Sound wave propagation characteristics 1) Sound source directivity: It is mainly related to the size of sound source and the wavelength of sound wave. When the wave length is larger than the sound source size, the sound wave propagates evenly in all directions; On the contrary, the sound wave propagates to the range of the sharp cone in front of it. For example, when we speak, the low-frequency part of the sound can spread evenly in all directions around the head because its wave length is larger than the size of the sound source; While the high-frequency part of speech is guided forward only by the speaker's mouth. Therefore, when we stand behind the speaker, the high-frequency components in the sound we hear will drop, and we often feel that we can't hear clearly. 2) Reflection and refraction of sound waves: When sound waves propagate in the air, they encounter hard obstacles (such as plane walls) with an aspect ratio of sound wavelength, which will cause reflection. The angle between the reflected sound wave and the normal perpendicular to the wall surface is equal to the angle between the incident sound wave and the normal. When sound waves encounter obstacles, in addition to reflecting sound waves, some sound waves will enter the obstacles. The number of incident sound waves is related to the characteristics of obstacles. If a hard obstacle is encountered in the propagation path, most of the acoustic energy will be reflected back and a small part will be absorbed by the obstacle; If you encounter soft and porous obstacles in the propagation path, most of the sound waves will be absorbed and a small part will be reflected. At this time, the sound wave enters another medium from one medium, and its propagation direction changes. We call this phenomenon refraction. 3) Diffraction and scattering of sound waves: When sound waves encounter a large obstacle, there is no sound wave near the edge behind the obstacle, but the rest of the sound waves are still propagating. We call this phenomenon diffraction. The degree of diffraction depends on the relationship between the wavelength of sound wave and the size of the object. In the process of sound wave propagation, if the diffraction produced by obstacles is irregular, it is called scattering phenomenon. For example, when sound waves pass through holes in obstacles, diffraction also occurs. At this point, the hole seems to be a new point sound source. When the wave length of the sound wave is longer than that of the hole, the sound wave passes through the hole and propagates in all directions. The sound wave with higher frequency has strong directivity and propagates forward from the hole. Therefore, when there is a sound source indoors, the sound wave will encounter objects such as walls and furniture, resulting in reflection and diffraction, and the sound wave will also spread to the outside through the cracks in doors and windows.