# The Sound Line of An Indoor Recording

- Oct 08, 2018-

Geometric acoustics is a science that analyzes indoor acoustic problems using the "sound line" perspective. The so-called "sound line" is actually an imaginary curve. It is emitted from the sound source and represents the direction of propagation of sound energy, regardless of the wave nature of the sound. It is used to study the indoor sound field, and mainly to understand the distribution of reflected sound after the sound waves are reflected by the reflecting surfaces in the closed space. Its theoretical basis is the Huygens principle. It is known from the basic theory of acoustics that the reflection of sound waves is the same as that of light waves, and they follow the same law of reflection. That is to say, similar to the assumption of geometric optics, the sound propagation path can be represented by a "sound line" similar to light, and the incident sound line, the reflected sound line and the normal line appear in the same plane, and the incident angle is equal to the reflection angle. . It must be noted that this is only true if the wavelength of the acoustic wave is much smaller than the linearity of the reflecting surface.

Generally speaking, when the reflection sound distribution in the room is studied by geometric graphic method, the longitudinal and cross-section of the closed space (room) can be used to understand the reflection of sound waves through the boundary surfaces (wall, ceiling and ground). In fact, after multiple reflections (usually two or three times) of sound waves, the distribution of reflected sounds is already very complicated and turbulent, and the practical significance is not significant, because it is almost close to random distribution at this time; The concept of "mirror" reflection is used to greatly simplify the drawing process. Now let's take the point source as an example. At this time, the sound wave radiated by the sound source can be approximated as a spherical wave, and thus can be represented by a bundle of sound rays. This kind of sound beam is reflected on a solid smooth and flat surface as if it were from a point behind the reflecting surface. The vertical distance from this point to the reflecting surface is equal to the distance from the reflecting surface to the sound source. Therefore, for a reflective surface, this can be thought of as a mirror image, and all reflections are emitted as if they were from a virtual source.

Through the geometrical graphical study of indoor sound waves, it is possible to find out the possible distribution of direct sounds and previous reflections. Thereby, the reflecting surface can also be modified to control these reflected sounds. The geometric acoustic method is simple and the image is intuitive, which is a common method of practical work. However, it is subject to many limitations and does not reflect the various characteristics of the sound field, so it is usually only used as an aid.