It is difficult for many beginners to master these knowledge. Next, the crown art sound-absorbing board explains some professional terms about acoustic processing in detail for you to help you master more knowledge of indoor acoustic design.

Decibel decibel

Decibel is one tenth of a bell and is usually used to express volume. Decibels don't express everything, it's just a ratio of two energy levels. Because we rely on our ears to sense volume, these ratios, which follow a logarithmic curve, are expressed in decibels, making many things much easier.

Here are some decibel numbers worth remembering: the minimum volume unit that the human ear can feel the change under normal circumstances is 1 decibel; when the speaker power is doubled, the result will be a significant increase of 3 decibels, and a doubling of the volume is a change of 6 decibels; if we want to double the volume, we need to increase the power of the amplifier to four times of the original.

Frequency

The source of sound is vibration, and vibration has frequency (symbol f), i.e. the number of vibrations per second. The unit is Hertz (Hz). Not all frequencies of sound can be heard by human ear. Only when the vibration frequency is 20Hz (say 16Hz) - 20000 Hz, can human ear have sound sense.

Infrasound is below 20Hz, ultrasound is above 20000hz, sound below 20Hz and above 20000hz will not be felt by human ear, and the most sensitive frequency of human ear is 100-3150hz. In architectural acoustics, the sound of 200-300hz or below is generally called low-frequency sound, the sound of 500-1000hz is called medium frequency sound, and the sound of 2000-4000hz or above is called high-frequency sound.

Absorption

In acoustics, sound absorption refers to that sound is not reflected, and sound waves are absorbed when they encounter soft materials. The absorption capacity of various materials is graded according to the absorption coefficient, which is determined by the relative sound energy absorbed by the material when the sound hits the surface.

Absorption coefficient

The sound absorption coefficient measures the relative sound energy absorbed by the material when the sound hits the surface of the object. It is usually a value from 0 to 1. The number multiplied by the surface area of the object is the percentage of the sound absorbed by the surface of the object. The unit of this percentage is sabins, which originated from the name of Wallace Sabine, a Harvard professor and acoustician.

A absorption coefficient of 1 means that the sound is completely absorbed after hitting the object surface, without any reflection. If the absorption coefficient of 0 means that the sound is completely reflected, without any absorption. Of course, the phenomenon of coefficient 0 is obviously impossible, and the case of coefficient 1 rarely occurs. In addition, because different materials have different absorption characteristics at different frequencies, the absorption coefficient will change with the change of frequency. In fact, many building materials have been measured for reference.

Anechoic silencing

Literally, silencing means no echo, no audio reflection. In essence, the closest thing to this situation is the open outdoors, but even in the outdoors, there are still sounds reflected from the ground and other objects. It's impossible to create an environment with absolutely no echo, because there's no material that can absorb the sound completely.

Compared with high-frequency sound, it is possible to create a near anechoic environment, but it is very difficult for low-frequency sound (the absorption depends on the wavelength, for example, a 100 Hz sound wave is about 10 feet long, so the thickness of the sound-absorbing material must be at least half of the wavelength to work, so create a large enough space and design enough sound-absorbing materials Absorbing low-frequency sound is obviously quite impractical. )

Damping radiation damping

In physics, this refers to the reduction of the amplitude of an electronic wave or a mechanical wave. In interior acoustic design, it can refer to induced vibration or some acoustic reflection. For example, installing some sound-absorbing materials in a wall or inside a loudspeaker can effectively insulate the sound or reduce the reflection.

Bass trap

Each closed space will have different resonance frequency with its size, so the size of the space directly affects the generation or disappearance of specific frequencies in different corners. The low-frequency sound-absorbing board is a low-frequency sound absorption equipment used to reduce the influence of the indoor retained sound wave, which is generally installed along the wall or in the corner.

The sound absorption characteristics of the low-frequency sound absorption board effectively prevent the low-frequency sound from interfering with each other in the room, so there will be more accurate response in the listening area. There are many shapes and sizes of bass sound-absorbing board, and many skills are required during installation. You can install it according to your own needs.

Helmholz resonator

Helmholtz resonators include a certain amount of air and an opening device leading to the outside. The internal volume of the speaker and its port are examples of resonators. The bottle is another example. When we blow it at the opening, it will produce a tone, which is the resonance of the generated air, and the pitch is related to the resonance frequency of the volume. In an open speaker box, the air echo from the driver is often used to enhance the resonant frequency of the front wave.

In terms of acoustics, we close a certain amount of air (such as a box) and put some holes or seams on its surface, so as to make a resonance system, which can be used to absorb (or more accurately remove) the non flowing waveform and problem frequency, and these problems are very prominent for a room. If one or two frequencies in the room are too strong, the resonator will be a very effective correction method.

Decay attenuation

In terms of audio, attenuation is a method of sound termination. The waveform envelope of any acoustic signal or an electronic instrument can be said to be composed of many elements, such as internal dynamics, tone extension, release and attenuation, which can define the characteristics of a signal or waveform envelope. The essence of envelope attenuation of all signals or waveforms can be changed according to the changes of factors such as time and attenuation amplitude.

Decay time

The attenuation time refers to the time taken for the sound pressure level of the reverberation to fall from the original length at the level of 60 dB (one millionth), sometimes called reverberation time. If you set the decay time carefully, you can get the mix wet as you want, and the sound will not get muddy or unclear.

Diffraction

Diffraction is the phenomenon that a sound wave changes when it encounters an obstacle or passes through a hole (sound wave, electromagnetic wave or light wave). Compared with obstacles, the shorter the wavelength is, the more likely it is to reflect and scatter around. Sound waves can also bend to fill an open space at the back of an object (which to some extent explains why you can hear the person next door but not see him when the door is open).

Diffusion diffusion

In terms of acoustics, diffusion is a key consideration, because it may cause serious problems due to the destruction of continuous reflection. In addition, it may also cause a closed space to sound larger than the actual one. In fact, in terms of acoustic processing, diffusion is a good alternative or supplement to absorption, because it will not reduce the sound energy, that is to say, it can effectively reduce the reflection in space or live performance venues.

Reverb reverberation

After the sound source stops vibrating, the residual sound in the room is called reverberation, sometimes people call it echo by mistake. In a large confined space, you can hear reverberation if you clap or play basketball. All the rooms can produce reverberation, but sometimes we can't find it. Reverberation is one of the main criteria for our subjective evaluation of the room. Our brain will make a judgment of the surrounding environment based on reverberation.

RT60

Abbreviation for revert time - 60dB, which is often used to represent a given reverberation time. In a large space, the time of reverberation dissipation can often reach 15-20 seconds. This means that in practice, the reverberation will take this length of time to dissipate into the ambient noise.

If the ambient noise is increased by 20dB, the time of reverberation dissipation will be reduced accordingly. The purpose of RT60 is to provide an objective method to measure the reverberation time. This parameter shows the time required for the reverberation to dissipate when the ambient noise is 60dB, or the time required for the reverberation to reduce to one millionth of the original volume.

Critical distance

When the volume of the sound source is the same as that reflected from other surfaces, that point is the critical distance. These reflex volume and time controls are a very important factor in creating an accurate listening environment.

STL (sound transmission loss, sound insulation)

It refers to the sound insulation effect that can be achieved in a certain scale or 1 / 3 scale by certain materials or separation materials in decibels. For example, the STL of 1 / 2-inch acoustic wall is 15dB at 125Hz. In fact, the transmission loss measured in the real environment is different from that measured in the laboratory. But there is a truth to be sure, even if a concrete wall is less sound insulation than the laboratory test results, it must be far better than a simple single-layer sound insulation wall.

SPL (sound pressure level)

Decibel is a unit to measure the relative size of sound intensity. Physicists introduced SPL sound pressure level to describe the size of sound: the pressure generated by sound through air vibration is called sound pressure, which is called sound pressure for short. The effective value of sound pressure is taken as logarithm to express the strength of sound. This value indicating the strength of sound is called sound pressure level, and the sound pressure level is represented by the symbol SPL, with the unit of decibel (DB) 。

Generally speaking, the minimum sound pressure that human ears can feel is 20 μ PA = 0dB SPL, which is the hearing threshold; when the sound pressure reaches 20 PA, which is 120dB SPL, people's ears will feel pain. Therefore, in acoustics or medicine, 20Pa = 120dB SPL is defined as the pain threshold. Working in this environment for a long time will cause damage to the auditory system.

NC Curve / contour noise standard

NC Curve refers to static noise and environmental noise in space such as auditorium. Curve or contour line is the mechanism of human ear to feel sound. In short, human ear has different sensitivity to different frequencies of sound. When the noise volume changes, the sensitivity will also change. NC curve is used to explain the mechanism, which can objectively represent the environmental noise level.

Because most of the environmental noise in auditorium and other spaces is caused by ventilation system, the frequency of noise is kept at a low level, and the human ear is relatively insensitive. NC curve can effectively measure the environmental noise standard of the whole spectrum, and its value range is generally between nc-15 and nc-70. Nc-15 represents a quieter environment. Generally, the environment meeting the standard is 30 dB SPL, and the noise frequency is lower than 80 Hz. Nc-20 is more noisy, but it is relatively quiet. Nc-25 ~ nc30 is too noisy for hearing.

NRC (noise reduction coefficient)

NRC is the abbreviation of noise reduction coefficient, which is usually used as a specific parameter to express the sound absorption efficiency of noise absorbing materials. Generally, it represents the mean value of Sabine coefficient of a certain material for 125hz-4khz band. The higher the value, the better the sound absorption effect. It is a more general standard. Compared with other noise reduction coefficients, it can not represent the sound absorption effect of a particular frequency band. For example, the blanket at home has a better sound absorption effect on the high-frequency noise, while the low-frequency noise is unsatisfactory.

Standing wave

Technically, this is determined by the mode of the room, that is, the vibration mode of the air in the room. Sound waves interfere with each other, resulting in higher SPL in some areas and lower SPL in others. The above regions become vertices and nodes respectively. Standing wave is easy to appear in the space where the distance between two reflecting surfaces is an integral multiple of half the wavelength of the frequency. For a given distance, there will be many frequencies that may produce standing waves. Standing wave will have an adverse effect on the sound effect of the room, but this can be avoided by the design of the house and the installation of sound-absorbing plates.

Node node

When the standing wave occurs, some specific points in the room become nodes. Because the peaks and troughs of the sound are completely offset, there is no sound wave at this point. The nodes are distributed about half wavelength, and the two sides of each node are vibration wave bellies. The antinodes are alternately distributed, so the sound wave is similar to a sine wave. Similarly, nodes can also be used to represent the intersection of cables. Ribbon cables often have nodes at two points and in the middle.

Flutter echo

The echo effect caused by multiple reflections of sound between two parallel reflecting surfaces, and the distance between the two surfaces is so large that the listener can hear the echo directly. In many cases, the sound effect is a kind of floating sound, because the time between these echoes is very short. In a small room, these echoes are so closely spaced that they are reflected in a tubular reverberation.

U-Boat

Now it mainly refers to the sound insulation material produced by auralex company for the construction of recording room. They are made of rubber and are U - shaped and filled with foam. The material is generally used for the surface of concrete slab to reduce the low-frequency vibration of the joint. In this way, the sound transmission can be greatly isolated. The room built in the above way is generally called "floating type", that is, the sound transmission at the connection of the house structure is disconnected.

Oblique room mode

Generally speaking, the room mode is irregular, which will cause sound waves to interfere with each other, resulting in resonance and cancellation effect. A sloped room pattern, typically consisting of six surfaces, four walls, and a roof and floor. Its sound effect is about 1 / 4 of axis mode and 1 / 2 of tangent mode.

Quarter space

The loudspeaker is placed in an ideal space, and the sound wave can freely spread in all directions. However, if the speaker is placed back to the wall, its sound wave can only propagate to a range of 180 degrees. If the speaker is placed in the corner, its sound wave can only propagate to a range of 90 degrees. However, in the above three cases, the volume is different: in the 90 range, the volume is 3dB higher than 180 degrees, and 6dB higher than in the free propagation.