1. Sound absorption of surface

When sound is projected onto a solid obstacle, most of the sound energy will be reflected by the surface of the obstacle; a small part will be absorbed by the obstacle and eventually converted into heat; the other small part will penetrate the obstacle. The relative share of these three parts depends on the smoothness of the surface of the obstacle, the proportion of the obstacle material and the shape and thickness of the obstacle. The sound energy reflection coefficient of smooth and hard surface is relatively large, generally more than 90%. The most common way to reduce the sound energy reflection is to increase the absorption and transmission of sound energy.

There are two physical mechanisms

Some soft and porous surfaces have good absorption properties.

2、Direct sound and reflected sound

The music from the stage reaches the audience in five ways:

The first is direct sound D, which is transmitted directly to the audience by the sound source in the form of spherical wave. At this time, the sound energy density, that is, the sound intensity, is roughly inversely proportional to the square of the distance. Since the audience's eyes are basically on the line from the stage sound source to his ears, it can be said that the audience who can see the stage sound source can also hear the direct sound from the sound source, and vice versa. In some concert halls, the audience can't see the sound source of the stage when they lean against their seats. In this way, they can't hear the direct sound. The closer they are to the stage, the greater the direct sound, and the farther they are away from the stage, the smaller the direct sound.

Second, the time delay and loudness (relative to the direct sound) of the reflected sound R1, R2, R1, R2 from the walls on both sides of the hall are related to the hall span, side wall facing material and surface shape. As far as seats on the central axis of the main hall are concerned. Because of symmetry, R1 and R2 arrive at the same time and the loudness is equal.

Third, the reflected sound from the ceiling is R3. The sound from the stage is transmitted to the ceiling and then reflected to the audience's ears, which is R3. The time delay, loudness and spectrum of R3 are related to the height of the hall, the inclination angle of the additional ceiling (sometimes called "floating cloud") and its specific structure.

Fourth, the sound reflected by the stage hood is R4. The sound on the stage is transmitted to the stage hood, and then reflected by the stage hood and then transmitted to the audience. R4 is related to the shape and material of the stage cover.

Fifthly, the sound is reflected many times. The sound from the stage is reflected many times through the side wall, floor, stage cover and ceiling of the hall, and then it is transmitted to the audience. Because the sound will be absorbed after one reflection (the frequency spectrum will also change somewhat), the more the reflection times, the weaker the loudness and the more disordered the direction. At the same time, due to the longer propagation path of multiple reflections, the time to reach the audience's ears is also more delayed. After so many reflections, the sound gradually melts into the reverberation and gradually disappears.

3.Initial time delay gap and reverberation

In terms of time, the time distribution of direct sound and all kinds of reflected sound is shown in the figure. Here, when the listener's seat is not in the central axis of the main hall, R1 and R2 are not equal. In the figure, the time gap between direct sound and R1 is called the initial time delay gap (usually in millisecond).

It is one of the four objective criteria for the sound quality of concert hall, and it is directly related to an important item in the subjective optimization evaluation - intimacy. If the gap is less than 20 ms, R1 and direct sound will form a sound with larger loudness and better sound quality; if the size of the hall is large and the gap is greater than 70 ms, R1 sounds like echo. After R4, there are more and more reflections, the more overlapped and the weaker. The intensity of reverberation decreases exponentially.

Reverberation is defined as the continuous sound in the room after the direct sound disappears. In order to measure the reverberation time, the reverberation time t is usually defined as the time (in seconds) required for reverberation and sound pressure level to drop by 60 dB. Because sound absorption is usually related to frequency. Therefore, reverberation time is generally related to frequency. Therefore, it is usually divided into low frequency reverberation (take frequency 67125250 Hz), medium frequency reverberation (take frequency 500 Hz or 500-1000 Hz) and high frequency reverberation (frequency ≥ 2000 Hz). Reverberation time is another of the four objective standards of concert hall sound quality. For the language hall, it requires clear language and short reverberation time, usually t500 - (0.5-1.2 seconds); but for the symphony hall, it requires a long reverberation time, t500 - (0.5-2.2 seconds), because it requires sound fullness (see the next section). SS reverberation time t500 can be determined by the following Eason formula.

reverberation time

(7-1) = where V is the volume of the hall (M3), is the average absorption coefficient, is the absorption coefficient of the i-th surface among the N surfaces in the hall, and S is the area of the i-th surface (M2). If the sound absorption of the audience is considered, it can be considered that each person is equivalent to an absorption area of 0.4m2 (a = 1), and the corresponding seat is no longer included in the absorption area. The 4MV term considers the absorption of high frequency sound by air.