between each tube and the next. After clamping one
end, it’s easy to insert spacers at the other. Fig. H: The
completed, clamped array. Fig. I: Wider spacers (such as
spaced grid of tubes dampened all sounds to a
noticeable extent. To be really sure, I would need
a sine wave generator capable of producing
constant-amplitude waves from 50Hz to 20kHz,
through a really good speaker, with an equally good
microphone feeding into a computer sound card
and audio software displaying at least 30 bands
7. Still, I was sufficiently intrigued to experiment further. I unscrewed one of the tube frames, removed
my cable-tie spacers, and inserted new spacers
made from 1"-diameter copper rod, as in Figure I.
I expanded the frame, reattached it, and then did
the same thing with the frame at the other end
(Figure J). I actually seemed to get more noticeable
audio effects through this configuration.
8. Lastly, I built a second filter using ½" conduit
spaced 1" apart in a triangular pattern. This didn’t
work at all — at least not on sound waves. Possibly
if I put it in a bathtub, it could block some ripples.
I haven’t tried this yet.
copper rods) can be substituted for cable ties.
Fig. J: After the wooden frame is expanded, it can be
retightened around the wider-spaced conduits.
There’s a serious side to all this. Anyone with
minimal skills and tools can investigate a phenomenon that has exercised the abilities of scientists in
heavily funded university laboratories. When Ph.D.s
studying phononic band-gap phenomena start writing their next grant applications, maybe they should
consider sending the work out to home hobbyists,
to tap our “distributed workshop capability” — just
as SETI uses distributed computer power donated
by space enthusiasts who hope to discover extraterrestrial messages.
After reviewing this article, materials scientist
Dr. Pierre Deymier (who coauthored the paper
cited earlier) suggests that plastic tubes may work
almost as well as metal tubes. Plastic tends to
flex, which would affect the tubing gaps, but large-diameter PVC pipes should be sufficiently rigid
while blocking a lower frequency range. Maybe a
phononic filter fabricated from 5" sewer pipes?
Time for a new experiment!
Charles Platt is a section editor for MAKE