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Reflection and reverberation time in musical acoustics

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Reflection and reverberation time in musical acoustics

Technical Knowledge
2018/11/14 15:40
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When sound is projected onto a solid obstacle, most of the sound energy will be reflected by the surface of the obstacle



1. Surface acoustic 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 portion will be absorbed by the obstacle and eventually converted into heat; 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 acoustic energy reflection coefficient of a smooth hard surface is relatively large, generally above 90%, and the most common way to reduce acoustic reflection is to increase the absorption and transmission of sound energy.
There are two physical mechanisms here:
Resonance sound absorption and porous sound absorption, some soft and porous surfaces, better absorption performance.
2, direct sound and reflected sound
The music that is uploaded from the stage reaches the audience's ears in five ways:
First, the direct sound D is directly transmitted to the listener's ear by the sound source in the form of an approximate 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 eyes of the listener are basically on the line connecting the stage sound source to his ear, it can be said that the audience who can see the stage sound source can be said. You can also hear the direct sound from the sound source; vice versa. In some seats in the concert hall, the audience can't see the sound source of the stage by leaning on the seat, so that you can't hear the direct sound. The closer to the stage, the louder the direct sound, the farther away from the stage, the smaller the direct sound.
Second, the time delays and loudness (relative to direct sound) of the reflected sounds of the walls R1, R2, R1, and R2 on the sides of the hall are related to the hall span, the side wall cladding material, and the surface shape. For the seat of the central axis of the main hall. Due to the 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. R3's time delay, loudness and spectrum and hall height, the angle of inclination of the additional ceiling (sometimes referred to as the "floating cloud") and its specific construction.
Fourth, the stage cover reflects the sound R4. The sound on the stage is transmitted to the stage cover, and then reflected by the stage cover and then transmitted to the audience. The R4 is related to the shape and material of the stage cover.
Fifth, multiple reflections, the sound from the stage is reflected by the side walls, the ground, the stage cover, the ceiling, etc. of the hall, and the sound in the ears of the listeners is heard. As a result of a reflection, the sound is absorbed (the spectrum also varies somewhat), so the more the number of reflections experienced, the weaker the loudness and the more chaotic and tending isotropic. At the same time, since the propagation path of the multiple reflection sound is long, the time to reach the listener's ear is also delayed. With so many reflections, the sound gradually dissolves into the reverberation and gradually disappears.
3. Initial time delay gap and reverberation
In terms of time, the time distribution of direct sound and its various reflected sounds 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, direct sound and R1 The time gap between them is called the initial time delay gap (usually measured in milliseconds).
It is one of the four objective criteria for the sound quality of a concert hall and is directly related to an important item in the subjective preference evaluation - intimacy. If the gap is less than 20 milliseconds, it sounds like R1 and direct sound will together form a louder, better sound; if the hall is large, making this gap greater than 70 milliseconds, it sounds like R1. After R4, it has belonged to multiple reflections. As more and more, the more overlapping, the weaker. The reverberation characteristic of the indoor sound is gradually dissolved, and the intensity of the reverberation sound is generally exponentially decreased.
The definition of reverberation is: the continuous sound of the room after the direct sound disappears. To measure the time of the reverb, the reverberation time T is usually defined as the time (measured in seconds) required for the reverberation and sound pressure levels to drop by 60 decibels. Since sound absorption is usually related to frequency. Therefore, the reverberation time is generally related to frequency. So usually divided into low frequency reverberation (take frequency 67,125,250 Hz), intermediate frequency reverberation (take frequency 500 Hz or 500-1000 Hz), high frequency reverberation (frequency ≧2000 Hz or more), reverberation time is music The other of the four objective criteria for hall sound quality. For the language hall, because the language is required to be clear, the reverberation time is short, usually T500-(0.5-1.2 seconds); but for the symphony concert hall, because the sound is very full (see the next section), the reverberation time Longer requirements: T500-(0.5- 2.2 seconds). The ss reverberation time T500 can be determined by the following Yisong 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 type of surface in the N kinds of surfaces in the hall, and S is the area of ​​the first type of surface (m2), if considering The sound absorption of the audience can be considered that each person is equivalent to an absorption area of ​​0.4 m2 (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.