Joe Bloggs

Chief of Testing

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Sound power measurement

What is sound power and why measure it?

A noise level or sound level is usually a sound pressure level. A sound pressure level is a measure of the small pressure fluctuations in the air superimposed on the normal atmospheric pressure. Noise or sound levels produced by a machine or a piece of equipment can easily be measured with a sound level meter.

A sound level meter shows the sound pressure level at the measurement position. The sound level depends on how far away the meter is from the machine, and on the measuring environment. For example, is the machine outdoors, in a large room or in a small room, does the room contain soft furnishings or are the walls hard and bare? This sound level is important because it relates to the loudness of the sound to a listener or bystander and to the potential for damaging effect on a person’s hearing.

A sound power level on the other hand is a measure of the total noise radiated by the machine in all directions. It is a property of the machine and is essentially independent of the measuring environment. As an analogy, a sound pressure level may be compared to the temperature at a point in a room, while sound power level is analagous to the power rating of an electric fire. The output power of the fire does not vary from room to room, but the temperature depends on the room size and the distance from the fire.

Because sound power levels of a machine are the same regardless of the room or environment in which they are used, they are usually required in sales brochures and manufacturers specifications. If sound power levels are 70 dB(A) or above they must also be specified in instructions and sales literature to comply with the EU Machinery Safety Directive or similar national regulations in the UK.

Sound power levels are useful to equipment manufacturers, buyers, users and installers. They are used to:

  • calculate the sound pressure level from a machine, or several machines, at a given position or distance in a given environment, such as a factory, workshop, office or in the home
  • compare the noise output from different machines, perhaps from different manufacturers
  • set specifications for the maximum permitted noise from a machine
  • compare machines before and after modifications to reduce the noise.

Sound power is measured in watts or picowatts, and sound power levels are traditionally given in decibels (dB re 1pW), where 0 dB corresponds to 1 picowatt. But frequently these days sound power level is given in bels (1 bel = 10 decibels) rather than decibels to avoid confusion with sound pressure levels. So a machine with a sound power level of 9.8 B (9.8 bels) is a machine with a sound power level of 98 dB.

Our capabilities

ISVR Consulting carries out sound power measurements using precision methods conforming to British (BS), European (EN) and International Standards (ISO), and to standards issued by Ecma International (formerly the European Computer Manufacturers Association).  The measurements are made in either our reverberation chambers, under reverberant conditions,  or in our large anechoic chamber under free-field conditions. Items measured include industrial machinery, electronic and business machines, and domestic appliances.