Tudy On Acoustical Simulation And Adjustment Methods Of Subjective Perception And Objective Law Of Electroencepha-Logram Variation Under Noise Exposure

The physiological and psychological effects induced by noise, especially low frequency noise (LFN), have been a hot research topic in the field of environmental noise. In this study, the acoustic simulation and sound adjustment on subjective annoyance under low frequency noise exposure, and the relationship between Electroencephalogram (EEG) variation and subjective annoyance under noise exposure, were researched.In the first part of the study, a plurality of typical noise sources from urban environment were recorded. Based on noise spectrum analysis, several characteristic frequencies involved high subjective annoyance were selected according to the relative subjective annoyance of pure tones with different frequencies at equal loudness levels. Pure tones with characteristic frequencies were synthesized into a series of noise samples. Paired comparison test was used for comparison of the subjective annoyance under actual noise and synthesized noise. The results suggest that the sound energy distribution of actual noise at different frequency bands should be considered in order to simulate the subjective annoyance induced by actual noise with composite noise more accurately. And composite noise should be combined with pure tones which induced high subjective annoyance and were located in frequency bands with the majority of sound energy, provided that the relative Sound Pressure Level (SPL) differences were remained among the characteristic frequencies of actual noise.In the second part of the study, structure-borne noise originating from a heat pump unit was selected to study the influence on subjective annoyance of low frequency noise (LFN) combined with additional sound. Paired comparison test was used for evaluating the subjective annoyance of LFN combined with different sound pressure levels (SPL) of pink noise, frequency-modulated pure tones (FM pure tones) and natural sounds. The results showed that, with pink noise of250～1000Hz combined with the original LFN, the subjective annoyance value (SAV) first dropped then rose with increasing SPL. When SPL of the pink noise was15～25dB, SAV was lower than that of the original LFN. With pink noise of frequency250～20000Hz added to LFN, SAV increased linearly with increasing SPL. SAV and the psychoacoustic annoyance value (PAV) obtained by semi-theoretical formulas were well correlated, moving in similar trends. This suggests that the PAV can be used to predict SAV variance of the LFN after adding pink noise, to search for the appropriate pink noise combined with LFN to alleviate subjective annoyance. When FM pure tones with central frequencies of 500Hz,2000Hz and8000Hz, or natural sounds (including the sound of singing birds, flowing water, wind, and ticking clock) were respectively added to the original sound, the SAV increased as the SPL of the added sound increased.In the third part of this study, a new kind of low-frequency noise source, main transformer noise in high-voltage substation, was selected to study the influence on subjective annoyance of LFN combined with additional sound. Paired comparison test was used for evaluating the subjective annoyance of LFN adjusted by combining with different SPL of narrowband pink noise, natural sound, and multiple natural sounds. Behavior disturbance test was also carried out on some adjusted noise samples. The results showed that, adding pure tone to product a beat, or adding the crowd conversation to mark the original sound, were all failed to alleviate the subjective annoyance. Adding narrowband pink noises with suitable frequency range may alleviate the subjective annoyance. Whereas it may due to that the narrowband pink noise sounds like the wind. Adding some sound of flowing water, especially that containing fewer low frequency component, can effectively reduce the subjective annoyance. However, the most appropriate sound level of water was not invariable. It should be determined according to the specific water sound. An additional birds singing cannot further improve the subjective perception of noise sample which added sound of flowing water already. Reducing the SPL of the specific tone in the original noise sample can reduce the subjective annoyance effectively. Whereas it mainly dues to the reduction of the whole noise sample SPL. The questionnaire result from behavior disturbance test indicated that, the reading disturbance was not improved significantly when subjects exposed to the adjusted samples, which can improve the subjective perception in paired comparison test. However, the dosage exercise test result from behavior disturbance test indicated that the brainwork index (AYP) rose, while the error rate fell when the subject exposed to the adjusted samples. This means the sound adjustment indeed reduce the negative impact on people’s thinking ability, which induced by LFN.In the fourth part of this study,70dBA white noise and pure tones at160Hz,500Hz and4000Hz were selected as the exposed noise sources. Electroencephalogram (EEG) signal of subjects was recorded and analyzed during the experiment. The relationship between variation and noise character, noise subjective annoyance, was also investigated. The results showed that, when the duration of noise was less than6s, Average Power of Electroencephalogram (APEEG) varied irregularly. When the noise lasted for5min, the sum of the relative APEEG of0and a wave in the frontal region exhibited some trends with the increase of the exposed noise frequency, and presented a positive correlation with subjective annoyance value. Accordingly, the subjective annoyance induced by noise can be estimated indirectly through determination of the sum of the relative APEEG of6and a wave in frontal region under noise exposure. Without reference to the fact that subjects were exposed to noise or not, the maximums distribution of APEEG of θ wave appeared in the frontal region, while the maximums distribution of APEEG of a wave appeared in the occipital region. And the relative APEEG of θ or a wave in each brain region exhibited basically consistent variation trends within the11min from1min before noise exposure to5min after that. More than two APEEG maximums of9wave appeared after noise exposure, and the time points of maximum occurrence moved forwards with the increase of noise frequency. The interval between two time points of maximum occurrence was reduced with the increase of exposed noise frequency. It is believed that the time for subjects to make steady stress responses tended to decrease following the increase of frequency. Furthermore, during the different noise exposures, the relative APEEG of θ wave in the frontal region decreased first and then increased with the increase of frequency, while the relative APEEG of a wave in the frontal region increased as the sound frequency increased.