What are the transmission modes of low-frequency noise?
1、 Low frequency noise:
The so-called low-frequency noise refers to the sound whose frequency is below 500 Hz (octave). Low frequency noise sources in residential areas mainly include five categories: elevators, transformers, water pumps in high-rise buildings, central air conditioners (including cooling towers) and traffic noise. The Institute of Environmental Pollution Control Technology of Zhejiang University tested and analyzed the acoustic instruments used for supporting equipment noise sources in typical residential areas in Hangzhou, and selected the noise sources of power supply system, underground garage, elevator equipment, heating system, drainage and water supply system, air conditioning equipment, and ventilation equipment. The results showed that 9 of the 12 typical noise sources were The frequency band where the loud pressure level is located is low frequency band and multi frequency band. Therefore, low-frequency noise becomes an influential and large noise source in residential areas.
2、 Low frequency noise transmission path:
Low frequency noise is mainly divided into structure sound transmission, air sound transmission and standing wave according to the transmission path, of which standing wave is harmful and serious. Structural sound transmission means that transformers, water pumps and central air conditioning hosts installed in the building transmit low-frequency vibration sound waves to each household through the basic structural girders and load-bearing beams of the residential building. Air sound transmission refers to the direct transmission of low-frequency noise through the air to residential areas. Refracted sound wave refers to the refracted sound wave formed by multiple reflections of low-frequency noise in the transmission process. The amplitude and intensity of low-frequency noise in the wave belly is strong, which is harmful to human health.
3、 Low frequency noise and high frequency noise:
Low frequency noise is different from high frequency noise. High frequency noise can rapidly attenuate with the distance getting farther or encountering obstacles. For example, the point source of high frequency noise can reduce by 6 dB every 10 meters. However, the low-frequency noise decreases slowly, and the sound wave is longer, so it can easily pass through obstacles and penetrate the wall to the human ear for a long distance. Vibration, low-frequency noise and general noise all have a common property, which is a kind of vibration wave and a way of energy transmission.