What Is a Reverberation?

(Reverberation) The length of reverberation time is an important acoustic characteristic of buildings such as concert halls, theaters, and auditoriums. Sound waves reflect off obstacles, so our world is full of reverberation.

reverberation

(Acoustic characteristics)

(Reverberation) The length of the reverberation time is

When a sound wave travels indoors, it must be reflected by obstacles such as walls, ceilings, and floors. Each reflection must be absorbed by the obstacles. In this way, after the sound source stops sounding, the sound waves have to be reflected and absorbed many times in the room before finally disappearing. We feel that after the sound source stops sounding, there are several sound waves mixing for a period of time (after the indoor sound source stops sounding, it still The existence of sound continuity). This phenomenon is called reverberation, and this period of time is called reverberation time.

For the lecture hall,

Reverb in the real world

Is there a place in this world where there is no reverb? Have! You get on the plane, fly to a height of 10,000 meters, and then jump down. At this time, you yell, there is no reverb, because you are in the air and there are no obstacles around, your voice will spread out infinitely. Without being reflected back. So there is no reverberation.

Another place where there is no reverberation is the acoustic laboratory. The walls, ceiling, and floor of the acoustic laboratory are specially treated. When the sound reaches the wall, it will be absorbed by the wall and not reflected back. Why is it absorbed? You can do a small experiment, find 100 needles, that is, the needles for sewing clothes, tie them together and straighten, and then you can look at the needle side of this bundle of needles, you will find that it is black because of the light After arriving at this side, after many reflections, it went straight to the inside and couldn't get out, so no light was reflected out, as if the light had been absorbed. The layout of the acoustic laboratory is similar to this, absorbing sound.

The recording studio is a half acoustic laboratory that can absorb most of the reverberation. The walls of the recording studio are irregularly arranged, and the surface is made of soft cotton products. Although it is not comparable to the bundle of needles, the sound reaches the wall and enters the messy cotton. Not going, so the reverberation in the studio is also very small.

With a loud roar in a room, how many reflections there will be, the answer is countless.

In this room, you slap and get a different sound

Is it a lot? This is actually a relatively simple reflection process. If there were more tables and chairs in this room, the reflection would be more complicated.

Close your eyes and yell, you can know what kind of environment you are in, outside or at home. Even if you yell at home, you can know in which room you are and where in this room. This is because each room has different reverberation characteristics due to different space sizes, different furniture placements, and different wall materials; at different positions in the same room, because your distance from the wall is different, so also With different reverb characteristics. You are familiar with these characteristics, so you can tell where you are by sound alone.

A seemingly novice question: Why do recordings and remixes need reverberation?

Why do recordings and remixes need reverberation? Answer: Because there is no reverb when recording.

Why is there no reverb when recording? A: Because the studio is reverb-free.

Why is the recording studio unreverberated?

In fact, professional recording studios have reverberation. They have a lot of plate-like materials, which can flexibly transform the room into various reverberation characteristics. But with the rapid development of digital recording technology, digital reverb effects can simulate real-life reverbs, so everyone simply made the recording studio without reverbs, and then used effects to simulate reverbs Effect, whatever reverb you want, there is reverb ... this is why recording studios, especially small and medium-sized recording studios and personal studios, are made without reverberation.

Artificial reverberation principle

In such a room, the teacher's voice has been reflected many times. If there are 5 sound reflection lines reaching the students' ears,

The above only lists 5 sound reflection routes, in fact, tens of thousands to tens of thousands. For the convenience of explanation, let's talk about these 5 items.

Every time the teacher speaks, the student actually hears 6 sentences: the first sentence is directly transmitted to the student's ear without reflection, and the next 5 sentences are the sounds that reach the student's ear through various reflection lines. These 6 sentences are very close in time. There is a timetable for the sound arrival in the picture. Note that the time unit is milliseconds (1 millisecond is equal to 0.001 seconds).

Because the time interval between these reflections is too close, the students cannot hear 6 sentences, but 1 sentence with a reverberation feeling.

The sound the students hear is a superposition of these 6 sounds.

This is just for the convenience of explanation. The real situation is the superposition of tens of thousands of sounds.

This is how the reverb effect works. Repeatedly superimpose the sound many, many times to get the reverb effect.

With such a thing, it is convenient to calculate in the future. No matter what the teacher says, as long as the teacher's voice is subjected to some calculation, you can get the effect of superimposing 6 sounds.

So what exactly is this "some" calculation? In mathematics, this is called "convolution" calculation. The English word is "convolution", which is to superimpose the teacher's voice according to the graph of the 6 pulses above.

This calculation is in no particular order. You can think of it as superimposing the teacher's voice according to the pulse diagram (sound wave); or you can think of the pulse sound wave as the teacher's voice Into sound waves consisting of countless pulses), and superimposed calculations are performed.

This pulse map, which is the sound wave containing 6 pulses, is the reverberation characteristic of this room from the teacher's podium to the student's seat. Acoustically, because this reverberation characteristic is obtained by impulses, it is vividly called "impulse response"-impulse response, referred to as IR.

The working principle of the reverb effector is to take the source sound and perform convolution calculation with the impulse response.

The IR with 6 pulses above is impossible in reality. Real-life IR often has hundreds, thousands, or tens of thousands of pulses.

Because the IR of all types of rooms has some common characteristics, some acoustics are made.

First of all, the first pulse of IR is called "direct sound", because this pulse is the sound that reaches the human ear directly from the sound source without reflection;

Secondly, it is stipulated that the last few obvious pulses of IR are called "early reflections". These sounds all reach the human ear after one or two or three reflections of the sound source. Due to the small number of reflections and the short sound line, they have Stronger energy and shorter latency.

Finally, it is stipulated that the numerous pulses behind the IR are called "late reflections". These sounds are sound sources that reach the human ear after numerous reflections. The number of reflections is long and the sound line is long, so it has weaker energy and longer delay. But there are so many of them, like the endless river.

IR in effects

As mentioned in the previous section, the reverb effect is a convolution calculation using IR and the sound source. So, someone will ask, is there an IR in the reverb effect? Is the IR of each effect the same? Where is this IR placed? In what form? If not, how did these IRs come from?

As mentioned earlier, reverberation is the convolution calculation of IR and sound source, so of course there is IR in the reverberation effect.

As everyone knows, the reverb effect of different effects is different, so the IR must be different.

Where is the IR? In what form? How did these IRs come about? Let's talk about it in detail.

Reverb effects are divided into three types like synthesizers: sample reverb, "algorithmic" reverb, and analog synthetic reverb.

(1) Sampling IR reverberation

Both Sony and Yamaha have oversampled reverbs, which are expensive. The software's sampled reverb effects include Acoustic Mirror from the well-known Sonic Foundry, and Room Simulator from Samplitude.

The IR of the sampled reverb is all wave files that are actually sampled. Can be stored in any memory, such as hard disk, CD-ROM, floppy disk, etc. Sony, Yamaha's hardware sampling reverb, also has a large capacity memory.

The IR of the sampled reverb is sampled from the recording. The easiest way to obtain the IR is to place a speaker at the teacher's position and a microphone at the student's position. The speaker plays a pulse and the microphone records. The recorded sound is IR, which is the reverberation characteristic curve from the podium to the student seat in this room.

The specific methods adopted by Sony, Samplitude, etc. are:

Where you want to get reverberation characteristics, such as the famous concert hall below, place speakers on the stage (which will be excellent speakers, of course), and place stereo microphones (excellent microphones) on the seats. Then a series of test signals are played. These signals are mainly pulses, and the full-frequency sine wave of various speeds is continuously scanned as supplement. The sound is recorded, and then IR is obtained after some calculations.

The IR obtained with this sampling method is extremely real.

Sampling the reverberation IR, not only the manufacturer can preset it for you, you can also make it according to the tools provided by the manufacturer. So it is infinite in number.

Sampled reverb can also fully replicate the effects of any other reverb effect.

Reverberation characteristics and various parameters

For the convenience of research, the reverberation is divided into several parts acoustically, and some idioms are prescribed. The first sound of the reverb is the direct sound, that is, the source sound, which is called dry out in the effect. The next few sounds that are clearly separated are called "early reflections." (Earlyreflectedsounds), they are all sounds that arrive after only a few reflections. The sounds are relatively loud and obvious. They are especially able to reflect the distance relationship between the source sound in the space, the ears, and the walls. The endless stream of sounds is called reverberation.

Most of the reverb effects will have some parameter options for you to adjust. Let's talk about what these parameters mean.

(A) Decay time

This is the total length of the entire reverb. Different environments have different lengths and have the following characteristics:

The larger the space, the longer the decay, and vice versa. The more empty the space, the longer the decay, and the shorter it is. The less furniture or other objects (such as pillars) in the space, the longer the decay; otherwise the shorter. The smoother and flatter the surface of the space, the longer the decay, and the shorter the opposite.

Therefore, the reverberation in the lobby is longer than the reverberation in the office; the reverberation in the unfurnished room is longer than the furnished room; the reverberation in the barren mountain valley is longer than the reverberation in the forest valley; The reverberation of the space is long ...

Many people like to set the reverberation time very long. In fact, the real reverberation time of some theaters and concert halls is not as long as we think. For example, the reverberation time of the Boston Concert Hall is 1.8 seconds, the New York Carnegie Hall is 1.7 seconds, and the Vienna Concert Hall is 2.05 seconds.

Here is a formula for calculating the reverberation time. You can use it to calculate the reverberation time of a room. Open the page

(B) the delay time of the front reflection (Predelay)

It is the time distance between the direct sound and the front reflection. It has the following characteristics:

The larger the space, the longer the Predelay; the shorter the space, the wider the space, and the longer the Predelay; otherwise the shorter

Therefore, the Predelay in the lobby is longer than the office; while the tunnel space is large, but it is very narrow, so the Predelay is very short.

If you want a wide open space, make Predelay bigger.

(Three) wet out

This is the size of the reverb effect. It has the following characteristics:

The wetout has nothing to do with the size of the space, but only with the amount of debris in the space and the material of the walls and objects. The softer the surface material of the wall and indoor objects, the smaller the wet out; The bigger the wall is not smooth, the smaller the wet out, the bigger the pits on the wall, the smaller the wet out, and the bigger the

Therefore, the reverberation of a packed car is much smaller than that of an empty car; the reverberation of a room full of furniture is smaller than that of an empty room; the reverberation of a room with a carpet is smaller than that without a carpet; the forest The reverberation in the valley is smaller than the reverberation in the barren mountain valley

High and low frequency cut-off (low cut / high cut)

This parameter is expressed in the form of EQ in some effects, such as Waves' RVerb.

This content is actually not directly related to the actual situation, it is only designed for the sound when we do reverb processing. However, it can also show that the high-frequency sound is more severely lost in the transmission. We have specific explanations later.

Generally, when processing, in order to clear and warm the reverberation sound, the low frequency and high frequency are removed. Only when expressing some sci-fi environments such as "cosmic sound", high and low frequencies are reserved.

Some effects also call this "color". For example, the effect of TC is color. Color is the feeling of "cold" and "warm", high frequency is cold, low frequency is warm. So these effects use color to indicate the cutoff of high and low frequencies, warm (red) means that the reverb sound is biased towards low frequencies, and cold (blue) means that the reverb sound is biased towards high frequencies. The EQ of Waves' RVerb shown above, which uses orange and blue-green to do those two points, is also for this purpose.

supplement:

The high and low frequency cut-off does not actually exist in reality. A common phenomenon in reality is that the reverberation of low-frequency sounds is larger than the high-frequency sounds in terms of sound size and decay time. This is because sounds of different frequencies have different abilities to bypass obstacles because of their different wavelengths. High-frequency sounds have short wavelengths and it is not easy to bypass obstacles. Low-frequency sounds have long wavelengths and are easy to bypass obstacles. In addition, they have different degrees of attenuation when propagating in the air (the higher the frequency, the easier it is to weaken) and the degree of absorption by the wall (the higher the frequency, the more easy it is to lose), so the reverberation time and size of the sound of different frequencies are different of. In the real world, in most small and medium spaces, the lower the sound has the longer reverberation time, the higher the sound has the shorter reverberation time, and it is impossible to do the reverse. How to reduce low-frequency reverb is a headache for any recording studio. There is only one case where the low-frequency reverberation is smaller than the high-frequency reverberation, which is a large space, and it is covered with obstacles and surfaces made of hard materials, such as those using hard plastic stools and concrete wall floors. indoor Stadium.

We can clearly see this pattern from the true IR spectrum of a concert hall.

Therefore, some reverb effects also have settings for the attenuation of sounds of different frequencies. But there are many effects that do not have this content.

(5) Different attenuation levels at different frequencies (Damp)

Then said above. This item is not available in some reverb effects. In addition, this item is basically not provided in the sampling reverb, because the characteristics of different attenuation levels of different frequencies of the sampling reverb are already included in the IR. For example, Waves RVerb provides this project. In addition, some effects have only one parameter setting, which is "damp" or "damping", which makes the high frequency decay faster. 8zo Uw $ iF & shy; Generally speaking, the high frequencies in the reverb are easily attenuated greatly. The larger the space, the more objects in the space, the less smooth the object and the wall surface, the more the high frequency attenuation. Only in the small and medium space, and the surface of the space is relatively smooth, the attenuation of high frequency is close to the low frequency. But when we do music mixing, sometimes for the sake of sound, we don't necessarily follow the natural law that high frequency is more likely to weaken.

(6) Different reverberation times for different frequencies

Some effects also provide different decay times for you to adjust. English is High-frequency decay and low-frequency decay, or other names, such as Ultrafunk Reverb can set different decay times. This feature is basically consistent with the previous damp. In general, the duration of high frequencies in reverb is definitely shorter than low frequencies. The larger the space, the more objects in the space, the less smooth the object and the wall surface, the shorter the duration of the high frequency, and the larger the gap from the low frequency. Only in the small and medium space, and the surface of the space is relatively smooth, the high-frequency time is close to the low-frequency. The above three frequency-related parameters are not provided by all the effects devices, some of them are provided, and some of them even provide two or even one. If you don't provide them all, you can use one of the other parameters instead of the ones that aren't provided, because they have similar characteristics.

(Vii) Diffusion

It is traditionally called Early reflections diffusion. We know that early reflections are a group of relatively obvious reflections. The closeness of these reflected sounds to each other is diffusion. The less smooth the wall (such as a carpet), the more the sound is scattered, the more the reflected sound, the closer they are to each other, the reverberation is continuous, and the sound is mild; the smoother the wall (such as glass) , The smaller the sound's degree of scattering, the less the reflected sound, and the more the distance between them, the reverb sound will sound closer to the echo, and the sound will be clear. Therefore, for some sustain sounds, such as organ and synthetic strings, a smaller diffusion can be used, and the sound is more beautiful. For pulse sounds, such as percussion, xylophone, etc., a larger diffusion can be used. Reverb is more smooth. Some effects also have diffusion as a parameter, but the specific definition is not the same. In some effects, diffusion refers to the irregularity of the reflected sound. The more irregular the shape of the space (such as in a cave or a church), the smoother the walls, the more irregular the appearance of the reflected sound, and the larger the diffusion; The more regular the shape (eg, unfurnished dwellings, empty classrooms), the smoother the walls, the more regular the appearance of reflected sound, and the smaller the diffusion.

(8) Reverb density

The meaning of this parameter is similar to diffusion, but it is only for the reverberation part after early reflection. Many effects do not provide density, but instead use diffusion to control the entire reverb.

(9) Room size

This should be well understood, the space can reflect the width and depth of the sound field. But different effects will have different algorithms on this. In addition, the sample reverb does not provide this parameter because the size of the space is already reflected in the IR.

(10) Early reflections level

That is the magnitude of the early reflections. Many effects allow you to independently adjust the sound of early reflections and the reverberation behind.

(11) Stereo width (Width)

Some reverb effects have such parameters. If you set this value to a large value, the effect will make a big difference between the left and right IR, and the stereo feeling will come out.