Saturday, August 30, 2008
  Experts

Item in the latest New Superstitionist. Not widely interesting, but fits into an area where I have some actual knowledge and experience, so caught my attention....

If you want to improve your soundproofing, try punching some holes through it. That's the counter-intuitive result of an experiment in which metal plates drilled full of holes were shown to transmit less sound than solid plates. ... The effect could help reduce noise in situations where air circulation is important to prevent overheating, such as large power transformers near homes.


In the details, turns out the experiment was using ultrasonic frequencies underwater, and the holes block only one frequency, with a wavelength equal to the distance between the holes. For the frequency they chose, the holes are about 3 mm in diameter, and about 7 mm apart.

This would be about the actual size of the pattern:

o o o o o o o
o o o o o o o
o o o o o o o

Okay, maybe it will work for a stealth submarine. I can see the possibility: sound passes through the holes, refracts outward on the other side, cancels at the centerline between each pair of holes.

(Though the article didn't state the purpose, a stealth sub is the only reason for research in underwater ultrasonic shielding.)

But if we try to extend this to regular sound in air, I'll guarantee it won't work.

Let's say we're trying to block the 60-cps hum of a transformer. The wavelength of a 60 cycle sound is about 18 feet. So we'd need to make a series of 18-foot diameter holes 18 feet apart in the housing of the transformer.

Now I'm looking through my front door at a residential transformer atop the nearest power pole. Appears to be a cylinder, about 4 feet tall and 2 feet in diameter. Okay, so we need to drill a pattern of 18-foot holes 18 feet apart on this 4-foot cylinder. Fetch my 18-foot brace and bit, and we'll get started. All right, that will take care of the 60-cps sound, and now we'll need to drill some 9-foot holes between the 18-foot holes to take care of the 120-cps harmonic, and then some 3-foot holes 3 feet apart to silence the 180-cps harmonic.

Even if they're talking about a much larger transformer, say about the size of a house, it's still impossible. You still won't have any enclosure left after opening those truck-sized holes.

Aside from this grotesque absurdity, the first rule of soundproofing in air is to make the enclosure perfectly airtight. After you've sealed and caulked all possible holes and cracks, you can then add various kinds of damping material. But if even a few air molecules can get through, you might as well skip the rest of the work.

Methinks these acoustical experts are not very good at math, among other things.
 


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Polistra was named after the original townsite of Manhattan (the one in Kansas). When I was growing up in Manhattan, I spent a lot of time exploring by foot, bike, and car. I discovered the ruins of an old mill along Wildcat Creek, and decided (inaccurately) that it was the remains of the original site of Polistra. Accurate or not, I've always liked the name, with its echoes of Poland (an under-appreciated friend of freedom) and stars. ==== The title icon is explained here. ==== Switchover: This 2007 entry marks a sharp change in worldview from neocon to pure populist. ===== The long illustrated story of Polistra's Dream is a time-travel fable, attempting to answer the dangerous revision of New Deal history propagated by Amity Shlaes. The Dream has 8 episodes, linked in a chain from the first. This entry explains the Shlaes connection.

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