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Sound Power and Sound Pressure

Sound Power is the ability of a device to make sound. It is purely a function of the device and unable to be measured directly. It has units of watts, and in decibel terms, dB re: 1pW.

The sound power is radiated into the acoustic field by sound intensity, a vector quantity, which is the flow of sound power per unit area. Its units are watts per square metre, or dB re: 1pW/m2. This radiation results in a sound pressure in Pa, or sound pressure level in dB re: 20uPa, which is what we, as receivers, perceive.

In a free field, the three parameters are very closely related, and we can determine sound power from measurements of sound pressure, or less commonly, sound intensity.

A problem occurs when there isn’t a free field, so corrections are made according to the acoustic environment – a typical example of this is correcting for the reverberation time of a room.

Many ISO standards exist (e.g. the ISO 140 series) for the determination of sound power, depending on the different types of test environment, and the required accuracy of the calculation.

GRAS endeavours to make these measurements very straightforward, by offering a choice of hemispheres for mounting microphones in test rooms according to the standards, or sound intensity probes for measurements of sound intensity in-situ, where the machine cannot be moved from its operating environment.

Microphone hemispheres are used for sound power measurements on everything from small sized personal electrical gadgets to office machines and IT products, household appliances, power tools and even small engines.

When conducting measurements, the characteristic dimension of the device being tested must be no more than half the measurement radius. Hemispheres can be equipped with 4, 10 or 20 microphones, either CCP or lemo connection, and are 1 metre or 2 metres in radius.


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