Going to war on noise pollution

Anyone who suffers sleep depravation caused by traffic, noisy neighbours and the tortured wailing of amorous cats may well rest easy with the development of a new class of resonant sonic materials.

The new materials, developed by a research team led by Professor Ping Sheng at the Hong Kong University of Science and Technology, is said to offer greater prospects in eliminating noises – especially at low frequencies under 500Hz- generated on a daily basis.

The new RSM’s are made up of series of small, structured spheres consisting of a solid core of metal with relatively high density and a coating of elastically soft silicone rubber. The whole structure is then encased in a hard matrix of epoxy.

The spheres vibrate inside the soft silicone as though they were suspended by springs. When a sound with a frequency close to that of the resonant frequency of the spheres enters the material, the spheres vibrate in their soft silicone sheaths, sending out sound waves in all directions.

A number of those waves interfere with the forward movement of the unwanted noise and quieten it. When testing the material the researchers could attenuate sound by 20 decibels at both 400 Hz, the resonance frequency of the spheres, and at 1400 Hz, the resonant frequency of the silicone layer.

Current soundproofing uses materials that resist vibration and transform sound waves into heat, but this doesn’t work well at low frequencies because the longer wavelengths pass relatively unimpeded through the thickness of the material.

The new RSM invented by Professor Sheng and his team makes control of low-frequency noises realisable. When used in soundproof structures, these materials only take up less than one-tenth of the thickness of traditional materials.

Professor Sheng’s new material may be employed in shielding off subsonic vibrations as well as ultrasound, and have potential applications in shock-protection of buildings and ‘ultrasonic cooling techniques.