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measuring the speed of sound?

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dmg

Tue Mar 02 2010, 12:03AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

I was wondering, how would one measure and display on an Oscillioscope the waveform of sound.
I had a simple system with a tone generator, an O-scope and a speaker with microphone and this was inside a glass resonant tube. this method did not work, as the Oscillioscope is not sensetive enough to detect anything from a microphone, and I would like to avoid using an amplifier, as I do not have a decent one for this...

does anyone know, or have tried a simular method to show and measure sound waves?
also, the tone generator will operate at fairly low frequencies (50-200Hz) but is capable of going into the beyond human hearing mega hertz range.

Myke

Tue Mar 02 2010, 01:03AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

The microphone probably isn't working if the o-scope can't detect anything from it. Either that or the mic is a resistive or capacitive mic which don't generate a voltage. You might try seeing if you can see the change in voltage drop across the mic when it's connected to a small voltage source.
You can get the wavelength of the sound by matching the phase of the source and receiver. Then move the receiver farther from the source until the wave is shifted by 360 degrees. This change in distance is the wavelength.

radiotech

Tue Mar 02 2010, 06:50AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Why dont you sprinkle some fine particulate matter *inside* the glass resonant tube and look at the nodal patterns made by the soundwave. A good book is Helmholtz "Sensations of Tone" You will appreciate how they made oscillographs and dealt with coils and condensers 150 years ago too.

dmg

Tue Mar 02 2010, 07:33AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

hmm... what about cork dust?...
and now talking about this, I wonder what will happen with a fluid?

well, this method will work, in theory that waveform's antinodal areas will have rapid vibrations (movement, whatever) of the powder, and the nodal areas should show no movement, this will give me a rough idea, but I would like to see the waveform on a scope, like myke has suggested, using a propper microphone connected to the scope is probably what I will do,

also, I might have on one channel of the scope (lets say A) will be the tone gennie's input (parraleled to speakers) and that will form one waveform, the other channel, (lets say B) will be connected to only the microphone, and will be attached to a rod, with a stopper on the tube to act as a close tube, the rod will be able to move back and forth inside said tube, and see what effect the microphone reads, in comparason to channel A, that has the original signal (tone generator's) input.

so bottom line is, I think If it works right, I should see one constant waveform (will be sinosoidal, no, I dont know how to spell it.) and that waveform will apear on the screen, along with the other waveform (the microphone's) and I will see how far or much the waveform is distorted, or any change in its appearance, in comparason to the waveform of the tone generator. I think I will be able to measure the distance of the waveform this way, by the distance between the 2 waveforms, since I will know the frequency of the tone generator, and it shouldn't be distorted, as its signal is fed directly to the oscillioscope. and the microphone will be distorted, as it is moved back and forth through out the resonant tube, but I will also try Radiotech's method, because, well... thats just freaking sweet.
but essentially, yeah, that is what I will try to do, rather then have the tone generator purely connected to the speaker, and the microphone connected to the O-scope, the tone generator will also be connected to one of the channels on the scope. (parraleled)

worse case senario, is that I hook one of the tubes openings to a mega subwoofer, have a cone to focus the sound and end up vibrating the tube off the table, promptly shattering it on the floor, not only bringing upon me the fury of the physics teacher, for shattering the tube, but also causing massive headaches and a barrage of unrelenting facepalms from the supply head, as they will need to order a new one, bearing the thought that it might only break again...

anyway... tomorow I will set it up and see if this method will work.

Mates

Tue Mar 02 2010, 09:25PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Just put the probe from the osciloscope close to the sound generator (I suppose the speaker it is electromagnetic device) and set the sensistivity higher. You should see the signal !

Bjørn

Wed Mar 03 2010, 12:17PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The wavelength alone will not give you the speed of sound. A small speaker will work as a suitable microphone.

Also remember that if you use an oscilloscope with two channels you need to understand how the triggering work or you might get an invalid result.

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