Intensity method for measuring sound power – a new assessment of field indicators

The sound power level characterises the total amount of sound emitted by a source. Due to this basic importance it
is used in many legal and other regulations in occupational health, environmental control and consumer protection. The sound power level is measured in accordance with about 10 different internationally harmonised basic standards. Here, quantities directly measured are either the sound velocity level on the source’s surface or the sound pressure level and the sound intensity level, respectively, in the surrounding airborne sound field.

Even though the use of the sound intensity level should be advantegeous in theory, the corresponding method is rarely used in practice. One reason for this is that the related measurement standards define a multitude of indicators and belonging criteria to assess the quality of the obtained measurement results. This essentially reasonable approach leads in practice often to the situation that no valid measurement result can be obtained by the intensity method. 

To improve this, the Federal Institute for Occupational Safety and Health (BAuA) remitted a research project to PTB [1]. Within this project, comprehensive calculations were performed, where the sound fields of monopoles and dipoles were superposed with external diffuse fields and plane waves. It turned out that one of the investigated three indicators really depicts the quality of the measurement result (Figure 1) whereas the other two indicators are unnecessary (Figure 2). These theoretical findings were underpinned by an extensive measurement program in which four different sound sources were measured in very different environments at PTB. Meanwhile, the investigations have been published [2] and are now the basis for a revision of the respective standards.

Figure 1. Deviation between the measured sound power level and the actual sound power level as a function of the criterion F_pIn - δ_pI0 for
a dipole whose sound field is superposed by an external semi-diffuse sound field. The standardised specifications F_pIn - δ_pI0 < -7 dB (grey) and F_pIn - δ_pI0 < -10 dB (black) effectively limit the deviation of the sound power level to values below ± 1.0 dB and ± 0.5 dB, respectively.

Figure 2. Deviation between the measured sound power level and the actual sound power level as a function of the criterion F_p|In| - F_pIn for
a dipole whose sound field is superposed by an external semi-diffuse sound field. The standardised specification F_p|In| - F_pIn < 3 dB leads to an unnecessary disqualification of correct measurement results which have already been validated according to the criteria in Figure 1 [F_pIn - δ_pI0  < -7 dB (grey) and F_pIn - δ_pI0 < -10 dB (black)].

Literatur:

[1] S. Brezas, F. Heisterkamp, and V. Wittstock. Practice-oriented simplification of noise emission measurement methods, subproject 2: Practical adaptation of the sound intensity measurement method. Report, Federal Institute of Occupational Health and Safety (BAuA), doi: 10.21934/baua:report20210914 (online), 2021.

[2] Wittstock, V., Brezas, S., Heisterkamp, F.: Sound power determination by intensity—Are field indicators and criteria in ISO 9614 meaningful? J. Acoust. Soc. Am. 155 (1), January 2024, 588-599, doi: 10.1121/10.0024361 (online)

Ansprechpartner:

Volker Wittstock, FB 1.7, E-Mail: volker.wittstock(at)ptb.de