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As an audio-visual room, it is necessary to have a good acoustic characteristics, such as reverberation time, resonance mode, sound-staining state, sound field uniformity, and the like. These indicators are closely related to the three-dimensional size of the room and the material texture of the interface between the walls, the floor, the ceiling, and the position of the speakers.
First, the acoustic requirements of the audio-visual room
This can also be called a hardware requirement. A room has three dimensions of length, width and height and has a minimum resonant frequency in each direction. The actual minimum resonant frequency in the room is determined by the length of the room and its wavelength is equal to twice the length of the room. For a room of 6m in length, when the speed of sound is 344m / s (room temperature 20 ° C), the lowest resonant frequency in the room is about 29Hz, which is the lowest frequency that can produce effective sound in the room. Even if the audio equipment can emit sound below the lowest resonance frequency, since it cannot form half a wavelength in the room and does not satisfy the resonance condition, it cannot generate resonance, and there is not enough effective sound pressure, so it is not optimal. effect.
The three-dimensional size of the room determines the presence of three fundamental natural resonant frequencies and harmonics that are integer multiples of the frequency of the three fundamental natural resonant frequencies that interfere with each other as they propagate through the room, creating a complex combined resonant frequency. Acoustically, the room can be regarded as a resonator. When the frequency of the sound source is consistent with the natural resonant frequency (normal frequency) determined by the three-dimensional size of the room, a standing wave will be formed and resonance will occur, which is the phenomenon of acoustic resonance. Sound field uniformity, acoustic staining, and frequency irregularities in the audio-visual room are all related to acoustic resonance. This resonance will add the color of the room's acoustic resonance to the original signal, causing sound pollution. Generally speaking, the loudness of a certain frequency or a certain frequency in the middle and low frequency is excessively strengthened, and the "squeaking" causes the signal reproduction loudness imbalance in this frequency band, which will greatly affect the listening effect. As for the high frequency band, the resonance distribution is relatively uniform, and the sound and dyeing is small, which is not enough to affect the overall listening effect. Therefore, the focus should be considered on the influence of the middle and low frequency bands.
In order to avoid or attenuate this harmful acoustic dyeing, the resonance frequency is evenly distributed, and a prominent isolated segment or a certain frequency resonant mode is avoided, and one method is to reasonably change the three-dimensional size of the room. It is conceivable to change the three-dimensional dimensions by appropriate interior decoration, making partition walls, ceilings or even removing some of the non-bearing walls (this rational transformation also takes into account the structural safety factors of the building, such as the intention to demolish a certain wall, It is best to ask for advice from construction professionals). Simply put, this is consistent with the principle of selecting the internal three-dimensional size when designing the speaker, that is, the length, width, and height cannot be integer multiples or too close. The specific sound field resonance modes, calculation formulas and related data are not listed one by one. Please refer to the related acoustic books if necessary. Here is a reference to the IEC29-B home listening room standard proposed by the International Electrotechnical Commission (IEC) with reference to European home listening rooms. Welcome to China Home Theater Network!
The other method is more reluctant, that is, it can be reasonably arranged while keeping the original three-dimensional size of the room, and the appropriate sound absorbing material is used to increase the interface damping of the wall surface, the floor, etc., so that a certain frequency band of the resonance is resonated. The intensity is reduced, and the resonance peak is flattened and widened, thereby reducing the effect of harmful resonance on the overall sound effect. However, this method has limited effectiveness and is only suitable for cases where the harmful resonance intensity is not large. Because if you want to strengthen the sound absorbing material to reduce the strong harmful resonance, it will also shorten the reverberation time, which will also make the sense of hearing worse, making the playback sound become dead, lack of color and anger.
Second, the reverberation time of the audio-visual room
This is another important parameter of the audio-visual room. Reverberation is produced by the multiple reflection characteristics of the sound in the room, which keeps the sound pressure in the room for a while after the sound source stops sounding. The reverberation time refers to the time required for the sound source to stop sounding and the indoor sound pressure is attenuated by 60 dB, that is, one millionth of the time to the original sound pressure stability value. If the reverberation time of the audio-visual room is too long, the sound emitted from the front of the sound source has not disappeared, and the sound behind it appears, which makes it easy for the bass to roar, resulting in a decrease in the clarity of the signal concentrated in the middle and high frequency bands, such as language dialogue, and even the sense of orientation disappears ( Because the sound image localization is obtained by the middle and high frequency direct sound of the left and right main speakers, the reverberation sound generated by the reflection is from a plurality of orientations, directly interfering with the direct sound and affecting the correct positioning of the sound image). If the reverberation time is too short, it will make the sound thin, dry, dull, lacking color and anger. Only when the reverberation time is moderate, the overall sound effect will be full, vivid, contagious and expressive.
The optimal reverberation time is not strictly a fixed value, even if it is specific to a listening room. It is based on the subjective experience and experience of the individual, and the reverberation time required to appreciate different program types is different. If you want to enjoy the film and television program, the reverberation time requirement is a little shorter, the sound effect will be more vivid, the character dialogue is clear, and the sound image is positioned. Appreciating pure music programs requires a slightly longer reverberation time, which sounds "comfortable", and classical music programs require reverberation time that is slightly longer than pop music. This is because different types of programs have different requirements for various aspects of post-mixing production, and a moderate reverberation time is required for playback to maximize the sound recovery.
As a home audio-visual room, it is generally a compromise to deal with the reverberation time, and then partially adjust by pulling or closing the movable curtain or tapestry hanging on the wall. A large number of experiments and data show that the reverberation time of the home audio-visual room is generally about 0.4-0.6s (500Hz). The calculation of the reverberation time usually uses the following formula: t=0.16V/α, where t is the reverberation time in seconds; V is the volume of the audio-visual room, the unit is cubic meters; α is the total indoor sound absorption coefficient. Table 2 shows the sound absorption coefficients of commonly used sound absorbing materials.
For example, an audio-visual room is 6.2 m long, 4.4 m wide and 2.8 m high. The ceiling and the ground are flat plastering surfaces. A 4m×3m chemical fiber carpet is placed in the middle of the ground. A 6m×3m velvet curtain (1cm away from the wall) is hung on the walls on both sides, and the wall opposite the screen is attached. 4.4m × 2.8m chemical fiber carpet. Try to estimate the reverberation time of the audition room (at 500 Hz). It is found from Table 2 that the sound absorption coefficient α1 of the bare wall and the ground is 0.02, α2 is 0.44 when the curtain is suspended from the wall 1cm, and the sound absorption coefficient α3 of the carpet is 0.28. The total sound absorption coefficient is:
总 total = 1 × 4.4 × 2.8 × α1 + (2 × 6.2 × 4.4 - 4 × 3) × α1 + 2 × 6.2 × 2.8 × α 2 + 4 × 3
×α3+4.4×2.8×α3=23.184
The net volume of the audio-visual room is: V=6.2×4.4×2.8=76.384m3, then the reverberation time is: t=0.16V/α total=0.16×76.384/23.184=0.53s?
It can be said that the reverberation time of this audio-visual room is more appropriate. If you consider the sound absorption factors of the audience and furniture, the reverberation time will be slightly shorter.
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