Transformer Noise Measurement Methods [includes discussion]

The present sound level index of transformer noise is discussed and compared with two of several other indexes considered by the Committee. These are loudness level and audibility. It is believed that the index of loudness level is more indicative of disturbance from transformer noise than is sound level. However, measurement of loudness level is more complex since it requires some form of frequency analysis and more or less extensive calculation. Furthermore, loudness level does not decrease following the law of spherical divergence even though the attenuation of the individual frequencies may follow this law. Therefore the loudness level has to be calculated for each distance of interest. The audibility index depends on the intensity of the individual frequency components above ambient noise. Data are presented which indicate when the intensity of the individual components will be great enough to make them audible. Determination of audibility is most precise when based on measurements involving frequency analysis. However, the limited data now available point to the possibility of using only the sound level meter with its A weighting network for this method. Measurements made with the sound level meter can be used to determine the distance betwen transformer and residence necessary for just achieving inaudibility. However, it is not to be inferred that inaudibility is necessary for satisfactory installation. Experience curves are presented which indicate that the level may be raised above inaudibility without giving rise to significant reaction.