IF YOU want to improve your soundproofing, try punching some holes through it. That鈥檚 the counter-intuitive result of an experiment in which metal plates drilled full of holes were shown to transmit less sound than solid plates.
Hector Estrada of the Polytechnic University of Valencia in Spain and colleagues placed 3-millimetre-thick metal plates in water and measured their ability to block ultrasound. Then they punched a pattern of 3-millimetre-wide holes 6 to 7 millimetres apart in the plates and measured their soundproofing ability again. Surprisingly they blocked the ultrasound more effectively.
Sound waves can be trapped and reflected by holes that are slightly smaller than their wavelength, but that鈥檚 not the whole story. The team found that the holes allow the plate to resonate in a way that causes the sound waves to interfere and cancel out. And it doesn鈥檛 seem to matter whether the holes are drilled randomly or in a pattern, as long as the distance between them corresponds roughly to the wavelength of the sound wave, Estrada says. The team鈥檚 work will appear in Physical Review Letters.
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鈥淭he holes allow the plate to resonate in a way that causes the sound waves to interfere and cancel each other out鈥�
So far, it has had a mixed response. Allan Pierce, an acoustics researcher at Boston University in Massachusetts, is sceptical and says he would need to see more data before he鈥檒l be convinced. But Gregory Kriegsmann of the New Jersey Institute of Technology in Newark, who has done similar experiments, agrees that resonant effects can result in counter-intuitive behaviour. 鈥淔unny things can start to happen,鈥� he says.
The effect could help to reduce noise in situations where air circulation is important to prevent overheating, such as in electrical transformers near homes. The new approach could lead to extraordinary improvements in soundproofing, claims team member Francisco Meseguer, also of the Polytechnic University of Valencia.
More work will be needed before the results can be applied to real-world applications. The team has yet to show that the effect works for plates in air instead of water, and at the much lower frequencies of audible sound.
