How to design the acoustic treatment of small rooms and the sound configuration of home theaters, Jardine Audio Company tells you:
Methods/steps
A, audio-visual room acoustic requirements
This can also be called hardware requirements. A room has three dimensions: length, width and height, and each direction has a lowest resonance frequency. The actual lowest resonance frequency of a room is determined by the length of the room, and its wavelength is equal to twice the length of the room. For example, in a room with a length of 6m, when the speed of sound is 344m/s (the room temperature is 20℃), the lowest resonance frequency in the room is about 29Hz, which is also the lowest frequency that can produce effective sound in the room. Even if the audio equipment can emit sound below the lowest resonance frequency, it can't produce resonance, and there is no effective sound pressure, because it can't form half a wavelength indoors and can't meet the condition of * * * vibration, so it can't get the best effect.
The three-dimensional size of the room determines that there are three basic natural resonance frequencies and harmonics of integer multiples of the three basic natural resonance frequencies. These sound waves interfere with each other when propagating in the room, resulting in complex combined resonance frequencies. Acoustically, a room can be regarded as a buzzer. When the frequency of the sound source is consistent with the natural resonance frequency (normal frequency) determined by the three-dimensional size of the room, a standing wave will be formed, resulting in * * * vibration, which is the acoustic * * * vibration phenomenon. The uniformity of sound field, sound dyeing and frequency irregularity in audio-visual room are all related to sound vibration. This * * * vibration will add the color of room vibration to the original signal, resulting in sound coloring. Generally speaking, the loudness of a certain section or several sections in the middle and low frequencies is over-enhanced and "buzzes", resulting in an unbalanced signal playback loudness in this frequency band, which will greatly affect the listening effect in serious cases. As for the high frequency band, the resonance distribution is relatively uniform, and the sound pollution is small, which is not enough to affect the overall listening effect, so we should focus on the influence of resonance in the middle and low frequency bands.
In order to avoid or weaken this harmful sound staining, make the vibration frequency of * * * evenly distributed, and avoid the prominent isolated frequency resonance mode in a certain section or several sections, one method is to reasonably change the three-dimensional size of the room. We can consider changing the three-dimensional size through proper interior decoration, partition walls, suspended ceilings and even removing some non-bearing walls (this reasonable transformation should also consider the structural safety of the building, and it is best to seek the opinions of architectural professionals if you plan to remove a certain section of the wall). Simply put, this is in line with the principle of selecting internal three-dimensional dimensions when designing a sound box, that is, the length, width and height should not be integer times or too close. Specific acoustic resonance modes, calculation formulas and related data are not listed one by one. If necessary, please refer to books related to acoustics. Here, the IEC29-B standard for home listening rooms proposed by the International Electrotechnical Commission (IEC) with reference to European home listening rooms is provided for reference.
The other method is reluctant, that is, it can be arranged reasonably while keeping the original three-dimensional size of the room unchanged, and appropriate sound absorption materials are used to increase the interface damping of walls, floors and so on. , so as to reduce the intensity of prominent * * * vibration in a certain frequency band, and flatten and widen the peak value of * * * vibration, thus reducing the influence of harmful * * * vibration on the overall sound effect. However, the effectiveness of this method is limited, and it is only applicable to the case of low harmful vibration intensity. Because if the sound-absorbing materials are blindly strengthened in order to reduce the strong harmful vibration, the reverberation time will be greatly shortened, and the listening sense will be deteriorated, so that the playback effect is lifeless and lacks color and vitality.
Second, the reverberation time of audio-visual room
This is another important parameter of the audio-visual room. Reverberation is caused by the multiple reflections of sound in a room, which makes the sound pressure in the room continue to be maintained for a period of time after the sound source stops producing sound. Reverberation time refers to the time required for the indoor sound pressure to decay by 60dB from the time when the sound source stops sounding, that is, the time to decrease to one millionth of the original sound pressure stability value. If the reverberation time in the audio-visual room is too long, the sound in front of the sound source will appear before it disappears, which will easily make the bass roar, leading to the decline of signal clarity concentrated in the middle and high frequency bands, such as language dialogue, and even the disappearance of the sense of orientation (because the sound image positioning is obtained from the middle and high frequency direct sound of the left and right main speakers, the reverberation sound generated by reflection comes from many directions, which directly interferes with direct sound and affects the correct positioning of the sound image). If the reverberation time is too short, the sound will become thin, dry and dull, lacking color and vitality. Only if the reverberation time is moderate, the overall sound effect will be full, vivid, infectious and expressive.
Strictly speaking, the optimal reverberation time is not a fixed value, even for a listening room. Based on personal subjective feelings and experience, the reverberation time required to appreciate different types of programs is also different. If the reverberation time is required to be shorter when enjoying film and television programs, the sound effect will be more vivid, the dialogue between characters will be clear, and the sound and image positioning will be better. However, it takes a little longer to enjoy pure music programs, which makes you feel "more comfortable". Among them, classical music programs require reverberation time to be slightly longer than pop music. This is because different types of programs have different requirements in all aspects of post-mixing, and appropriate reverberation time is also needed in playback to maximize the sound effect.
As a family audio-visual room, reverberation time is generally handled in a compromise way, and then some adjustments are made by opening or closing movable curtains or tapestries hanging on the wall. A large number of experiments and data show that the reverberation time of family audio-visual room is generally around 0.4——0.6s (at 500Hz). The calculation of reverberation time usually adopts the following formula: t=0. 16V/α, where t is reverberation time in seconds; V is the audio-visual indoor volume, in cubic meters; α is the total indoor sound absorption coefficient. Table 2 gives the sound absorption coefficients of commonly used sound absorption materials.
For example, an audio-visual room is 6.2m long, 4.4m wide and 2.8m high. The ceiling and the ground are smooth plastered surfaces, with a 4m×3m chemical fiber carpet in the middle of the ground, a 6m×3m velvet curtain (away from the wall 1cm) on both sides of the wall, and a 4.4m×2.8m chemical fiber carpet on the wall opposite the screen. Try to estimate the reverberation time (500Hz) of the listening room. From Table 2, it is found that the sound absorption coefficient α 1 of exposed wall and ground is 0.02, α2 of curtain hanging from wall 1cm is 0.44, and α3 of carpet is 0.28. The total sound absorption coefficient is:
Total α =1× 4.4× 2.8× α1+(2× 6.2× 4.4-4× 3 )× α1+2× 6.2× 2.8× α 2+4× 3× α 3+4.4. α total = 0.16× 76.384/23.184 = 0.53s It can be said that the reverberation time of this audio-visual room is appropriate, and it will be slightly shorter if the sound absorption factors of the audience and furniture are considered again.
One of the purposes of setting up an audio-visual room is to better appreciate the real sound effects.
Third, the placement of speakers.
When you spend a lot of thought and money to decorate the audio-visual room according to the above requirements, if the speakers are not placed properly, then the result is only four words: "All previous efforts were in vain." One of the purposes of setting up an audio-visual room is to better appreciate the real sound effects, which has many similarities with Hi-Fi appreciation. Regarding the placement of speakers, I only briefly mention some more principled precautions here.
Firstly, the acoustic characteristics of the left and right sides of the audio-visual room are required to be as symmetrical as possible (not necessarily visually symmetrical), that is, the acoustic reflection or absorption of the left and right sides of the room is as equal as possible. Generally, it is required that the distance between the two main speakers should be greater than 2.5m or the included angle between the main speakers and the audience should be at least 30 (the best angle for pure music appreciation is 60, while the home theater generally requires about 45 due to the matching of audio and video), so as to obtain a better stereo sound field, but the distance or angle should not be too large, otherwise the positioning of the image and the audio and video may be inconsistent. For example, the sound and image on the left or right side of the screen are located far away from the screen, which is beyond the approximate reasonable range, which will cause disharmony between auditory effect and vision.
The acoustic characteristics of the audio-visual room are directly related to the position of the speaker in the room to some extent. The position of the main speaker in the room is mainly determined by the listening experience. Put the main speaker in the corner, away from the wall or the side wall for audition, and choose a position where you can get more uniform sound field vibration. In addition, tilt the main speaker back a few angles, try to listen to the main speaker a little higher, and see if the effect will be better (sometimes this change will be more obvious, and you may get quite good results).
In view of the low directivity requirements of surround speakers, the specific placement requirements are not very strict, and they are generally placed on the left and right sides behind the audience, but the audience should not be too nearsighted, and the height of the speakers should be appropriately higher than the human ear. The spacing between two surround speakers is usually slightly smaller than the spacing between the main speakers. When the surround speakers are in place, it is generally necessary to adjust the surround delay of the AV amplifier, so that the sound effects of the front and rear speakers can be integrated and perfectly integrated, and satisfactory overall effects can be obtained.
The position of the center speaker is relatively fixed and should be on the same vertical line with the center of the screen. It is best to keep the tweeters of the main speaker and the center speaker at the same level with the audience's ears. If the conditions are not met, the height difference between the center tweeter and the tweeter of the main speaker should not exceed 30cm. If the difference is too large, the sound and image height of the main speaker and the center speaker will be obviously inconsistent, which will cause the sound and image of the moving object on the screen to become messy and unnatural, and the audio-visual effect will be greatly reduced. In addition, it is worth noting that when selecting the center speaker, the tweeter should use the same unit as the main speaker as much as possible. Only in this way can the center speaker and the main speaker be well matched in frequency response curve, directivity and phase, which also meets the basic technical requirements of Dolby AC-3 and THX.
Subwoofer is commonly known as "subwoofer". Because its playing frequency is generally below 120- 150 Hz, special attention should be paid to the placement of subwoofers in the audio-visual room. It must be placed in multiple indoor locations for audition. Choosing a suitable location can minimize indoor sound staining and distortion. Generally speaking, the subwoofer should not be placed in the corner, near the wall or the center of the audio-visual room, because these positions are easy to cause strong vibration.