Reflection and reverberation time in music acoustics

1. Surface sound absorption

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 energy; another small part will penetrate the obstacle. The relative share of these three parts depends on factors such as the smoothness of the surface of the obstacle, the specific gravity of the obstacle material, and the shape and thickness of the obstacle. The sound energy reflection coefficient of a smooth and hard surface is relatively large, generally above 90%, and the most common way to reduce sound wave reflection is to increase the absorption and transmission of sound energy.

2. Direct sound and reflected sound

The music from the stage reaches the listener's ears in five ways:

First, the direct sound D, which is directly transmitted to the ears of the listener by the musical sound source in the form of approximately spherical waves. At this time, the sound energy density, that is, the sound intensity, is roughly inversely proportional to the square of the distance. Since the listener's eyes are basically on the line from the stage sound source to his ears, it can be said that any audience who can see the stage sound source You can also hear the direct sound from the source; vice versa. For some seats in some concert halls, the audience can't see the sound source of the stage when leaning on the seat, so you can't hear the direct sound. The closer to the stage, the greater the direct sound, and the farther away from the stage, the smaller the direct sound.

Second, the time delay and loudness (relative to direct sound) of the reflected sounds R1, R2, R1, R2 of the walls on both sides of the hall reaching the listener's ear are related to the hall span, the wall covering material and the surface shape. As far as the seats in the central axis of the main hall are concerned. Due to symmetry, R1 and R2 arrive at the same time and the loudness is also equal.

Third, the ceiling reflects sound R3. The sound on the stage is transmitted to the ceiling, and then reflected, reaching the listener's ear is R3. The time delay, loudness and frequency spectrum of R3 are related to the height of the hall, the angle of inclination of the additional ceiling (sometimes called "floating cloud") and its specific structure.

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

Fifth, multiple reflections. The sound emitted from the stage is reflected by the side walls of the hall, the ground, the stage cover, the ceiling, etc., and then the sound in the ears of the listener is transmitted. As a result of a reflection, the sound is absorbed (the frequency spectrum will change somewhat), so the more reflections you experience, the weaker the loudness and the more chaotic and isotropic directions. At the same time, due to the longer propagation path of multiple reflections, the time to reach the listener's ear is also more delayed. After so many reflections, the sound gradually dissolves into the reverberation and gradually disappears.

3. Initial time delay gap and reverb

From the time point of view, the direct sound and the time distribution of it and various reflected sounds are shown in the figure. Here, when the subject's seat is not in the central axis of the main hall, R1 and R2 are not equal. The time gap between them is called the initial time delay gap (usually measured in milliseconds).

It is one of the four objective standards for the sound quality of a concert hall, and it is directly related to an important item in subjective optimization evaluation-intimacy. If the gap is less than 20 milliseconds, R1 and direct sound will sound together to form a louder and better sound quality; if the size of the hall is large, so that the gap is greater than 70 milliseconds, it sounds R1 is echo. After R4, it belongs to multiple reflection sounds. The more and more classics, the more overlapping, the weaker. It gradually dissolves into the unique reverberation of the room sound, and the intensity of the reverberation generally decreases exponentially.

The definition of reverberation sound is: the continuous sound in the room after the direct sound disappears. To measure the reverberation time, the reverberation time T is usually defined as: the time (measured in seconds) required for the reverberation sound and the sound pressure level to drop by 60 dB. Because sound absorption is usually frequency dependent. Therefore, the reverberation time is generally related to the frequency. So it is usually divided into low frequency reverb (take frequency 67, 125, 250 Hz), intermediate frequency reverb (take frequency 500 Hz or 500-1000 Hz), high frequency reverb (frequency ≥ 2000 Hz), reverb time is music Another of the four objective criteria for hall sound quality. For the language hall, due to the requirement of clear language and short reverberation time, usually T500- (0.5-1.2 seconds); but for the symphony concert hall, due to the requirement for sound fullness (see the next section) Longer requirements: T500- (0.5-2.2 seconds). The ss reverberation time T500 can be determined by the following Ison formula.

Reverberation time

(7-1) = where V is the volume of the hall (m3), the average absorption coefficient is the absorption coefficient of the first surface of the N kinds of surfaces in the hall, and S is the area of the first surface (m2), if you consider The sound absorption of the listener can be considered as equal to the absorption area of 0.4m2 (a = 1), and the corresponding seat is no longer included in the absorption area. The 4mV item considers air to high frequency sound

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