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SOUND SUCKER, "ACOUSTIC ILLUSION"
(c)1997 William Beaty
The device below is a good sound absorber. It probably absorbs certain
frequencies much better than others. At first glance you'd think
this would be boring. However, the psychological effects are VERY
INTERESTING. When you hold the thing up near one ear, you feel as if:
the air pressure is falling
invisible pillows are drifting around your head
your ears are about to pop
you are going deaf
your head is changing size
the size of the room is shrinking
you are about to faint
The effects AREN'T really huge, but they are significant. They seem to
vary from time to time, so if nothing happens, try again in some other
location. Also note that if you play around with this thing for awhile,
the effects don't go instantly stop after you put it down. Perhaps it
convinces your brain that your ears have changed shape?
INSTRUCTIONS:
Buy a couple large boxes of Dixie(tm) coffee stirrers (a few thousand
total.) These look like short, hollow, plastic soda straws, except that
they are less than 1/8" in diameter. (Don't use the kind that look flat,
they must be round and hollow.) Obtain a straight-sided container, such
as a peanut butter jar or a tall, very wide coffee mug. I think the
straws need to seal flat against a flat surface. I used a cardboard box
with a square of cardboard placed in the bottom. Place the stirrers in
the box or jar. Shake them around and pack more in until you have a solid
cylinder-shaped array of straws, looking like an insect eye.
Go into a room which has significant broad-band environmental noise. Close
your eyes and bring the open end of the straws-matrix up near one ear.
Move it away. Wave it around your head. WEEEEEIRD! Part of the
background sounds in the room are sucked up by the device, and it feels as
if your ears are filling up with invisible cotton. You really gotta try
this. Sometimes nothing happens, but when it works right, the effect
is very creepy.
JOKE:
Build two of these. Sneak up behind someone and carefully hold the open
mouths of the jars near their ears. The victim should exhibit a strong
response! NOTE: I FINALLY TRIED THIS AND IT DOESN'T WORK. IT SEEMS TO
ONLY WORK IF ONE EAR IS USED. WHEN YOU PUT IT ON BOTH EARS, YOU FEEL
NOTHING ODD. Perhaps your brain ignores bilaterally symmetric illusions
in "audio space?"
OTHER THINGS TO TRY:
I haven't tried duplicating the effect with normal-sized drinking straws.
It might work, though the effect is probably stronger (and higher in
frequency) with the shorter, smaller diameter straws. Also, you need to
use a fairly large diameter container, like 5" diameter or more.
IDEA: Try building a sort of rotating "Radiometer" with several "paddles",
with one surface of each paddle covered with a drinking-straw array. One
side of each paddle will act as a reflector, while the other behaves as a
good absorber at resonance. Place a needle-bearing in the center of the
assembly so the paddles can rotate horizontally. Now play a very loud
sound in the room, with the frequency at the resonant absorption band of
the drinking straws, and perhaps the "pinwheel" will begin to turn
because of radiation pressure differences.
Try binding the straws together with tape or rubber bands and remove the
jar. The effect becomes much less. Obviously a lot of sound is passing
through the straws even though the ends of the array constitute a jump in
acoustic impedance. The ends of the straws must require plugging before
the "black hole" acoustic illusion effect occurs, so that no sound comes
from the other side of the array, and so that reflected sound is given two
chances to experience friction with the straw surfaces.
I wonder what it would be like to be near an entire wall of straws?
Obtain a 2ft cardboard box, many thousands of coffee stirrers, then
fill the bottom of the box with on-end stirrers. Maybe trim down
the edges of the box to match the length of the straws. If the "black
hole" effect seems less, it might help to include a wood or plexiglass
plate in the bottom of the box behind the straws. This will provide
a solid acoustic reflector (the cardboard might be too leaky for sound.)
THEORY:
This phenomenon possibly involves the "energy sucking
antenna" effect. A small resonator, if the Q is fairly high, will
build up a strong wave which can act as "anti-sound" and deliver energy
into the resonator. Once resonance has built up in the straws, they will
efficiently absorb energy in a region 1/4 wavelength in all directions.
They should behave like a large "pillow" which absorbs sound and converts
it into frictional heat along the walls of the straws. This will only
occur at the fundamental and at the harmonics. The box of straws acts as
a "comb filter", and in theory it punches a series of slots in the
spectrum of broadband noise.
WHAT THE HELL?
After playing with this device for many minutes, for me the "floating
pillows around your head" effect did not stop! I put the device away and
went into another room, and for about half an hour I kept noticing changes
in my hearing, as if someone behind me was still waving the stupid device
around my ears. I've heard that human ears are supposed to generate
sound. Perhaps they employ a feedback system, and perhaps this plastic
narrow-band absorber confuses the brain subsystem which performs my sound
localization. Or perhaps my brain simply accomodated to the presence of
the device (as when the physical shape of our ears is accidentally
altered.) Then, when the device was removed, my acoustic perceptions did
not return instantly to normal. The acoustic equivalent of "phantom limb
effect?"
Or did I just give myself even *more* brain damage? ;)
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