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Diffraction grating for sound



Digby Willard <dwillard@MAIL.CENTRAL.STPAUL.K12.MN.US> wrote:

As long as we're talking about double source interference for sound, I
thought I'd share something I noticed at our last fire drill. There's a
fire alarm buzzer right outside my room. As I was leaving the room
last, I
noticed that the buzzer got louder and softer and louder ... as I walked
past it.

I took a look around to actually find the buzzer, and found it in the
wall,
covered by a metal grate whose holes were about 1 cm square, separated
by
about .5 cm. Haven't done any math yet to determine if my observation
matches theory.

The grate is cross-hatched, so presumably one could also get a variation
in
volume by changing altitude.

Digby Willard
St. Paul Central High School
***********************************************************
I think you'll find that for the wavelengths involved, the grating size
and spacing are not the domnant factors. If you play a sound system
test CD that has various frequency pure sinusoidal waves on two stereo
channels on a two-speaker boom box (speakers about one to two feet
apart) INSIDE a room, you'll find the same effect of maxima and minima
separated by a few FEET all through the room. For sounds in the range
of 2000 Hz (with a wavelength of about 6 inches), the interference
pattern for two point sources a foot or two apart is on the order of a
few INCHES apart along a line parallel to the front of the boombox.
The observed spacing of maxima and minima most likely comes from
reflections from all the walls, ceiling, and floor of the room with
each other and with the sound coming directlyfrom the speakers.

If your fire alarm is inside a hallway, the reflected sounds will
produce similar effects. So I doubt the grating is significant in your
case.

Peter Vajk
St. Joseph Notre Dame HS
1011 Chestnut Street
Alameda CA 94501