Sound power and intensity

The central quantity of this section is the sound power: the total acoustic energy per second a source radiates. Expressed in decibels as the sound power level, it is the figure that goes on a datasheet, feeds a room or outdoor prediction and is checked against noise-emission limits.

Sound Power chooses between the standardised routes and closes the job with the ISO 4871 emission declaration, and each route has its own page: Sound Power by Pressure Methods for the enveloping surface of ISO 3744/3746 and the precision anechoic grade of ISO 3745, Sound Power in the Reverberation Room for the direct and comparison methods of ISO 3741, Sound Power in Situ by Comparison for the ISO 3747 comparison against a reference sound source where the machine works, Sound Power in a Duct for the ISO 5136 in-duct method that measures a fan inside its own test duct, and Sound Power by Intensity Scanning for the on-site scanning of ISO 9614-2 and its ISO 9614-3 precision grade, and Sound Intensity for the discrete-point power summation of ISO 9614-1, which stands still at each position rather than sweeping the surface.

One route does not measure sound at all. Sound power from surface vibration (ISO/TS 7849) estimates the radiated power from the surface-averaged velocity level and a radiation factor, which is what remains when the machine cannot be moved to a qualified room and its environment is too noisy for an enveloping surface: Part 1 gives an upper-limit value from the velocity alone, Part 2 an engineering value once the radiation factor has been estimated properly. It also answers a slightly different question from the acoustic routes — it characterises what the structure radiates, and stays blind to sound escaping through openings, intakes and outlets — and it is the natural bridge to the structure-borne pages, since the same surface velocity is what Vibration and structure-borne sound measures.

Behind the intensity-based routes sits sound intensity itself: the signed power flux that can localise sources and separate them from background noise, measured with a two-microphone probe per IEC 61043 and qualified by the ISO 9614-1 field indicators, covered in Sound Intensity (p-p).

If you are here to measure a machine, start with Sound Power and let its decision guidance pick the route; read Sound Intensity (p-p) when that route involves an intensity probe, and go to Sound power from surface vibration when the machine cannot leave its installation and the background is too high for any pressure method. The determined power level is what the quieting measures of the Noise control pages are judged against.

• Sound Intensity (p-p): two-microphone sound intensity per IEC 61043, with the ISO 9614-1 field indicators and its sound power determination at discrete points.
• Sound Power: choosing the determination method and declaring the noise emission per ISO 4871.
• Sound Power by Pressure Methods: the enveloping surface of ISO 3744/3746 and the precision anechoic grade of ISO 3745, and the sound energy level of a burst over the ISO 3744/3746 enveloping surface.
• Sound Power in the Reverberation Room: the direct and comparison methods of ISO 3741, for sound power and for the sound energy of a single event.
• Sound Power in Situ by Comparison: the ISO 3747 engineering and survey comparison against a reference sound source in a reverberant environment, for the machine that cannot leave its installation, with the sound energy level of an impulsive source.
• Sound Power in a Duct: the ISO 5136 in-duct method for fans, with the sampling-tube flow and modal correction of Annex A and the plane-wave relation.
• Sound power from surface vibration (ISO/TS 7849): the radiated power from the surface-averaged velocity level and the radiation factor, with the Part 1 upper limit and the Part 2 engineering value.
• Sound Power by Intensity Scanning: the on-site scanning of ISO 9614-2 and the ISO 9614-3 precision grade.

• The determination methods start after the facility and the probe have been qualified. ISO 3745’s free-field qualification of an anechoic or hemi-anechoic room, ISO 3741’s reverberation-room qualification (eigenfrequency counting or a reference-source comparison) and the IEC 61043 residual-intensity test of a probe-and-analyser chain are all assumed, not performed: the library warns on the coarse advisory criteria the standards state explicitly — the Table 1 minimum volume, the position count, an inter-position spread above 1.5 dB, the ISO 3744 K₂ validity — and grades a residual-intensity index you measured yourself. The C₃ meteorological correction of ISO 3745 likewise needs an air-absorption coefficient you supply; it is not computed from ISO 9613-1 here.

• One route stops short of its standard: on the vibration route, the measurement clauses 5 to 7 of both parts of ISO/TS 7849 are laboratory practice rather than code, and only the single-machine radiation factor of Formula 8 is implemented, so a batch or family determination needs an already-averaged value. And nothing here reduces a machine’s emission: quieting a source is Noise control, and a declared emission value is the input to that work, not its result.