Standards Development
This standard will specify the method for measuring the far-field sound pressure radiated from a UAS and the procedures for using those measurements to calculate the sound power level and the sound pressure level directivity for use in noise mapping.
The standard will also propose that the overall A-weighted sound power level is used to quantify the noise impact of a UAS.
Only the hover condition will be considered at this time. In the future, the standard should be extended to include straight and level flight, take-off, landing and other flight modes.
The reason why this project is necessary is: there is demand from the UAS industry for a standardised method and metric for quantifying the noise produced by a UAS. A method for measuring the sound power level produced by a UAS in hover is described by European Commission Delegated Regulations 2019/945 and 2020/1058. This method is based on ISO 3744. It requires that the measured overall Aweighted sound power level produced by the UAS is reported and the regulation places limitations on the sound power level based on the weight of the UAS. This standard presumably already has buy-in from industry in Europe. However, the method described in the standard requires further development as it:
Requires that testing is performed on a UAS hovering an unspecified distance above an acoustically rigid plane. Sound pressure level measurements are then made on a (hypothetical) surface enclosing the UAS. The noise level of a UAS will presumably be significantly influenced by operation close to ground level due to recirculation and the ground effect. Microphone placement and wind protection should also be clearly specified in order to ensure the microphones are in the acoustic far-field, do not affect the inflow onto the UAS and are not affected by wind. Guidance should be provided on appropriate heights and positions of the microphones in order ensure this.
Allows testing outdoors and within hemi-anechoic chambers. For testing outdoors, wind is an issue. For testing in hemi-anechoic chambers, recirculation may be an issue. Guidance on how to mitigate/avoid these effects should be given.
It may also be preferrable to allow testing in a fully anechoic chamber rather than a hemi-anechoic environment. This should be considered and methods developed to allow it.
The standard should reference the UAS noise measurement methods described in ISO 5305.
Noise mapping requires that the sound power and directivity of a source is known. It will be useful to specify a standard method of collecting this data for UASs. Although only hover will be considered initially, the noise produced in the hover condition should be similar to that produced during slow flight. The standard can be updated in the future to include methods for measuring directivity in other flight conditions.
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