I think that this is an example of a slightly "scrambled" idea that m=
ay have come from a standard demonstration. The demonstration is thi=
s: Two people are sitting at opposite ends of a table. One person s=
cratches or taps on the table lightly with a fingernail. The other p=
erson will barely be able to hear the sound when sitting upright. The=
scratching or tapping will be heard clearly, and loudly, when the pe=
rson puts his or ear down on the table.=20
In general, sound travels faster in solids than in air, and in rigid =
(hard) materials, sound waves can go farther in solids before dissipa=
ting. Hence the NOT RECOMMENDED trick of putting one's ear to the r=
ailroad tracks to listen for the oncoming train. Someone in Chicago w=
as killed that way a few years back - he put his ear to the third (el=
ectric) rail. Others have been hit by trains. Do NOT try this! Stic=
k with tables.
The standard explanation: Sound travels in solids as well as in air. =
=20
The usual conclusion made by observers: Sound travels *better* in sol=
ids. Or it's louder (by observation).
What's *really* going on: The loudness of a sound is a function of t=
he energy carried in the sound wave. When one taps or scratches on t=
he table, more energy is transferred to the table than to the air abo=
ve the table. When one listens with an ear to the table, the sound I=
S 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 ca=
n be verified by putting an ear next to the tap, then listening at th=
e opposite end of the table, where the sound will be softer than befo=
re. 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 th=
e ear is in contact with the table. =20
Vickie Frohne
-----Original Message-----
=46rom: 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
=20
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. Ca=
n
anyone point me to some more information about this (preferably on th=
e
web)?
Alison Innes
Education Coordinator
Bell Homestead National Historic Site
519-756-6220