Standing wave and low frequency problem in small room acoustic processing

The standing wave is caused by the reflection of the wall and is reflected back when the sound is transmitted to the wall through the air. The sound of the reflected sound of some frequencies is exactly the same as the source sound, then the sound of this frequency will be strengthened, so the sound of this frequency will become larger, and the sound of some frequencies will be exactly the same as the source sound. It is the opposite direction of vibration, so the sound at this frequency is weakened.

Almost any room has standing waves, but the degree is light and heavy. The walls are parallel to each other, the ceiling and floor are parallel to each other, and there are no large obstacle rooms in the room, usually with severe standing waves. In different locations in the room, there are different standing waves. Let's take a picture of a measured picture. In a small, empty, long-shaped room, use a speaker to play a sweeping signal, at the position of the sound engineer. The measurement microphone is recorded in the corner of the back of the room to measure the standing wave of the same.

The size of the house is different, and the situation of standing waves is different. In theory, the standing wave phenomenon in a large room is flatter than in a small room, as it is in practice. The figure below shows the standing wave condition (roughly schematic) caused by the sound reflection of two different sized rooms. The above is the sound wave interference caused by the big room, the following is caused by the small room. Relatively speaking, the sound interference of a large room is much smoother. This is one of the reasons why the acoustics of a large studio is better than a small studio.

Foreign acoustic experts recommend that each room have at least 70 cubic meters to ensure high-quality sound reproduction. The length, width and height of such a room are almost 4*5.5*3 meters.

The ratio of length to width of a right-angled house will also bring about different standing waves. The worst thing is that the length, width, and height are the same or integer multiples, so that the sound interferes in all three directions, causing a more intense standing wave. The best case is that the length, width and height are different, so that the standing waves of the sound in three directions cancel each other out. The resonance curve in two different spaces is given in the following figure. The volume of both spaces is the same, but the ratio of length, width and height is ideal. The following is the case where the ratio is not ideal. of.

Thanks to the acoustic experts who have calculated the best length, width and height ratio for right angle houses for us. as follows:

The low-frequency wavelength is very long, and the high-frequency wavelength is short. According to the physics knowledge, the standing wave problem mainly occurs in the frequency region. The higher the frequency, the standing wave is getting lighter. (This is lighter than the standing wave of the small room in the large room mentioned above, which is a reason). It can be seen clearly from this picture.

Usually, the standing wave in the low frequency area of ​​our studio is very serious. It is serious enough to completely affect our judgment on music. You think that a bass sound is too heavy. In fact, it is not heavy. It is a standing wave. You mistakenly think that this tone is very heavy, and you think that a certain sound is too light, but in fact it is not light, because the reflected sound and the direct sound cancel each other out, making you mistakenly think it is very light.

For most of us, there are two ways to solve the standing wave fundamentally:

1. Changing the direction of sound reflection can be achieved by changing the shape of the wall inside the house or by adding a reflector.

2. Eliminate the reflection sound. As I said before, the standing wave problem mainly occurs in the low frequency region. As long as the low frequency standing wave is solved, the whole problem is basically solved. Therefore, the method of solving the standing wave actually becomes:

Someone said, I put a plate for dinner on the wall and can't change the direction of the sound? wrong! This only changes the reflection of high frequency sounds, but does not change the reflection of low frequency sounds. The low-frequency wavelength is very long, from one to two meters to a few meters, or even a dozen meters, low-frequency sound will easily bypass this plate. According to the results of previous physics research, we know that only large obstacles can affect the direction of the low frequency, so we can't take small things to try to influence the low-frequency propagation route, but we must use big things. What kind of thing is the "big" thing?

Directly making the wall and ceiling into a special shape is the same as the professional studio. Let's compare the professional studio and the ordinary room. Below are the top views of two different rooms. The left picture shows the sound reflection in the professional studio control room. No matter how the reflection is, there will be no reflection (the main reflection 0 can reach the position of the sound engineer; the right picture is the ordinary room) In the case of sound, the sound can be directly reflected to the position of the sound engineer in many ways.

Therefore, in a professional studio, a single reflection (primary reflection) does not reach the position of the sound engineer at all, thus greatly avoiding the standing wave phenomenon. In an ordinary room, there are many reflections (primary reflections) at various angles that can reach the position of the sound engineer, with severe standing waves.

Below is a plan for a typical large studio. It can be seen that the control room and the studio are all irregularly shaped.

The ceiling is also irregular, and usually the front end of the control room is designed to prevent sound from forming standing waves between the heavens and the earth.

Close up of a ceiling

You can also use the giant baffle, all the walls in this studio are all skewed and the ceiling is also oblique

All the walls in the studio are not parallel, a circular arc-shaped wall in the Mastering studio to prevent standing waves. Use irregular angles and shapes to avoid standing waves.

The following studios, the huge baffles on the ceiling, can effectively eliminate standing waves.

Low frequency reversal will only bring standing wave problems, but also low frequency reverberations that are difficult to eradicate. Many friends may find that the low frequency of your room is so turbid and heavy, this is the low frequency reverberation. Low frequency reverberation is especially serious in empty small rooms.

Some people have said that this is easy, using a sound-absorbing sponge or cotton wool. Ok, then let's experiment to see how these sound absorbing materials absorb low frequencies. The following figure shows the absorption of sounds of different frequencies for rock wool of different thickness (the sound absorption capacity is N times stronger than that of the sponge). We can see that no matter how good the medium-high frequency absorption capacity of these fiber cottons, the low frequency below 150Hz There is almost no sound absorbing ability.

This is also the rock wool with the best sound absorption. What about sponges, egg boxes and other items? Their absorption capacity for low frequencies below 150 Hz is almost zero.

The low-frequency wavelength is very long. I have already emphasized it in the front. Imagine: two or three meters, six or seven meters long sound waves "flying in and out" in your house, no one can stop. Therefore, things that can absorb low frequencies must also be "big" things, which require a lot of space.

The size of the board determines how much frequency it can absorb. The larger the frequency, the lower the frequency that can be absorbed. Generally, a 1800*500mm board can absorb lower frequencies, and a 1200*300mm board can absorb higher frequencies.

In some studios, there are ways to do this:

These hidden low-frequency sound absorbing devices are very effective at absorbing low frequencies.

The second is to arrange this special low-frequency sound-absorbing panel device, the outermost layer is the composite main board, then the fiber cotton and air layer, and finally the surrounding is sealed with the main board, the whole device is made into a closed cavity, the air layer The thickness and density of the board determine how much frequency the sound can absorb, and the thicker it absorbs the lower the frequency.

The following are widescreen sound absorbers designed by Beijing Jingmeikang Acoustics Technology Co., Ltd., all kinds of specifications, the thickness of the air layer determines how low they can absorb. The fiber cotton inside is a hard high-density fiberboard, which is not ordinary loose cotton.

This studio is equipped with a wide range of widescreen sound absorbing bodies. You can see that these sound absorbers are very large and very different from those that can only absorb medium and high frequencies.

After the low frequency sound absorption, the wave phenomenon is significantly improved. The red line is the standing wave before the low frequency is absorbed, and the blue line is the case after the low frequency is absorbed.

This is a small sound-absorbing panel that was not used to absorb low frequencies, but if it is placed diagonally in the corner or a little distance from the wall, its absorption of low frequencies is suddenly enhanced.

Look at the different sound absorption effects of different placement methods. Red is to place the sound absorption board diagonally at the corner. Blue is attached directly to the wall.

The installation is equally simple and can be hung directly on the nail or attached to the mounting accessory that comes with it. It can save a lot of construction costs compared to conventional products. As long as you have the usual tools, you can complete the installation yourself. No professional construction team, no messy dusty construction site, no need to worry about neighbors complaining about construction noise. You will quietly complete your great transformation.

Some of these huge widescreen sound absorbers can effectively absorb low frequencies. Note that it is not that its surface absorbs low frequencies, but that the entire device as a whole absorbs low frequencies.

Large cavities can absorb low frequencies. As seen above, the so-called low frequency sound absorbing panel has an air layer inside, that is to say it is a small cavity. Then the large cavity is stronger, and the sound absorption of the low frequency is better. You may notice that there is no special bass absorption device in the professional studio, but there is no low frequency reflection problem. This is precisely because there are huge cavities in the walls of professional recording studios.

Below is a control room plan for a typical medium-sized studio. It can be seen that there are many huge cavities inside. One of the functions of these cavities is to absorb low frequencies.

Therefore, professional studios do not need special frequency low-frequency sound absorbing devices to solve the bass problem.

This is also a stand-alone corner-mounted bass trap designed by Jing Mei Kang Acoustics Technology Co., Ltd. The three different specifications determine its extraordinary low-frequency absorption capability, with a lower absorption limit of 40HZ or even lower.

When sound waves enter a closed space, there are two distinct forces at work. For a listening room, the intermediate frequency and high frequency, their wavelength is much smaller than the room size, and will not cause ROOM MODES. These mid-high frequency sound waves illuminate the room like a beam of light, or rebound like a billiard on the countertop. The high-five in the listening room will excite the mid-high frequencies and behave as an audible, loud, clapping sound with the applause. The low frequency wavelength is much larger than the size of the room, so it is very different. Superimposing and canceling the sound pressure causes standing waves. These frequency peaks appear as very high sound pressures that converge at the same frequency, causing an audible anomalous frequency response. Excessive low frequency delay masks the instantaneous sound pressure and, more importantly, masks the low frequency details.

No room can get rid of Room Mode, but using low frequency traps can improve low frequency response and eliminate transient bursts and delays. Although each room's Room Mode has a unique pressure zone, all Room Modes will appear in the corner. Low-frequency sound wells upgrade high-quality sound systems with their superior performance.