Sound power measurement

What is sound power and why measure it?

A noise level or sound level is usually a sound pressure level, a measure of the small pressure fluctuations in the air superimposed on the normal atmospheric pressure. Noise levels produced by a machine or a piece of equipment can be easily measured with a sound level meter. The 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, and 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 and to the potential damaging effect on 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 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 power of the fire does not vary from room to room, but the temperature depends on the room size and distance from the fire.

Sound power levels are useful to equipment manufacturers, buyers, installers, and users for

Sound power levels must also be specified in instructions and sales literature for equipment conforming to the EU Machinery Safety Directive.

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 (formerly the European Computer Manufacturers Association). The measurements are made in special test chambers in either reverberant or free-field conditions. Items measured include industrial machinery, electronic and business machines, and domestic appliances.

Reverberant field method

The direct methods described in ISO 3741 or ISO 3742 are for steady broad-band noise sources, or steady discrete-frequency or narrow-band noise sources, tested in reverberant conditions.

The noise source being measured is placed in the Reverberation Chamber. Octave band or one-third octave band spectra of the noise emitted are measured using either a single microphone on a rotating boom or a number of microphones at static positions. (For narrow-band testing a number of source positions may be specified by the Standard). The chamber is characterised by measuring its reverberation times in the relevant frequency bands. The Sound Power Level (dB re 1 pW) in these bands is then computed from the noise spectra and the reverberation times.

Measurements made in conformity with this International Standard should result in standard deviations not exceeding those shown in Table 1.

Table 1

One-third octave band centre frequencies, Hz Standard deviation, dB
100 - 160 3.0
200 - 315 2.0
400 - 5 000 1.5
6 300 - 10 000 3.0

Free field methods

If directional information is required then the measurements must be made under free-field conditions. Tests are carried out according to ISO 3745 in the Large Anechoic Chamber, usually over a reflecting plane (semi-anechoic conditions). Ten or more microphone positions are used over an imaginary hemispherical measurement surface. Octave or one-third octave spectra are measured at each microphone position and the Sound Power Level computed.

Measurements made in conformity with this International Standard should result in standard deviations not exceeding those shown in Table 2.

Table 2

One-third octave band centre frequencies, Hz Standard deviation, dB
100 - 630 1.5
800 - 5 000 1.0
6 300 - 10 000 1.5

Measurements may be made in frequency bands outside these ranges but will have significantly higher standard deviations.

(Standards are referred to here by their ISO number, but the British and European standards are generally identical, for example, the technical content of international standard ISO 3745 is identical to BS EN ISO 3745, but the BS version has a national foreword.)