ISO series of standards for sound power determination

For the purposes of this guide, the set of ISO sound power standards consisting of the '3700' series, comprising six standards, and ISO 9614, comprising three parts are referred to as the 'ISO basic series'.

This set of standards differ by Grade of accuracy, Test environment , and measurement method. Below is a simplified comparison of the standards in the basic series. The table below is based in information in ISO 3740, which provides guidelines in the use of the basic series. Please refer to this standard for a more detailed comparison of the standards.

 

The ISO series of basic sound power determination standards
ISO Standard	Measurement Method	Test Environment	Grade of Accuracy
3741	Reverberation	Reverberation room	Precision
3743pt2	Reverberation	Special reverberation room	Engineering
3744	Direct	Essentially free field-field over reflecting plane	Engineering
3745	Direct	Anechoic Hemi-anechoic room	Precision
3746	Direct	Essentially free field-field over reflecting plane	Survey
3747	Comparison	Essentially reverberant	Engineering Survey
9614	Intensity	Almost any	Precision Engineering Survey

 

Further information

All the ISO Standards, can be purchased from ISO online catalogue

Basis of methods for sound power determination

The sound power level can be determined by one of four basic methods, Intensity, Direct, Reverberation or Comparison. A simplified overview of the methods is outlined below. In practice, depending on the grade of accuracy required, there may be additional corrections to account for factors such as background noise and sound reflections.

Intensity method

This is the most direct method for measuring sound power, but unlike the other methods that requires a measurement of sound pressure using a microphone, it requires the use of an intensity probe that additionally measures particle velocity.

The sound power is derived using the assumption that all the radiated sound energy propagates through a hypothetical surface enveloping the noise source.

Direct method

This method is the most common method for determining sound power emission. The method is based on the the assumption that for a sound wave propagating from the noise source, into a free field, the intensity is in phase with the pressure. Like the intensity method, the sound power is derived from measuring the sound radiating from the noise source through a hypothetical enveloping surface.

The sound power level, Lw, is approximated as:

Lw = Lp + 10Log(A)   dB

Where Lp is the time averaged sound pressure level averaged over the enveloping surface and A is the area (m²) of the enveloping surface.

It should be noted that although the direct method relies on the assumption of an essentially free-field, a correction accounting for boundary reflections may be applied in some circumstances in the form of environmental correction factor, k₂.

Reverberation method

This method assumes that the sound energy emitted from the noise source is uniform throughout the room and is therefore only suitable for use in reverberation rooms. The sound power level, Lw, is approximated as:

Lw = 10log(T) + 10Log(V) - 14   dB

Where T is the reverberation time (s), V is the volume of the room (m³) and the constant, 14, is derived from the 'Sabine' formula.

Comparison method

For this method, the sound power level is determined by comparison with the known sound power level of a Reference Sound Source (RSS). The sound power level, Lw, is calculated from

Lw = Lw_(RSS) - Lp_(RSS) + Lp_(ns)   dB

Where

Lw_(RSS) is the sound power level of the RSS derived from its calibration,
Lp_(RSS) is the time averaged sound pressure level of the RSS, averaged over the enveloping surface measured in the test environment,
Lp_(ns) is the time averaged sound pressure level of the noise source, averaged over the enveloping surface measured in the test environment.

The comparison method may be based on the Direct or the Reverberation method.

Grades of accuracy

The table below shows the ISO basic series categorized by grade of accuracy. The table lists the measurement uncertainty as quoted in each standard, for A-weighted, and 1kHz 1/3 octave band levels. Other frequency bands may have larger uncertainty values depending upon the standard. The table provides only a simplified guide. Refer to the respective standard or ISO 3740, for further details.

The uncertainty values quoted refer to the standard deviation of reproducibility which is defined in ISO 7574pt1 :1985 as the standard deviation of noise emission values obtained from the repeated application of the same noise emission measurement method on the same noise source at different times and under different conditions.

Grades of accuracy and Uncertainty
Grade of Accuracy	Precision			Engineering				Survey
Description	Requires greatest effort. Provides a thorough description of the sound field.			Provides sufficient information to take engineering action.				Requires least effort. Only A-wt or C-wt levels required
ISO Standard	3741	3745	9614	3743	3744	3747	9614	3746	3747	9614
Uncertainty A-wt (dB)	0.5	-	-	2	1.5	1.5	-	3,4a	4	4
Uncertainty 1 kHz band (dB)	1.5	1	1	2	1.5	-	1.5	-	-	-

Note on table: a - Dependent upon value of environmental correction factor, k₂

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