Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



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
|