Electrical & Electronic

2-2001  It's so simple

Vibration power cancels sound

Problem. Vibrating objects generate noise. This is a particular problem with large machines such as power transformers. The main mechanism is often the vibration of the casing displacing air periodically. The displaced air has nowhere to go and is compressed, resulting in a pressure fluctuation or sound.

It is very difficult to prevent the casings of such machines vibrating. Surrounding them with sound deadening material is liable to lead to problems with heat dissipation. Vibration isolation of the casing from the source of the vibration is often very difficult, and impossible in the case of power transformers which use oil circulated by natural convection to cool the windings.

Solution An American company, Adaptive Audio of Annapolis, has come up with a device called a vibration excited sound absorber, or ‘SoundSpring’. It consists of a surface element connected to the surface of the machine by a spring-like coupler. Relative motion of the machine casing and the SoundSpring surface causes the volume of occupied air to change.

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As the surface of the machine moves forward, it displaces air. At the same time, the surface of the SoundSpring moves backwards and creates space for the air to move into. Instead of the displaced air compressing the surrounding air and generating sound, it moves into the space created by the SoundSpring.

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In practice, the enclosure, spring and seal are all combined as stainless steel bellows. A typical design is five inches (127mm) in diameter, with four convolution bellows and a flat radiating surface. It is tuned to absorb sound at 120Hz for reducing US power transformer noise. Noise reduction is approximately 21dB at this frequency.

Applications Tests with a 3.3m x 1.4m x 2.6m transformer showed that a reduction of 15dB in the noise requires 42 sound absorbers while 69 reduces noise by 20dB. Adding too many noise absorbers can lead to a reduction in effectiveness. Optimum positions may be picked by computer analysis based on an understanding of the deflection behaviour of the casing.

Other targeted applications include power generators, pump housings, substation reactors, and propeller aircraft cabins. The method also works in fluids other than air, for example, under water. More information can be found at www.soundabsorbers.com . TS