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13/03/2008
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Tom Shelley reports on a step forward in noise measurement that not only reduces work, but also means problems get pinpointed quickly
A new design of noise meter – with two noise channels – is cutting the amount of work needed to discover the attenuation of sound insulating barriers.
But the benefits don’t stop there. Combined with the use of a special probe with two-phase matched microphones, it can also be used to measure sound power levels – even in far from ideal conditions – and rapidly pinpoint where particular noise signals are coming from.
This is expected to be of great assistance to an engineer making measurements to establish whether a machine is likely to meet European Union compliance requirements. More importantly, it may well greatly accelerate the process of locating causes of problems during the design phase, with a view to eliminating them quickly.
Keith Vickers, automotive and industry applications specialist with Bruel and Kjaer UK, points out that two channel working has already been introduced with its hand-held 2260 meter but its capabilities are being extended with the 2270 model, which he says “has a more intuitive interface and the ability to voice annotate measurements and use a built-in camera to link images of noise sources with measurements”.
The use of two channels allows direct measurements of attenuation – so a microphone can be on one side of a noise barrier with a second on the other. But, more importantly, it enables its use with the company’s Sound Intensity Probe Kit, currently able to work with the 2260, and “eventually to go on the 2270”, which contains two phase-matched microphones, mounted face to face with a spacer between, and allows some very clever things to be achieved.
According to Vickers, when the noise source is at 90 degrees to the axis of the probe, the measured value goes to minus infinity. Finding a noise source or a source of anything from maxima, which are likely to be varying all the time, is very difficult. However, finding a null point or minimum is much easier.
“The other use”, he says, “is for measuring sound power level.”
Sound power level limits, as opposed to noise limits are specified by various European Union regulations, so they are important for designers and developers. Conventionally, this requires making measurements in either an anechoic chamber or in open field conditions far from other noise sources. All that has to be done with the probe kit, he explains, is to “create a surface that encapsulates the sound source, make measurements with the probe, so that its axis is normal to the surface, and multiply the measured noise levels by the area”. This removes the effects of extraneous noise, so the measurements can be made almost anywhere.
Pointers
* System includes a camera, so that an image of the noise source can be associated with recorded sound level measurements
* By combining the instrument with a phase-matched dual microphone probe kit, it is possible to pinpoint noise sources using a null technique
* It is also possible to use the same kit to make sound power level measurements under non-ideal conditions, cancelling out the effects of extraneous sound
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Author
Tom Shelley
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