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Name: Dee W.
Status: other
Age: 50s
Location: N/A
Country: N/A
Date: Thursday, November 28, 2002


Question:
Everything else being equal, does sound travel better (meaning here,louder) when the air is humid or not? Ditto, when the air is hot versus cool? Ditto, when the barometric pressure is high or low? We have gotten much confusion over whether air density a) helps sound travel louder (as measured by decibels at recipient) because more molecules to travel through or b) hurts sound volume because of resistance and drag -- this is all from what started as "simple" science project. Any help will be much appreciated. Cannot seem to find answers to these basics anywhere in texts or Internet.


Replies:
Your topic is like "The Normandy Invasion" -- starts off "simple" and turns into a massive undertaking. Your failure to find web sites may have been due to a common problem -- You did not select the key search terms. You are asking about the attenuation of sound, not its speed, which is a factor but not the only one. Search these terms: "acoustic attenuation" "sound attenuation" "acoustic impedance" "sound impedance" and you will find a number of sites that discuss the topic(s). Here are some I found.

GENERAL

http://www.pa.op.dlr.de/acoustics/acoustics_content.html

LARGE SITE, MANY LINKS

http://www.ndt-ed.org/index_flash.htm

SOUND ATTENUATION

http://www.silex.com/pdfs/Sound%20Attenuation.pdf

INCLUDES A CALCULATOR FOR VARIOUS MEDIA

http://www.cnde.iastate.edu/ncce/UT_CC/Sec.2.6/Sec.2.6.html

Vince Calder


FOLLOW UP ON THIS INQUIRY, FOUND AN EXTENSIVE ONLINE TEXT ON THE SUBJECT OF ACOUSTIC ATTENUATION

http://www.sfu.ca/sonic-studio/handbook/index.html

Vince Calder


Dear Dee,

Here is a quick answer to your questions from Paul G. Hewitt's book, Conceptual Physics, fourth edition, copyright 1981.

"The speed of sound depends on wind conditions, temperature, and humidity. It does not depend on the frequency of sound; all notes travel at the same speed. The speed of sound in dry air at 0 degrees C is about 330 meters per second, nearly 1,200 kilometers per hour. Water vapor in the air increases this speed slightly. Sound travels faster through warm air than through cold air. This is to be expected because the faster-moving molecules in warm air bump into each other more often and therefore transmit a pulse in less time. For each degree rise in temperature above 0 degrees C, the speed of sound in air increases by 0.6 meters per second. In water, sound travels about four times as fast as it does in air, while in steel, the speed of sound is about fifteen times as great as in air."

You might be able to find this textbook in your local library. I have also found the web site Askjeeves.com to be a reliable source for answers to science questions. It usually can provide links to reliable web sites.

Good luck on your "sonic" adventure.

Martha Croll



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