Re: [Phys-l] frequency and wavelength of sound in air

[curtis osterhoudt <flutzpah@yahoo.com>]

The "fingerprint" explanation is OK, though of course it's using a "it is the way it is because that's how it is" explanation. You might want to think about what sound actually _is_ in this case: pressure fluctuations in the air (or water), which lead to alternating compressions and rarefactions. If the sound in the air has a certain frequency (the compression->rarefaction->compression cycle takes the same amount of time for a given tone), and these compressions and rarefactions impinge on any other medium, that medium _can't_help_ but to feel the effects at exactly the same frequency. 

The point(s) where the sound goes from air to water (or between any two media) is necessarily driven at that frequency, and will respond appropriately. Barring nonlinearities, the response in the second medium must have the same period as that of the sound in the first medium. 

   In the human vocal tract, standing waves are set up, with frequencies determined mainly by the medium in the tract (air, usually), the length of the tract, and the frequencies (the plural here is important) at which the tract is driven (determined primarily by the vocal folds). The speed of sound in Helium is higher than that in air, and this means that for a given tract length (l), the frequencies which are supported (given by f = c/l) are generally higher than in air. The timbre of a voice is shifted higher, and more energy resides in higher frequencies in a helium-filled tract. A pretty good explanation may be found here: http://phys.unsw.edu.au/phys_about/PHYSICS!/SPEECH_HELIUM/speech.html . 

 /************************************
Down with categorical imperative!
flutzpah@yahoo.com
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________________________________
From: Julie Quah <juliequah@gmail.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, April 4, 2009 7:37:37 PM
Subject: Re: [Phys-l] frequency and wavelength of sound in air

Hey there, thanksnfor the bery quickmrespond from yoi. More
Questioms if you don't
Mind - Why does sound frequency not change when travelling from one  
medium (eg air) to another (eg water)? Is it because the sound  
frequency is like the finger print for each wave ? Then why do we hear  
sound pitch increase when it travels through helium, which is another  
different medium as compared to air?

Sent from Julie Quah's Iphone


On Apr 5, 2009, at 9:02 AM, curtis osterhoudt <flutzpah@yahoo.com>  
wrote:

> Hi, Julie,
>
>   The "normal" speed of sound in water is about 1.48 mm*MHz (1.48 mm/ 
> microsecond; 1480 meters per second), depending on the water  
> temperature, etc. The "normal" speed of sound in air (STP) is about  
> 0.343 mm*MHz (343 meters per second), depending on various things.  
> Of course, there is some frequency dependence (dispersion) on these  
> numbers, and shock waves can be much, much faster.
>
>   If you're asking whether the frequency increases when sound enters  
> water from air, the answer is very nearly always "no" (it's a good  
> student exercise to explain why this is). But because the speed is  
> higher in water than in air (usually), the wavelength increases.
>
> /************************************
> Down with categorical imperative!
> flutzpah@yahoo.com
> ************************************/
>
>
>
>
> ________________________________
> From: Julie Quah <juliequah@gmail.com>
> To: Phys-l@carnot.physics.buffalo.edu
> Sent: Saturday, April 4, 2009 6:56:43 PM
> Subject: [Phys-l] frequency and wavelength of sound in air
>
> Just wondering, if the speed of sound in water is higher than that  
> in air,
> Which of  its, frequency or wavelength, (or both) increases in water?
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> Forum for Physics Educators
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