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CD
Insert a pair of 3" #8 deck screws
at each of the 4 corners and tighten
to secure the frame around the tubes.
2
2
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Expanded to
maximum size
"
2
2
E
F
Figs. C–E: Assemble a frame to hold a bundle of
conduit. For each piece, drill the 2 end holes 1½" deep
with a 1" drill bit, and the 4 side holes with a y" drill
The only remaining problem was how to set up a
6× 10 array of tubes with a consistent and precise
separation of 2mm. Here’s how I tackled it.
Building a Phononic Filter
1. I sawed 6 pieces, approximately 20" in length,
from each 10' length of conduit.
2. I decided to put spacers between the tubes at
each end of the 6× 10 array, and then clamp them
with adjustable frames. For spacers I used cable
ties, since they were just under 2mm thick and
could be easily trimmed to fit. For the frames I used
pieces of ¾"× 1½" oak, cut and drilled as shown in
Figure C and assembled as in Figures D and E. The
advantage of this design is that it will expand to
allow larger tubing spacers if you want to vary the
parameters of your experiment in the future.
3. After assembling the frames loosely, I stood
them opposite each other and started stacking the
pieces of conduit between them, inserting spacers
as I went along, as in Figure G.
80 Make: Volume 15
bit. Fig. F: A band saw makes swift work of cutting 10
conduits at once. Fig. G: Lengths of conduit stacked
between the frames. Pieces of cable ties maintain a gap
4. After laying in all the conduits, I tightened the
frames till they gripped securely enough for me
to handle the array (Figure H).
5. Now I was ready for the audio test. Ideally this
should have been done in an anechoic chamber —
an insulated room in which all sound is absorbed
and none reflected. Unfortunately, I don’t happen
to own an anechoic chamber, so I nailed together
a couple of 1' square frames of 2× 4 pine, stood one
frame in front of my filter and the other behind it,
and draped a blanket over the whole thing to make
a sound-absorbing tunnel.
6. I placed a speaker from my stereo at one end of
the tunnel, and the second stereo speaker outside
the tunnel, so that I could use the balance control
on my stereo to switch between the speakers for an
A–B comparison. Then I played an audio test CD to
generate a series of tones, and placed my ear to the
band-gap filter.
It seemed to perform as specified, attenuating
some frequencies, although of course the close-
Illustrations by Charles Platt
