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Thunder drums

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Iron belss

Out of the three elements of the drum: 1) the tube, 2) the membrane and the 3) spring, all of them perform a unique role in generating the sinister thunder sound. These sounds are low in frequency, pretty loud, growing and lowering in intensity like a real, distant thunder.

Thunder drums Simple sound

Sounds of a rod

Closed flute

Wave modulation can be done with two iron bells.

E. Rajch1), A. Kamińska1), Ł. Jasiński1) G. Karwasz1,2)

1)Institute of Physics, Pomeranian Pedagogical Academy, 76-200 Słupsk, POLAND 2)Faculty of Engineering, Trento University, ITALY

It is not so easy to obtain a “clear”, i.e. monocromatic sound. It turns out, that any (higher quality) wine or beer glasses generate almost one-frequency sounds.

Sounds are the easiest to detect wave phenomena. Human ear is able to recognize single instruments in a symphonic orchestra, by detecting harmonic components characteristic to an instrument, i.e. performing Fourier analysis.

Fourier analysis even one hundred years ago was a difficult, mechanical task.

So-called König analyser was a series of empty metallic spheres, resonating at characteristic frequencies. Ten years ago, digital oscilloscopes

http://polly.phys.msu.su/~zeld/oscill.html were needed to do this same. Today it is free software and cheap microphone connected to the PC input.

Evolution in time of the “thunder”; higher frequencies fall down quicker than lower ones – the sound velocity is lower for low frequencies.

Then, the low frequencies fall and rise, like a distant reflection of a thunder from clouds and mountains.

1)

Virtual Oscilloscope Sir Wiliamm

Thomson Harmonic analyzer (Science Museum”, Londyn)

f1=429 Hz

The fundamental pitch for closed tube

L=0.19 m, f1=429 Hz

f1=0/4L

3f1 5f1 7f1

Wine glass (125cm3) f=1290 Hz

1290 Hz

290 Hz

Large brandy glass (4000 cm3) f=290Hz

The "music" is due to radial deformation of the vessel:

squeezing it in one dimension, pulling in the other.

If water is added to the glass, the frequency of the sound gets lower: now a bigger mass (water+glass) vibrates.

„Musical glasses” were popular in 18th century Europe. From those times comes Glass harmonica, invented by Beniamin Franklin.

Glass harmonica has 28 glass bowls with diameter increases from 70 mm to 160 mm, which were easier to use instead tuning glasses by filling them water to insure the availability of all the tones

required by the music.

Several 18th century composers wrote for the glass harmonica,

including Mozart, Johann Friedrich Reichardt, Karl Leopold Röllig, Johann Abraham Peter Schulz, and Johann Gottlieb Naumann.

Now, you probably understand that what is nice or bad is just mathematics.

The two bells alone have close frequencies, as shown by

Fourier transform.

Played together, they sum-up to a modulated sine-wave see to the left.

Our ear makes Fourier

deconvolution and does not like it.

Almost every glass sings: you must insist on delicate rubbing it with a wet finger. The frequency depends quite little on the vessel form, and more on this glass type.

Glass Armonica, probably early 1760's

The Bakken A Library and Museum

König analyser

(Karl’s University, Prague)

A metal rod creates a variety of sounds, depending on the mode of excitation.

Open flute

In open tube (blown at its end) arrows are formed at both ends.

Frequencies scale like 1:2:3 etc. i.e.

in "octave" intervals

1) even the lightest blowing generates higher harmonics But:

2) Exact scaling does not hold for the strongest blow - the dominating frequency belongs to the "closed tube"

series.

The fundamental pitch for open tube f1=0/2L

f1=587 Hz

2f1 3f1 4f1.

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