Amplitude modulation (AM) sound, so called swish sound, is generally contained in wind turbine noise (WTN) and it causes serious annoyance in the areas around wind farms. Therefore, the methods to assess the characteristics of this kind of sound should be investigated in both viewpoints, physically and psycho-acoustically. Regarding the former problem, a practical method to evaluate the magnitude of the AM using common acoustic measurement instrumentation is proposed in this paper. That is, the sound pressure level difference between the levels measured by using FAST and SLOW dynamic characteristics of a sound level meter is calculated for the measurement time interval under investigation and then the cumulative distribution function of the level difference is calculated. From the result, the value of 90% range is obtained as an indicator for assessing the AM. Statistical data evaluated by using this indicator for AM sounds contained in actual WTNs were obtained through the field measurements performed nationwide across Japan.
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