Key words: Experimental study on low-frequency noise reduction of indoor ornamental plants?
China Library Classification Number: TB53 Document Identification Number: A Document Number:1672-3791(201) 04 (a)-0000-00?
It is generally believed that only noise with high sound level will pollute the indoor environment, such as road traffic noise, commercial activity noise and industrial production noise [1]. With the progress of science and technology and the improvement of people's material and cultural living standards, the use of various power equipment that directly provide convenient services for people's lives, such as range hoods, fluorescent screen ballasts, outdoor air conditioners, refrigerators, computers and other equipment, has greatly changed the indoor noise environment. Some low-frequency and low-noise environmental noise is becoming the main component of indoor noise. On the basis of briefly expounding the source of low-frequency noise and indoor sound field theory, the influence of ornamental plants on indoor noise sound pressure level, indoor reverberation time and noise reduction is analyzed, which provides experimental basis for indoor noise reduction by using ornamental plants. ?
1 Definition and source of low-frequency noise?
Low-frequency noise refers to noise dominated by low-frequency components, and there is no obvious boundary compared with intermediate frequency and high-frequency noise. Researchers usually define noise in different frequency ranges as low-frequency noise according to their own research needs. Generally speaking, for environmental noise, the lower limit of low-frequency noise is defined as the frequency of 20Hz, which is the lower limit of human audible frequency. However, there is no unified conclusion on the upper limit of low-frequency noise, which is usually between100 and 250 Hz [2]. Typical indoor low-frequency noise pollution sources are mainly some low-frequency mechanical vibrations, such as elevators, transformers, underground garages, combustion, exhaust fans, household appliances, tap water pumps and so on. And low-frequency noise generated by electromagnetic excitation. Among them, household appliances are the main source and important factor of indoor low-frequency noise. The acoustic characteristics of low-frequency noise are that the central frequency corresponding to the peak sound pressure level on the noise spectrum is often in the low frequency band, and the sound pressure level in the middle and high frequencies is relatively low, and the spectrum is low. ?
2 indoor sound field theory [3-5]?
When the sound source is placed indoors, the sound receiving point not only receives the sound energy directly radiated by the sound source, but also receives the sound energy reflected by objects such as walls. The sound field distribution at this time is much more complicated than the outdoor free sound field. Generally speaking, according to the propagation process of sound field, sound field can be divided into direct sound field and reverberation sound field. ?
2. 1 sound pressure level?
Sound pressure is a measure of sound size, and sound pressure level is a logarithmic form of the ratio of sound pressure to reference sound pressure. For a point sound source with sound power, the sound pressure level of the direct sound field with distance is:?
( 1)?
Where: sound power level of point sound source; Is the directivity factor, and the point sound source is placed in infinite space, on rigid infinite plane, at the intersection of two rigid planes and at the intersection of three rigid planes. ?
Let the reverberation sound field be an ideal diffusion sound field, and its corresponding sound pressure level is:?
(2)?
Where is the room constant in. ?
Then the sound pressure level of the total sound field obtained by the superposition of the direct sound field and the reverberation sound field is:?
(3)?
2.2 reverberation time?
Reverberation time refers to the time required for the acoustic energy density to decay to one millionth of the original, that is, 60dB. The modified reverberation time is:?
(4)?
Where, is the room volume, and the unit is the unit; Is the surface area of the room, in units; Is the average sound absorption coefficient of the room, that is, the ratio of absorbed sound energy to incident sound energy; Is the attenuation coefficient of sound intensity in air. ?
2.3 Sound absorption and noise reduction?
Using the relationship between sound absorption coefficient and reverberation time, we can get that the amount of sound absorption and noise reduction is:?
(5)?
Where, and are reverberation time before and after installing sound absorption device indoors. ?
3 Experimental test and result analysis?
The experiment was conducted in a closed indoor environment. The prestige VS302USB dual-channel acoustic analyzer is used to simulate the noise signal and measure the reverberation time, and the AWA62 18B noise statistical analyzer and AWA572 1 filter are used to analyze the octave spectrum. There are four kinds of ornamental plants used in the experiment: green radish, African jasmine, equisetum and yemen iron. ?
As can be seen from Figure 1, the average sound pressure level of indoor noise is reduced to some extent when ornamental plants are placed indoors. The influence of green radish on noise sound pressure level is the most obvious. When the rising power of sound source is constant, the sound pressure level of reverberation sound field is determined by the room constant or the surface area of absorption surface. When ornamental plants are placed indoors, the leaves of the plants absorb sound waves, thus increasing the surface area of the absorption surface, increasing the indoor constant and reducing the sound pressure level of the sound field. The larger the leaf surface and body shape of plants, the more obvious the noise sound pressure level drops. ?
As can be seen from Figure 2, the reverberation time of the room has changed to some extent after the ornamental plants are placed in the room. Repeated experiments show that the reduction of reverberation time has good repeatability, which shows that indoor plants have a certain absorption effect on sound waves. From the reverberation time effect, the larger the blade size, the more obvious the impact on the room reverberation time. Substituting the monitoring results in Figure 2 into Equation (5), the noise reduction can be obtained, as shown in Figure 3. As can be seen from the figure, ornamental plants have an obvious effect on reducing indoor noise, among which ornamental plants with large size, large leaf area and large leaves have the most obvious effect on reducing indoor noise. ?
4 conclusion?
After ornamental plants block the propagation path of indoor low-frequency noise, the sound pressure level of noise can be reduced to some extent, and the reverberation time in the room will also change, resulting in indoor sound absorption and noise reduction effect. Noise reduction is affected by factors such as plant size, leaf size and number of leaves. ?