Lee, S et al, Annoyance Caused by Amplitude Modulation of Wind Turbine Noise
Annoyance caused by amplitude modulation of wind turbine noise
Seunghoon Lee, Kyutae Kim, Wooyoung Choi and Soogab Lee
August 9 2010
A listening test has been performed to investigate the relationship between human annoyance and the amplitude modulation of wind turbine noise. To obtain sound samples for the listening test, sound from a 1.5 MW wind turbine in Korea was recorded. The strength of the amplitude modulation of the sound samples was defined in terms of the modulation depth spectrum, which was approximated by assuming that the sound samples are sinusoidally amplitude-modulated. The stimuli for the listening tests were created by reducing the modulation depth spectrum of the sound samples. A total of 30 participants were involved in the listening tests. The results of the listening tests indicate that the equivalent sound level and the amplitude modulation of wind turbine noise both significantly contribute to noise annoyance. © 2011 Institute of Noise Control Engineering.
Wind turbines produce sound with levels that fluctuate periodically at the blade passing frequency. This amplitude modulated sound is clearly perceptible at locations near the wind turbines, whereas the perception of this sound becomes difficult at large distances from the wind turbines. However, some residents living near a wind farm have claimed that in some circumstances this sound is perceived at a distance of more than 1 km from wind turbines. Recent studies have also shown that the amplitude modulation can be heard even at large distances if the background noise level is quite low.
Several previous studies have argued that the amplitude modulation of wind turbine noise may increase noise annoyance. Van den Berg reported that residents living at 500 m and up to 1900 m from a wind farm expressed annoyance due to wind turbine noise. The residents could hear a low pitched thumping sound especially at night, which is periodic at a blade passing frequency. The author maintained that this amplitude modulation may increase annoyance. Moreover, Pedersen and Persson Waye performed a field study to evaluate the prevalence of annoyance caused by wind turbine noise. The result of their study suggested that wind turbine noise is more annoying than other community noise sources with the same A-weighted sound level. They mentioned that one of the reasons for this result could be the presence of the amplitude modulation of wind turbine noise.
Indeed, amplitude modulated sound is generally known to be more annoying than un-modulated sound. Kantarelis and Walker examined the source of difference between the annoyance of diesel and electric train noises. They suggested that the amplitude modulation in diesel engine noise is the reason for the extra annoyance. The annoyance caused by the diesel train noise decreased as the modulation depth was reduced from 13 dB to 5 dB. Furthermore, Bradley investigated the influence of amplitude modulated low-frequency sounds from heating, ventilation, and air conditioning (HVAC) systems on annoyance, finding that annoyance is correlated with both the sound pressure level and the amplitude modulation of the noise from HVAC systems. Thus, the amplitude modulation of wind turbine noise is also thought to increase the annoyance of wind turbine noise.
However, for wind turbine noise, only a few experiments have been performed on the influence of amplitude modulation on annoyance. Persson Waye and Őhrstrőm8 performed a listening test to evaluate annoy- ance from five wind turbine noise sources and analyzed the relation between noise annoyance and psycho- acoustic parameters such as loudness, sharpness, tonal- ity, fluctuation strength and modulation. The results show that the annoyance ratings were significantly different for the different noise sources, but none of theparameters, including modulation, could explain the differences in annoyance ratings. The reason may be that this experiment was not designed to evaluate additional annoyance caused by the amplitude modula- tion of wind turbine noise.
Thus, our study performs a listening test to examine the annoyance caused by the amplitude modulation of wind turbine noise. Two kinds of amplitude modulated sounds from a 1.5 MW wind turbine are used for the listening tests. The strength of amplitude modulation is quantified by measuring the modulation depth spectrum of the recorded wind turbine sound assuming that the wind turbine sound is sinusoidally amplitude-modulated. The stimuli for the listening test were designed by reduc- ing the modulation depth of the recorded wind turbine sound.