100
Percent Sound Transmission
75
50
25
0
0 2. 5
AB
5 7. 5 10 12. 5 15 17. 5 20
Frequency (kilohertz)
Band-Gap Filters
Photograph from Fundación Juan March de Madrid
First let’s resolve any confusion between phononic
and photonic. Photonic (with a T) band-gap filters
are a hot item in telecommunications research. For From Lab to Hardware Store
reasons rooted in quantum theory, if you shine light Finding a practical application for phononic band-into a certain type of crystalline structure, some gap filters has been a challenge, because they must
of the photons are blocked, even though they’re be large and made from rigid materials, which tend
smaller than the holes in the structure. The band of to be expensive. If appropriate patterns could be
light frequencies emerges with a gap in it — hence designed into buildings, they could permit ventilation
the term band-gap filter. while canceling noise, especially near highways and
Interestingly, the filtering effect is very scalable. in industrial environments. But for our purposes,
It should even work the same way with ocean waves let’s just see if we can make one work.
as it does with light waves, leading some scientists Most of the necessary info can be found in papers
to propose that a pattern of concrete columns could such as “Phononic crystal with low filling fraction
prevent beach erosion by blocking wave motion, and absolute acoustic band gap in the audible
while still allowing tides to flow without interference. frequency range: A theoretical and experimental
Somewhere between the tiny wavelengths of light study” by Vasseur, Deymier, Khelif, et al., published
and the huge wavelengths of the ocean, we find in Physical Review E, May 2, 2002.
vibrations that are audible and may be described as It’s not what anyone would describe as beach
phononic (with an N). A tone of 10kHz, up near the reading, but buried in its text is a valuable take-home
limit of human hearing, creates waves of air pres- message for those of us who like to build stuff. The
sure about 1. 4" ( 3.5cm) apart. To build a “crystal scientists blocked significant segments of the audio
structure” that will block such audio waves, we need spectrum using copper tubes 28mm in diameter,
nothing more sophisticated than a grid of rods. spaced 2mm apart. Figure B shows a detail copied
Intuitively, it seems that all audio frequencies from one of their sound filtering graphs.
should pass through gaps between the rods without For less than $40 I carried home a bundle of 10
any problem, but theory says that they can’t, and lengths of ¾" steel electrical conduit, each 10' long.
indeed they don’t. This was first verified in 1995 when For mysterious reasons known only to licensed
scientists from the Materials Science Institute of electricians, the so-called ¾" conduit is not ¾" in
Madrid noticed that a public sculpture by Eusebio diameter, on the inside or the outside. Its outside
Sempere (Figure A), consisting of vertical rods about diameter is about 0.92", which I thought should be
3cm in diameter and spaced about 10cm apart, close enough to 28mm to achieve at least some
looked like a band-gap filter. Sure enough, when they filtering effect.
dragged some test equipment to the site, they found
that some sounds were slightly attenuated.
Make: 79
