Sound power level measurement in diffuse fields for not movable sources or emitting prominent discrete tones
Résumé et principaux résultats
Article présenté au Congrès Français d'Acoustique d'avril 2018.Determination of sound power level (Lw) using diffuse field is widely used in industry and certification, especially for heating, ventilation and air conditioning (HVAC) machines. A hard-walled or reverberation test room is less expensive to build than anechoic or hemi-anechoic rooms and it is suitable to manage the large range of ambient conditions required for air-conditioning units. The associated international standards are ISO 3741 (precision, grade 1), ISO 3743-1, ISO 3743-2 (engineering, grade 2) and ISO 3747 (survey, grade 2 or 3). In practice, laboratories (and industrials) face difficulties to carry out tests at grade 1 and even grade 2 because many machines have spectra with discrete-frequency components (pure tones) at either low or medium frequencies with a possible impact on the A-weighted level. The qualification of the room according to Annex D of ISO 3741 is not always possible. Then, the sound measurement of those sources requires several microphones and source positions which may be unrealistic. In addition, many kind of machines cannot be moved (wall hung boilers, fans, ducted HVAC units). Their noise measurement according to ISO 37431 also requires source moving if the machines have large directivity or discrete-frequency components. For this purpose, the standard title includes "small, movable sources". Despite these constraints, some product standards (e.g. ISO 13347-2) allow the use of ISO 3743-1/2 even if the machines are not movable, accepting a result bias to improve reproducibility. This paper illustrates these difficulties, with actual examples encountered by the authors. Various options are studied to improve the situation. For instance, the possibility to merge the different standards with a system of more or less severe criteria is evaluated, the accuracy grade becoming a consequence of the used means and the obtained uncertainty result.
Thème
Acoustique générale et vibroacoustique
Mots-clés
Acoustique, Mesure de bruit, Normalisation
Auteurs
BESSAC François