies below 5 kHz, and less than 3 dB for frequen cies between 5 and 10 kHz, increasing to 5 dB for frequencies between 10 and 20 kHz. For the standard stimulator these errors are less than 1 dB under 5 kHz, less than 1.5 dB between 5 and 10 kHz and up to A dB between 10 and 20 kHz.
The error in reproducibility between both stimulators turned out to be less than 2 dB for frequencies under 5 kHz and less than 4 dB for frequencies between 5 and 10 kHz.
The error in reproducibility of the phase is about 45 degrees at 10 kHz for both stimulators. This error increases with frequency. In all subsequent experiments the sound stimulator calibration curves (amplitude and phase) are used for correcting the sound stimulus measurements. This means that all meas urements are relative to the sound pressure as measured in front of the mem brane.
2.6 Signal processing
The LDV meter is applied to the measurement of the acousto-mechanic trans fer functions of the middle ear. The input signal is the sound stimulus as measured by the probe microphone. The output signal is the displacement of the structure of interest. The transfer function is presented by its amplitude and phase characteristic as measured at discrete frequencies.
In processing the LDV signal and sound signal, the same instruments are used in order to cancel the effect of phase distortion and amplitude distor tion as introduced by filtering, amplifying etc. The output of the signal tracker and probe microphone are both analyzed by means of two lock-in ampli fiers arranged in quadrature. Routinely, 25 to 30 different pure tones were applied over the frequency region of 170-12000 Hz.
The experiments are fully controlled by means of a PDP LAB 8 computer. This computer controls the setting of stimulus frequency, attenuators, filters and time constants of the lock-in amplifiers. The outputs of both lock-in am plifiers are measured by means of the computer AD converters.
For most experiments the sound pressure level is measured and adjusted at a level of 80 dB SPL. The attenuators are varied until the output of the lock-in amplifiers are at 50% of their range. At that moment the time constant of the lock-in amplifiers is switched from 1 to 10 seconds and an accurate
meas-urement is made, A time constant of 10 seconds corresponds to a noise band width of 0.025 Hz. This measurement is repeated and the mean result is ac cepted when the two measurements are within 10%. If both results differ too much this procedure is repeated. In case the interferometer signal becomes too weak, this is detected by the frequency tracker by means of a drop-out signal which is read by the computer. Depending on the length of the drop-out time, the operator is warned to readjust the laser interferometer and the measure ment is repeated partly or in full. Under normal conditions a full transfer function of the tympanic membrane or middle ear structure could be measured within 20 minutes. In some experiments periodic noise or periodic pulses (20 ms repeat time) were used for sound stimulation. Measurement signals from the LDV and probe microphone were averaged using a PAR 4202 averager. The number of samples was 1024; the number of averages was 1000 to 5000. Periodic noise or periodic pulses were generated using a signal recycler, which was synchro nized to the clock of the averager. The signal recycler was loaded from the computer.
The periodic (white) noise signal was computed from a low pass filtered maximum length sequence of 1023 points resampled to 1024 points. The spectrum was flat within 2 dB up to the filter corner frequency of 15 kHz. In some cases the amplitude spectrum above 5 kHz was increased by 10 to 20 dB in order to increase the signal-to-noise ratio at the higher frequencies. The drop-out signal was used to inhibit the signal averager so that responses during a weak photodetector signal were rejected and did not contribute erroneously to the result.
The results from the averager were interfaced to the computer and their spectra were computed. The transfer function (amplitude and phase) was calcu lated and corrected for the calibration characteristics from the probe micro phone and frequency tracker.
The pure tone method using lock-in amplifiers and both the periodic pulse and periodic noise methods gave results identical to within 1 dB.
2.7 Concluding remarks
In the present version of the instrument no special attention was paid to obtaining a high spatial resolution (spot diameter = 0 . 3 mm; depth of focus = 0.2 m m ) . In the present application this was not requested. A high spatial resolution can be obtained by expanding the laser beams. This requires the use of more accurate optics in order to maintain coherence. Moreover, the internal
reflections within the instrument will become more troublesome, more easily generating a spurious carrier signal, which will in turn produce a distorted output signal, in particular for low reflecting objects.
The sensitivity of the instrument is determined by the noise level. For lock-in detection at a commonly used time constant of 10 seconds, the noise level is 0.0005 nm for frequencies between 1 and 40 kHz. For a detection cri terion of 20 dB S/N this gives an instrument sensitivity of 0.005 nm; for a time constant of 30 s this becomes 0.003 nm. This sensitivity can be compared to a sensitivity of 0.0003 nm (time constant of 30 s and 20 dB S/N) for the homodyne interferometer as applied by Khanna (1986). The difference is that our sensitivity applies to a measurement in which only diffusively scattered light is used. Khanna's measurement applies to a test setup consisting of a mirror reflecting microphone diaphragm. In the LDV meter the use of dif fusively scattered light has the advantage that the instrument need not be aligned perpendicular to the reflecting surface and, moreover, that it is not necessary to attach a mirror that can affect the vibration of the object.
The LDV sensitivity is limited by the noise generated from the frequency tracker. For objects which reflect well, the sensitivity can be increased further by reducing the frequency tracker generated noise. For objects which do not reflect well, the sensitivity will be limited by photodiode shot noise. The signal-to-noise ratio when limited by shot noise can be improved by in creasing the laser power.
The accuracy which can be achieved when measuring a transfer function is determined by the accuracy of the LDV meter and by that of the sound measure ment. The latter of these is the most dominant. The total inaccuracy (repro ducibility) will be in the order of 2 dB for frequencies under 5 kHz and in the order of 4 dB for frequencies between 5 and 10 kHz. For the phase the re producibility error is in the order of 45 degrees at 10 kHz. The effect of a non-constant physiological condition of the preparation must be added to this. In general, the condition of a preparation will degrade with time. This is caused by drying out of tissue and degeneration of tissue proteins (lysis). This will affect the reproducibility of the measurement. Experience has shown that this is within 1 dB for a measurement repeated within 1 hour and that this increases to within 4 dB for 6 hours.
2.8 References
ap-plied to tympanic membrane vibrations in cat. J. Acoust. Soc. Am. 69, 744-750.
Dragsten, P.R., Webb, W.U., Paton, J.A. and Capranica, R.R. (1976): Light-scattering heterodyne interferometer for vibration measurements in audi tory organs. J. Acoust. Soc. Am. 60, 665-671.
Eberhardt, F.J. and Andrews, F.A. (1970): Laser heterodyne system for measure ment and analysis of vibration. J. Acoust. Soc. Am. 48, 603-609.
Hill, B. (1977): Design of a laser interferometer for measurement of extremely small biological motions: Application to crayfish giant axon. Stanford University, Ph.D. Thesis.
Khanna, S.M., Tonndorf, J. and Ualcott, W.W. (1968): Laser interferometry for the measurement of submicroscopic displacement amplitudes and their phases in small biological structures. J. Acoust. Soc. Am. 44, 1555-1565.
Khanna, S.M. (1986): Homodyne interferometer for basilar membrane vibrations. Hearing Research 23, 9-26.
Khanna, S.M., Johnson, G.U. and Jacobs, J. (1986): Homodyne interferometer for basilar membrane vibrations. II. Hardvare and techniques. Hearing Research 23, 27-36.
Hichelsen, A. and Larsen, 0. (1978): Biophysics of the eusiferan ear. J. Comp. Physiol. 123, 193-203.
Oldengarm, J., van Krieken, A.H. and van der Klooster, H.U. (1975): Velocity profile measurements in a liquid film flow using the laser Doppler tech nique. J. Phys. E. Sci. Instrum. 8, 203-205.
Taub, H. and Schilling, D.L. (1971): Principles of communication systems. McGraw-Hill Kogakusha, Ltd.
Tonndorf, J. and Khanna, S.M. (1968): Submicroscopic displacement amplitudes of the tympanic membrane (cat) measured by a laser interferometer. J. Acoust. Soc. Am. 44, 1546-1554.
CHAPTER 3
MECHANICS OF THE NORMAL HUHAN MIDDLE EAR
In this chapter the Laser Doppler Velocity meter is applied to the meas urement of mechanical vibrations of the tympanic membrane and middle ear ossi cles in the human ear. The results concern post mortem measurements on tempo ral bone preparations. The preparations permit relatively simple access to the middle ear. Measurements are reported for the tympanic membrane (umbo), for the stapes footplate at several locations and for the tympanic membrane (umbo) when the malleus is disconnected from the incus, stapes and inner ear.
This chapter is reprinted from: Vlaming, M.S.M.G. and Feenstra, L. (1986): Studies on the mechanics of the normal human middle ear. Clin. Otolaryngol. 11, pages 353-363.
Studies on the mechanics of the normal
human middle ear
M. S. M. G. VLAMING AND L. FEENSTRA*
Applied Physics Department,- Biophysics Group. University of Technology. Delft and *EST Department. Free University Hospital. Amsterdam, the Netherlands
Accepted for publication 26 February 1986
VLAMING M. S. M. G. & FEENSTRA L. (1986) Clin. Otolaryngol. 11, 353-363 Studies on the mechanics of the normal human middle ear
The middle ear was studied in temporal bone preparations using a laser-Doppler interferometer. For measurements at a sound level of 80 dB SPL this method proved to be very reliable, as was shown by good reproducibility of results in experiments over more than 6 hours. The vibrations of the tympanic membrane and stapes footplate were studied from-200 Hz to 10 KHz and the results demonstrate a piston like movement of the stapes footplate up to 120 dB SPL. The damping effect of the normal ear is located mainly at the footplate/cochlea level and the middle ear cavity
per se does not contribute significantly to the stiffness of the middle ear system.
Keywords mechanics middle ear laser iitterferometry footplate
