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Five Problems That Home Theater Acoustic Design Needs to Solve

Author:Leeyin Acoustics  Date:2018-05-13  From:  Hits:24

The acoustic design of the home cinema is not easy as acoustic treatment on walls and floors. How much sound is absorbed and the absorption of each band have strict rules. This requires measurement and calculation of the original room, combining laboratory data and actual data of various sound-absorbing materials for reasonable selection and matching, at the same time designing the right amount and location, then you can achieve better room acoustic characteristics. The acoustic indicators of the standard home theater acoustics solution are as follows:


•Reverberation time (t20) matches design expectations, and ensure that the overall sound field in the room meets the requirements.

•The difference of reverberation time from 125Hz to 8000Hz must not exceed 0.05s, and ensure that the room frequency response curve is straight.

•Below 500Hz and less than two standing wave points, and it must be ensured that the position is outside the 1.5s lateral to the front of the seat area and 2s behind, avoid obvious visible wave defects and interfere with audiovisual.

•No sound staining, tremor echo generation and other acoustic defects, avoid obvious acoustic defects interfere with audiovisual.

•Eliminate resonance to interfere with audiovisual, avoid obvious acoustic defects interfere with audiovisual.


1.Acoustic treatment

Talking about acoustics, we must mention the reverberation and reverberation time. Reverberation time is an important evaluation index of hall sound quality or indoor sound quality, reverberation time has a significant impact on human listening performance, so it is still one of the most important reference values to describe indoor sound quality so far. Reverberation time depends on the building structure of the room, top structure, wall material, decoration, etc, seating arrangements and other indoor fixtures can affect the reverberation time.



The 3D dimension of the room determines that there are harmonic wave  frequencies with 3 basic natural resonance frequencies and integral multiples of three fundamental natural resonant frequencies. These sound waves interfere with each other as they spread through the room, which generates complex combined resonant frequencies. Acoustically, the room can be considered as a resonator. A standing wave will be formed and produce resonance when the sound source frequency coincides with the natural resonance frequency (the normal frequency) determined by the three-dimensional dimension of the room, this is the phenomenon of acoustic resonance. The sound field uniformity, sound staining, and frequency irregularities in the audiovisual room are all related to acoustic resonance.



This resonance will add the color of the room acoustic resonance to the original signal, causing noise. The general performance is that the loudness of a certain frequency band or segments in the low-middle frequency is excessively strengthened. The acoustic treatment is to make reasonable arrangements and use proper sound-absorbing material without changing the original three-dimensional dimensions of the room, to increase the damping of interfaces such as walls, floors, and ceilings, reducing the intensity of a prominent frequency band resonance, flatten and widen the resonance peak, reduce the impact of harmful resonances on overall sound effects.


2. Sound absorption processing

Sound absorption is the phenomenon of energy loss after a sound wave hits the surface of a material. Sound absorption can reduce indoor sound pressure level. There will be different sound absorption coefficients at different frequencies. People use the sound absorption coefficient frequency characteristic curve to describe the sound absorption performance of the material at different frequencies. To reduce echoes generated by the sound waves hitting the wall surface, we can perform a variety of acoustic treatments on walls, such as diffused wall surface or sound absorption treatment. In general, sound absorption is more commonly used and effective. However, sound absorption materials with large sound absorption coefficients should be used. The end wall behind the main speaker set should carry out strong sound absorption treatment. The average sound absorption coefficient should not be less than 0.6 in the frequency range of 125-4000Hz and the sound absorption coefficient should not be less than 0.4 at 125Hz frequency.


According to ISO standards and national standards, the frequency range of the sound absorption coefficient in the sound absorption test report is 100-5 KHz. The value obtained by averaging the sound absorption coefficient of 100-5 KHz is the average sound absorption coefficient, the average sound absorption coefficient reflects the overall sound absorption performance of the material. Noise reduction coefficient NRC is often used in engineering to roughly evaluate the sound absorption performance in the language frequency range. This value is the arithmetic mean of the sound absorption coefficient of the material at four frequencies of 250Hz, 500Hz, 1000Hz, and 2000Hz, rounding to 0.05. Materials whose NRC less than 0.2 are considered as reflective materials, and materials with an NRC greater than 0.4 are considered sound-absorbing materials. When it is necessary to absorb a large amount of sound energy to reduce indoor reverberation and noise, it is often recommended to use high sound absorption materials. Centrifugal glass wool is a high NRC sound-absorbing material, The NRC of a centrifugal glass wool with 24cm/m3 of 5cm thickness can reach 0.90.


In fact, there are many kinds of acoustic panels. Distinguished from the appearance and material, there are wooden acoustic panels, fabric acoustic panels, wood wool acoustic panels, strip-shaped acoustic panels, diffusion-type acoustic panels, dry-hanging decorative panels, three-dimensional wave boards and other types.



3.Diffusion processing

The so-called diffusion is a phenomenon in which sound is reflected, refracted, and diffracted irregularly in many directions, diffusion makes the indoor sound field even. Sound diffuser can improve indoor sound field distribution, reduce the sound level difference between the front and rear rows, adjust the balance of sharpness and fullness, eliminate echo and suppress acoustic feedback. Generally the purpose that we use sound diffusers is hoped that the deficiency of a certain sound band in the room can be supplemented by using a specific material diffuser to reach a state of balance. When designing and configuring sound diffusers, it should be noted that:


• The diffuser cannot be a sound absorber at the same time.

• The material of the diffuser should be as large as possible and have a certain stiffness, such as concrete, plastered masonry, marble, granite, etc.

• If you use wood, then you should use solid hardwood. For example, teak, oak, rosewood or paste multi-layer combination board on hardwood surface. Avoid using hollow plates made of thin plates such as plywood, pentane, etc, because this will result in strong absorption of low frequencies.

•Avoid casting diffusers with plaster, although the production cost is low, it produces metallic sound staining, which is not good for sound.

•The diffuser must have enough diffuser geometry to be comparable to the acoustic wavelength, so that it will have a good diffusion effect.

 

4. Standing wave processing

What is Standing Wave? The scientific explanation is wave formed by two waves whose frequency and amplitude are the same, the vibration direction is the same, and opposite spreading directions. Waves continue to advance as they propagate through the medium, so they are called traveling waves. After the above two wave stacks, the waveform does not advance, so it is called the standing wave. In a regular room, if you do not do any acoustic design, then when the speakers in the room are playing, the sounds we hear include harmonic sounds formed due to standing waves in addition to the horn's direct sound and the wall's reflection. In the past we constantly stressed that the low frequency standing wave in the small room determines the overall tone of the low frequency. Only the effect of low frequency standing wave is reduced, the low frequency effect can be improved. So eliminating the standing wave phenomenon in the room is mainly for the low-frequency part.


5. Shading and soundproofing treatment

Shading is one of the more important parts of home theater systems, but this has a very close relationship with the properties of the room. If there is no window in the room, then we can ignore this part. If there is a window, then we should not only consider adding shading materials, but also add noise insulation or other acoustic material around these windows. The second thing that needs to be said is that the proportion of soundproofing of home theater doors in the overall space is also very important, which maintain good sound pressure indoor and not exposed, and it does not affect daily life.


Acoustic design and processing is a systematic project, you can have a good listening environment only by combining the above points.


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