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



...here's my conceptual interpretation (same idea as Chris below, but in a 'count pulses' description. Imagine you're using a speaker to drive a set of sound waves. Each time you touch two wires together you create ONE movement of the speaker in a single click -- something happened that made a single high pressure and low pressure zone - a square pulse or click - move away from the front of the speaker. Do this over again regularly in time for ten clicks and you have a chain of happenings -- ten peaks and ten troughs.

Now say these ten pulses cross a boundary -- into say water. The water surface will experience 10 peaks and ten troughs in a series of compressions -- 10 clicks will impact the water surface. The number doesn't change to nine or eleven -- there are still ten total in that time interval. Clicks don't replicate or unhappen. Hence there will be 10 compressions and 10 rarefactions
moving onwards in the water under the surface impacted by the air.

But water is denser than air, so the waves can travel faster and on arrival each click is transmitted a farther distance from the surface before the next one arrives at the surface than it could in air -- so our ten clicks in water have a new longer spacing in the water as well as a new speed. New spacing means new wavelength. Same number of clicks in same time interval means same frequency.

If you think of a wave a a string of happenings in time then why frequency is preserved seems obvious I think.

Dan M

Dan MacIsaac, Associate Professor of Physics, SUNY-Buffalo State College
222SciBldg BSC, 1300 Elmwood Ave, Buffalo NY 14222 USA 716-878-3802
<macisadl@buffalostate.edu> <http://PhysicsEd.BuffaloState.edu>
Physics Graduate Coordinator and Dept Chair pro tem




On Apr 4, 2009, at 11:51 PM, Jack Uretsky wrote:

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?
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Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
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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General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley



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