Does wind affect the loudness of sound? I.E. Will a sound register
louder/softer/the same at decibel meters (or other sound measuring
device) that are equidistant from the sounds origin with all thing
being equal, except for a 20 mph wind blowing constantly, from behind
one meter across the sounds origin, through behind the other meter?

MMcgrath --

Sound is changed in many ways as it moves through the air but yes, it
is louder or more intense downwind.

As the Encarta article below notes, sound is a disturbance in the
atmosphere.  Once the ?disturbance? occurs it will be moving through
the air at 1,129 feet/second (68 degrees Fahrenheit).  It will be
moving faster ? at 29.3 feet/second (fps) faster downwind and 29.3 fps
slower upwind.  It won?t make much difference at 1 meter (3?) but it
will at 200 yards.

At 200 yards, sound will arrive downwind  0.1 seconds faster and
travel about 55? less.  There is less absorption of the sound wave ?
but there?s also two other factors at work.  There is also less
distortion of the sound wave and a higher frequency, which may make it
appear louder.

Encarta
?Sound ? Physical Characteristics?
http://encarta.msn.com/text_761560639___2/Sound.html


A U.K. research company specializing in acoustics notes all of the
different aspects of sound traveling through the atmosphere in the
following article.  Note, in particular, the section on ?Atmospheric
Effects,? which notes the absorption of sound by molecules but also
shows in a diagram how ?winds will increase sounds downwind from a
source and reduce them upwind. This is not solely a result of the
velocity effect, but also because the spherical wave-front is deformed
by the prevailing wind.?

Square One Research
?Sound Propagation?
http://www.squ1.com/index.php?http://www.squ1.com/sound/propagation.html


You may remember the Doppler Effect from physics classes.  It is used
extensively in medical equipment (for ultrasound measurements) and in
astronomy (where light shifts its frequency depending on how fast a
star is moving away from the Earth).

This article shows how frequency (which you?ll hear as pitch) changes
in a moving sound wave.  The increased frequency may also be
interpreted by your ear as being louder.

Wikipedia
?Doppler Effect? (July 24, 2005)
http://en.wikipedia.org/wiki/Doppler_effect


There?s a very complete discussion of all of the common effects on
sound in this page by James B. Calvert, an emeritus professor of
engineering at the University of Denver.  It deals with several other
special effects that occur with sound ? but you may want to start with
the ?Sound Outdoors? section:

University of Denver
?Sound Waves? (Calvert, Sept. 24, 2003)
http://www.du.edu/~jcalvert/waves/soundwav.htm


Google search strategy:
Sound intensity downwind upwind
Sound absorption downwind upwind
Doppler effect

Best regards,

Omnivorous-GA

---

Clarification of Answer by omnivorous-ga on 01 Aug 2005 15:41 PDT

MMcgrath --

This sentence should read:
"At 200 yards, sound will arrive downwind  0.1 seconds faster and
travel about 110? less."  It's traveling about 55' less downwind --
and 55' more upwind because of the 1.93 and 1.83 seconds spent in each
direction.  20 mphs is 29.3 feet per second, so it's about 55' each
way -- for a total of 110' difference.

My apologies for that calculation error.

Best regards,

Omnivorous-GA

---

Clarification of Answer by omnivorous-ga on 01 Aug 2005 17:36 PDT

MMcgrath --

Thanks so much for the extra sum -- and the comments.  But after I
posted the first Clarification Request, I realized that I'd done the
math backwards in the sound calculations.

The third paragraph should be replaced by the following:

"Sound will arrive downwind  0.03 seconds faster and travel about 33
fewer feet.  There is less absorption of the sound wave because the
wave has traveled less distance and had energy absorbed by fewer air
molecules ? but there?s also two other factors at work.  There is also
less distortion of the sound wave and a higher frequency, which may
make it appear louder."

Note that racecar is correct below about a decibel meter not detecting
any change -- but the frequency will change and your ear would likely
just the downwind sound to be louder, just because humans tend to hear
higher frequencies better.

Best regards,

Omnivorous-GA

This will help me win my bet, so I think it is a terrific answer.
Seriously, there are a lot of good sources here, and that is
important. You can pretty much find a site that will say anything on
the Internet today, so credible sources is key to any answer. The
response was well thought out and well organized as well. Thanks.

"There is also less
distortion of the sound wave and a higher frequency, which may make it
appear louder."

This is false.  If the sound meters are fixed relative to the source,
the wind does not cause a shift if the frequency detected.

Racecar --

I don't believe that you are correct unless describing a no-wind
situation.  The sound going towards a downwind microphone is traveling
at 1,158 fps and towards an upwind microphone at 1,100 fps and the
Doppler calculation indicate a frequency shift would occur:
http://en.wikipedia.org/wiki/Doppler_effect

No, the source isn't moving, but the wind is causing a relative movement.

Best regards,

Omnivorous-GA

The Doppler calculation assumes the observer is stationary with
respect to the medium.  In this case they're both moving with respect
to the medium but they're stationary with respect to each other -
there's no effect on the wavelength.

Ian G.

Respectfully, the comments by both omnivourous and iang are incorrect.
 When source and observer are not moving relative to each other, but
both are moving relative to the medium, there IS an effect on the
wavelength, but none on the frequency.  As omnivorous says, the sound
travels slower (relative to the ground) upwind than downwind, but the
wavelength also shortens, and the frequency doesn't change.  Think
about it this way: frequency is the number of wave crests that arrive
in say a second.  Let's say the frequency is 100 hertz.  That means
every second 100 waves leave the source.  Now the receiver is a
certain distance away.  If fewer (or more) than 100 waves arrive at
the receiver every second where do those extra waves go?  In the case
where source and receiver are moving relative to each other, the extra
waves go to fill up the (changing) distance between the source and
receiver.  But in this case, the distance between source and receiver
doesn't change, so the number of wave crests between them doesn't
change, and there can be no shift in frequency.  Omnivorous, if this
doesn't make sense to you, I'm sure I can dig up some online
references.

Racecar

My apologies, you're right!

Ian G.