 The Clarinet BBoard
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Author: S.Koumas
Date: 2001-04-30 20:14
Hi,
I was just wondering how sound is measured... i know its measured! I know it is measured in Frequencey and Amplitude, but how do they measure them both? Can someone enlighten me on this subject?
Cheers
S.Koumas!
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Author: Jo
Date: 2001-04-30 21:02
The frequency is just the measurement of the sound wave. Since the sound wave can't be seen, they usually use machines and computers and whatnot to look at the waves. Same with Amplitude. To be honest, I am not really sure what they did to figure any of it out back before any of the machinery was invented.
Phythagoras discovered that dividing a guitar string in half will double its frequency. It's the same for other things too- organ pipe, yard stick... etc. I suppose the entire thing started then and he was pretty smart and found all of this out without computers.
Good luck!!!
Jo
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Author: Bill
Date: 2001-04-30 21:06
I'll take a try at a crude explanation.
A microphone is connected to a computer. The input is sound amplitude/volume which changes with time. You could play a long "steady" open G, or other note, which would have close to the same amplitude as time advanced, or you could vary the amplitude by playing louder and softer.
The computer uses a mathematical technique called a (Fast) Fourier transform to convert from the time domain to the frequency domain. The output can be displayed as amplitude spikes, with spikes representing the the frequency spectral content of the input, e.g., a clarinet being played. You see a series of spaced vertical lines with different heights. Each spike is a frequency representing the fundamental frequency and the harmonic frequencies. Different instruments would show different harmonic content.
The following freeware tuner progam has a conventional tuner, and also displays the frequency spectrum. You can also see a frequency spectrum displayed on the web page.
http://www1.ocn.ne.jp/~tuner/tuner_e.html
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Author: Bart Hendrix
Date: 2001-05-01 17:05
A sound wave is a longitudinal (or compressional) wave that requires some material through which to travel (space adventure movies not withstanding, sound cannot travel through a vacuum). By rapidly interrupting a moving air column (as in speech or playing a clarinet) there is a series of high compression areas separated by low compression areas moving outward from the source. With a clarinet or voice, the compressions and rarefactions (low pressure areas) are further shaped by the instrument or oral and sinus structures (but that has little to do with the answer to your question).
The number of compressions that pass a given point in one second is referred to as the frequency and our ear/brain system translates that into pitch. To measure the frequency, an electronic system just needs to be able to count the number of compressions that pass its sensor in one second.
The amplitude (volume) is simply the difference in pressure between compressions and rarefactions. If a sensor is able to detect and measure the pressure changes caused by a passing sound wave, it can measure the amplitude. Amplitude can also be considered an expression of the amount of energy caried by the wave.
The rest is just electronics translating the measurements into something we can understand as observers.
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Author: Gordon (NZ)
Date: 2001-05-02 12:44
There are many ways of doing the measuring, both electronic and non-electronic.
For frequency (pitch) we could just compare it with a matching note on a piano and look up the frequency (vibrations per second) for that note in a book.
Or we could sit the speaker playing the note in water and measure the length of the ripple waves produced. Frequency = velocity of sound, in this case in water (which can be looked up in a text book), divided by the wavelength.
It is sometimes difficult to explain what is happening in electronic congtraptions because there is nothing to see....
Amplitude (volume) is related to the amount of energy in the sound, so this sound can be picked up with a microphone and the energy sent along a wire to an electronic storage device and measured. The storage and measuring device could work something like this:
Imagine water (representing the sound energy) going in an eratic fashion into a storage tank which has a slow leak. The higher the level of water in the tank, the more the water must be coming in, so we can measure how fast the water is coming in by how high the level is. This sort of thing is done with electronic contraptions.
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Author: Gordon (NZ)
Date: 2001-05-02 12:49
I didn't really finish. The measurement could then be sent to a meter (like a car speedo) so the needle shows us how much sound volume there is. This whole thing would be called a DB meter, or decibel meter, with volume being measured in decibels.
Or we just let the sound into our ears and our ear and brain do all the fancy electronics stuff and declares the sound loud or soft!
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