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[Phys-L] Re: sound travelling through a solid



not so certain that's all; the sound in the table is confined to the
table (impedance miss match at the surfaces to confine, yes), while in
the air one has the much larger volume of the room or four Pi vs << two Pi.

How the sound is transmitted to the ear drum is a complication. I think
the pinna-wood is a close match. Holding ear just above table not much
better than a meter above. The pinna vibrates to the closed cavity
(table at one end drum tother).

More complication. When I snap a finger nail across an edge of my
PowerBook subjectively, unfortunately, it's equally loud ear above or
against the top. The pitch, however is much higher above than thru the
Ti. I suppose one hears the low mass finger nail above, while thru (ear
against the cover) a portion of the cover plate (high mass) was set to
vibrating.

the mechanism of crating sound ina piece of lumber is, I suspect,
complicated, perhaps a Sci. Faire project?

bc, table Physicist, not yet.

RAUBER, JOEL wrote:

Or perhaps to put it more succinctly, its more about impedance matching
than anything else?

________________________
Joel Rauber
Department of Physics - SDSU

Joel.Rauber@sdstate.edu
605-688-4293



| -----Original Message-----
| From: Forum for Physics Educators
| [mailto:PHYS-L@list1.ucc.nau.edu] On Behalf Of Frohne, Vickie
| Sent: Friday, February 25, 2005 11:42 AM
| To: PHYS-L@LISTS.NAU.EDU
| Subject: Re: sound travelling through a solid
|
|
| I think that this is an example of a slightly "scrambled"
| idea that may have come from a standard demonstration. The
| demonstration is this: Two people are sitting at opposite
| ends of a table. One person scratches or taps on the table
| lightly with a fingernail. The other person will barely be
| able to hear the sound when sitting upright. The scratching
| or tapping will be heard clearly, and loudly, when the person
| puts his or ear down on the table.
|
| In general, sound travels faster in solids than in air, and
| in rigid (hard) materials, sound waves can go farther in
| solids before dissipating. Hence the NOT RECOMMENDED trick
| of putting one's ear to the railroad tracks to listen for the
| oncoming train. Someone in Chicago was killed that way a few
| years back - he put his ear to the third (electric) rail.
| Others have been hit by trains. Do NOT try this! Stick with tables.
|
| The standard explanation: Sound travels in solids as well as in air.
|
| The usual conclusion made by observers: Sound travels
| *better* in solids. Or it's louder (by observation).
|
| What's *really* going on: The loudness of a sound is a
| function of the energy carried in the sound wave. When one
| taps or scratches on the table, more energy is transferred to
| the table than to the air above the table. When one listens
| with an ear to the table, the sound IS louder than the sound
| observed while listening in the air. But the sound does NOT
| get louder as the sound travels in the table. This can be
| verified by putting an ear next to the tap, then listening at
| the opposite end of the table, where the sound will be softer
| than before. The only reason why the sound is louder when
| one puts an ear to the table is that more of the energy is
| directed into the ear when the ear is in contact with the table.
|
|
| Vickie Frohne
|
| -----Original Message-----
| From: Forum for Physics Educators on behalf of Alison Innes
| Sent: Fri 2/25/2005 10:53 AM
| To: PHYS-L@LISTS.NAU.EDU
| Subject: sound travelling through a solid
|
| I recall reading on the listserve a while ago that the
| commonly held idea that sound is louder when it travels
| through a solid is incorrect. Can anyone point me to some
| more information about this (preferably on the web)?
|
| Alison Innes
| Education Coordinator
| Bell Homestead National Historic Site
| 519-756-6220
|