[Home Theater Network HDAV.com.cn] 1. Indoor Sound Field When two people walk and talk and walk into the building from the outside, people will feel that the mutual conversation is not the same as the outdoor, and the voice of the conversation will become louder. What is the reason? The indoor sound field is compared with the free field or the outdoor sound field without reflection sound. On the one hand, because the indoor surface produces a lot of reflected sound, the sound energy is gradually increased, and finally the stable sound energy is achieved. The sound energy is much improved, which makes the indoor speech louder than the outdoor. On the other hand, after the indoor sound source stops sounding, the sound in the room does not disappear immediately, but gradually decreases. In general, the impact of the room on the sound is mainly reflected in the following four aspects: (1) causing a reflected sound; (2) changing the sound quality; (3) Increasing the energy density due to the excitation of the normal wave; (4) The distribution of sound in space is changed. Under different conditions and different requirements, different analysis methods can be used to analyze the indoor sound field—geometric acoustics, statistical acoustics, and wave acoustics. The geometric acoustic analysis method is similar to geometric optics. The sound line diagram can be used to analyze the special shape of the room and the effect of the reflection on the sound quality. Statistical acoustic analysis methods are mainly in practice, through a large number of statistical analysis of data to derive some formulas, and use these formulas to carry out the sound quality design of the hall; and the analysis method of wave acoustics, through the study of acoustic characteristics, can reveal the indoor sound field In essence, although the simple principle of guiding sound quality design cannot be proposed, it can not only explain the range of application of geometric acoustic methods and statistical acoustic methods, but also explain some acoustic processes that do not satisfy the conditions of diffuse sound field. 2. Indoor geometric acoustic analysis method When the frequency of the sound wave is high, that is, the wavelength of the sound wave is smaller than the scale of the reflective surface in the room, the concept of light in geometric optics can be used to regard the sound wave propagation as the sound energy propagating along the sound line, ignoring the wave performance of the sound wave. It is a method of geometric acoustics. The method of geometrical acoustics is very effective in studying the effects of various reflecting surfaces in a room, and it is also possible to check whether there is acoustic focusing in the room. The geometric acoustic method uses conditions in which the size of the reflecting surface or obstacle is much larger than the wavelength of the acoustic wave. At this time, the sound wave can be reflected by the geometric optical reflection law whose reflection angle is equal to the incident angle. Figure 1 The spread of indoor sound Figure 1 shows the cross section of a rectangular room. When the sound source is sounded at S, the listener R in the room first hears the sound directly from the sound source, that is, the direct sound SR, and then hears it in turn. The primary reflected sound SAR, SBR, which is reflected by the sound source through a wall, is reflected by the two walls and reaches the secondary reflected sound SCDR, which is reflected by the three walls and reaches the three reflections of the listener... until the sound is reflected multiple times. Since each surface of the room absorbs a part of the sound energy when it is reflected by the sound, the intensity of the reflected sound will gradually decrease as the number of reflections increases. In addition, the distance from the reflected sound to the ear is longer than the direct sound, so the time for the reflected sound to reach the listener lags behind the direct sound, and the more the number of reflections, the longer the lag time. Figure 2 shows the pulse response waveform of the room measured by the Guangdong Guild Hall. It can be seen from the figure that the intensity of the signal received by multiple reflections on the wall is weakened. Figure 2 The measured impulse response waveform of the Guangdong Guild Hall According to the different classification of the reflecting surface, the common types of reflection are: plane reflection, concave reflection and convex reflection. The plane reflection is shown in Figure 3 (a). The sound source S emits several sound lines and is incident on the reflection interface. The sound lines are reflected at these different incident points. The sound lines can be extended behind the interface and they will gather at a point. , that is, the sound image point. The sound image point S' is located on the vertical line from the sound source S to the interface, and the distance from the interface is equal to the distance from the sound source to the interface. For the observer in the room, all reflections of the sound source on the interface appear to originate from this sound image point S'. S' is also referred to as a virtual sound source. The concave and convex reflections are shown in Figure 3 (b) and Figure 3 (c). It can be seen that the convex surface has obvious scattering effect on the incident sound wave. In the sound quality design, the correct use of the convex curved surface will contribute to the uniform diffusion of the sound field. Figure 3 reflection type The characteristic of the concave reflection is that the sound is concentrated in a certain area or the sound focus point appears, which results in uneven distribution of the sound field, and should be prevented in the sound quality design. It should be noted that for low frequency sound (63 Hz ~ 125 Hz), the wavelength is 5.4 m - 2.7 m. In a small room where each surface size is smaller than the wavelength of the acoustic wave, the law of geometric reflection will not be applicable. In addition, when sound waves encounter small-scale obstacles in a large room, or smaller-sized reflectors, they will bend, that is, form significant acoustic diffraction, but will not cause significant diffuse reflection. For medium frequency sounds (500 Hz - 1 000 Hz), the wavelength range is 68 cm - 34 cm, and the geometric laws are still followed when encountering large ceilings and walls. However, when encountering components and decoration processes that are close to the wavelength of the acoustic wave, diffusion reflection is formed. For high-frequency sound (2000 Hz - 8000 Hz), the wavelength range is 17 cm ~ 4 cm, when most indoor components will form a clear sound shadow. In addition, the general ceiling and wall still follow the law of geometric reflection. Links: "Speaking about indoor acoustic design and acoustic processing points" More fresh and fun home theater information, please pay attention to home theater network http:// (WeChat: cnhifi), the country's most influential home theater audio player interactive media website. Wireless earphone,Mono Earphone, Wireless Bluetooth Earphone Shenzhen PuFang Technology Co., Ltd. , https://www.hytelus.com
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