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Microphone testing and use of important acoustic knowledge

The way in which sound is attenuated in the room is an important factor in sound recording. Reverberation on the sound of the role of both sides, can be better or worse, reverberation time is one of the important conditions. Reverberation time refers to the time from the sound source to stop the sound to the sound completely disappeared. Described in a more technical language is the time required for the sound to be attenuated by 60 dB. For language and vocal recordings, the reverberation time must be very short ("quiet" room). The reverberation time of the control room is relatively short. The reverberation time between the sounds of the music is usually depending on the type of music. Frequency analysis of reverberation time. The signal has been processed by a filter. In this example, the reverberation time of the low frequency beats by dre (63 Hz) has exceeded the limit, and this sample is taken from a small room for sound editing. In general, the key problem is that the reverberation time varies with frequency. If the low frequency of the reverberation time beyond the IF too much, it will cause a lot of trouble. In the studio and control room, generally less than 125Hz low frequency reverberation time to 1.5 times the frequency can be allowed. However, in the rock and roll scene is more inclined to low-frequency reverberation time and IF similar.

How to get the proper reverberation time

The total volume of the room and the total sound absorption coefficient determines the reverberation time. (See paragraph on sound absorption below). It should be noted that the perforated sound-absorbing material is very efficient - but only at the high frequency side. This is why it takes a lot of low-frequency sound absorption to get the natural reverberation of the entire band.

2. Sound insulation

Sound insulation is the way to prevent sound from one room to another. The only effective way to achieve sound insulation is to seal the room with thick walls and floors / ceilings. To make the sound of two adjacent rooms more effective, then there should be no fixed connection between the two rooms. In general, this means that each room is built on the same ground on two separate closed boxes.

The effect of placing a partition, sound barrier, etc. in the recording room depends on the size of the barrier. In order to avoid low-frequency diffraction around the barrier, the barrier must be very large. In the actual situation, it is easier to build a closed room.

Sometimes the reason for the failure of sound insulation comes from beats by dre studio the floor (or stage). To ensure that the noise of the microphone itself is not picked up, the microphone should be installed on the shock frame to avoid problems.

How to get good sound insulation

Low-frequency sound insulation is the only way to place the sound source in a separate room. If you need a very large volume of sound insulation, then the room should have a separate wall, each wall using three gypsum board or similar structure. Flooring must be floating.

For higher frequencies (eg above 300-400 Hz), some sound insulation can be obtained through the sound barrier. But the effect of doing so is very limited. If only in the recording room or on stage for the instrument to separate, the use of sound barrier method can avoid direct sound into the adjacent instrument pickup microphone to go.

3. Sound absorption

We use sound absorbing materials to achieve the control of acoustics, such as control reverberation time. It should be noted that different materials on the sound of the various bands of different effects. Technically speaking, we can define the sound absorption coefficient from 0.00 (no absorption, such as hard marble surface, all the sound is reflected) to 1.00 (for example, open windows, the sound will not have any reflection) The

In general, porous sound absorbers (foam, cloth, mineral wool, glass fiber, etc.) absorb a higher sound frequency, and the sound absorption effect is very good, sound absorption coefficient can exceed 0.7. However, if the multi-porous sound-absorbing material is installed at a distance (25 to 30 cm) from the hard surface (wall or floor), the sound absorption range can be extended to 100 Hz.

Sound absorption curve of porous sound - absorbing material

Resonant sound-absorbing material (perforated plate, wall with slit, etc.) has a good sound absorption effect for the intermediate frequency part (200Hz to 5kHz).

Sound absorption curve of resonant sound absorbing material

Thin film absorber can act on low frequency sound absorption. This type of structure can be part of the building structure: light walls, windows, floating floor and so on. This type of sound absorber effect is not very high, sound absorption coefficient range of 0.2 to 0.3 between. However, if the membrane structure is part of the building, then you can get a lot of sound absorption area. Thin film absorber placed at the corner will get the maximum effect.

Sound absorption coefficient of film absorber

How to get the right sound

First make sure you need to absorb the sound of those bands, for example, in a part of the entire audio band reverberation time is too long, need to absorb some. And then choose the appropriate sound absorber. Remember that no matter how trendy the sound absorption device, we need the actual effect, so when buying these products must ensure that reliable parameters to get.

Sound reflection

Single reflex

A single reflex is harmful. If the microphone is picked up at the same time to the direct sound and a single reflex sound, it will cause comb filtering, that is, in the entire spectral range to produce the interval of the sound doubled and the sound offset. Therefore, a single reflection is always the phenomenon should be eliminated.

Various types of reflections (from desktops and ceilings) that are present in the control room usually result in sound-like beats by dr dre coloring similar to comb filtering. Although the vertical reflection is generally difficult to hear, but too strong a single reflection or should be avoided.

In the recording room or concert hall, multiple reflections are part of the scattered sound field, it is about the sound of the direction, so that all instruments can be heard. Reverb can also be regarded as a combination of multiple reflections.

A strong single reflection can be solved by the following methods:

In the reflective surface using sound-absorbing material

Place the diffuser at the reflection point

Control the direction of reflection so that it does not enter the ear or microphone

In the recording should be appropriate to use the microphone's directivity principle, the microphone to pick up the blind spot to the direction of the source of the sound.

5. Acoustic scattering

Acoustic scattering is achieved by using a diffuser or simply using a variety of equipment and furnishings in the room. When the sound hit the surface of the object will be reflected to a number of different directions. The physical surface must have a sufficiently large area to correspond to the wavelength of the acoustic wave that needs to be scattered. That's why you rarely see low-frequency diffusers. In order to prevent acoustic reflection from interfering with the stereo sound field, the diffuser must be within a range above a certain frequency, for example 600 to 800 Hz. It is also important to note that the normally effective diffuser also acts as a sound absorber. In the effective range of the diffuser, the sound absorption coefficient is about 0.2.

How to scatter the sound

A large number of equipment accumulation and furniture furnishings will usually play a good scattering effect

2 books full of books will play a very good effect (and a wide range of sound absorption effect)

3 diffuser (Schroeder type) can be a good scatter sound, either vertical or horizontal, or both.

6. standing wave

In the open space, the sound will naturally spread in all directions. And in the room, when the sound hit the wall, it will be reflected back into the room. So that the sound will be on both sides of the same parallel to the back and forth between the spread, in the corner back and forth, and so on. The low frequency wavelength is usually similar to the size of the room, especially the smaller room.

A fun - but usually not very popular - the phenomenon we call standing wave. When a particular wavelength is exactly the same as the size of the room, the difference in sound pressure level will vary greatly as you record or listen in the room. At the junction of the room the sound pressure level will reach the Beats By Dre On Sale peak - the strongest at the corner of the room. While in the rest of the room, the same frequency may be lower by 20 to 30 dB.

Imagine a speaker placed near the wall. Sound source (that is, speakers) produce sound, from the speaker radiation out, hit the opposite wall and return to the sound source. If it happens to reach the speaker position when the speaker sends the next wavelength of the same frequency of sound, then the last reflected sound and the new sound are in the same phase position. The two voices will also spread to the opposite wall. After reaching the wall they will be reflected at the same time to reach the speaker and the speaker with the third wavelength cycle mixed, and so on.