Source: https://jmrplens.github.io/phonometry/devices/emission/ # 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](https://jmrplens.github.io/phonometry/devices/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](https://jmrplens.github.io/phonometry/devices/sound-power-pressure/) for the enveloping surface of ISO 3744/3746 and the precision anechoic grade of ISO 3745, [Sound Power in the Reverberation Room](https://jmrplens.github.io/phonometry/devices/sound-power-reverberation/) for the direct and comparison methods of ISO 3741, and [Sound Power by Intensity Scanning](https://jmrplens.github.io/phonometry/devices/sound-power-intensity/) for the on-site scanning of ISO 9614-2 and its ISO 9614-3 precision grade. A fourth route does not measure sound at all. [Sound power from surface vibration (ISO/TS 7849)](https://jmrplens.github.io/phonometry/devices/vibration-sound-power/) 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 four 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](https://jmrplens.github.io/phonometry/vibration/) 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)](https://jmrplens.github.io/phonometry/devices/intensity/). If you are here to measure a machine, start with [Sound Power](https://jmrplens.github.io/phonometry/devices/sound-power/) and let its decision guidance pick the route; read [Sound Intensity (p-p)](https://jmrplens.github.io/phonometry/devices/intensity/) when that route involves an intensity probe, and go to [Sound power from surface vibration](https://jmrplens.github.io/phonometry/devices/vibration-sound-power/) 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](https://jmrplens.github.io/phonometry/noise-control/) pages are judged against. ## Pages in this section - [Sound Intensity (p-p)](https://jmrplens.github.io/phonometry/devices/intensity/): two-microphone sound intensity per IEC 61043 with the ISO 9614-1 field indicators. - [Sound Power](https://jmrplens.github.io/phonometry/devices/sound-power/): choosing the determination method and declaring the noise emission per ISO 4871. - [Sound Power by Pressure Methods](https://jmrplens.github.io/phonometry/devices/sound-power-pressure/): the enveloping surface of ISO 3744/3746 and the precision anechoic grade of ISO 3745. - [Sound Power in the Reverberation Room](https://jmrplens.github.io/phonometry/devices/sound-power-reverberation/): the direct and comparison methods of ISO 3741. - [Sound power from surface vibration (ISO/TS 7849)](https://jmrplens.github.io/phonometry/devices/vibration-sound-power/): 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](https://jmrplens.github.io/phonometry/devices/sound-power-intensity/): the on-site scanning of ISO 9614-2 and the ISO 9614-3 precision grade. ## What this section does not cover 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 is absent by design: **ISO 9614-1's discrete fixed-point power summation is not implemented at all**, and only its Annex A field indicators are, reused by the two scanning parts. 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. Finally, nothing here reduces a machine's emission: quieting a source is [Noise control](https://jmrplens.github.io/phonometry/noise-control/), and a declared emission value is the input to that work, not its result.