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

Hi all-
This is a teacher's net. Whith that in mind, I have read the various explanations of the physics of sound crossing a boundary - a topic that I think I have thoroughoy understood for at least 50 years. Reading the explanations, and trying to understand them from the viewpoint of a twenty-year old college student, left me torouglhly confused. I urge others to reread these explanations from a similar viewpoint and see if they can't come up with more picturesque and compelling explanations.
Regards,
Jack

On Sat, 4 Apr 2009, curtis osterhoudt wrote:

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
************************************/




________________________________
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?
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


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