Stereo headphones use the binaural effect of the human ear to give people an immersive feeling. When the sound source (including complex cluster signals) is biased toward the left or right ear, it deviates from the center of the two ears. On the front central axis, there is a difference in the distance between the sound source reaching the left and right ears, which will lead to differences in sound level, time, and phase of the sound reaching the two ears. This small difference is perceived by the human ear, transmitted to the brain, and compared and analyzed with the existing auditory experience stored in the brain to determine the direction of the sound.

Binaural Effect

What is the working principle of artificial stereo?

To talk about artificial stereo, we have to start with some interesting characteristics of the ear. Human hearing has the following physiological and psychological characteristics:

Time difference effect

If the left ear hears the sound first, then the listener will feel that the sound comes from the left ear (the one who heard the sound first) direction of the ear) and vice versa. This phenomenon is called the time difference effect between left and right.

The time difference effect is one of the important basis for our auditory identification of the direction of the sound source (the location of the sound). Its principle is: the ears are on both sides of the head. If a sound comes from directly in front of the listener (central axis.), then the distance from the sound to both ears is equal. Therefore, the listener feels that the sound comes from directly in front; If the sound comes from the left ear of the listener, then the left ear will hear the sound before the right ear, so the listener will feel that the sound comes from the left side in front. In other words, the larger the sound source deviates from the central axis of the front, the greater the listening time difference between the left ear and the right ear.

Some people may ask, in the two cases where the ears are on the head, the diameter of the head is about 20cm, and the speed of sound at room temperature is 344m per second, then the time difference between a sound that deviates from the square in front of the head and reaches the two ears is It is very tiny. Can the human ear distinguish it? It is doubtful. But experiments and practice have proven that doubts are unfounded.

Sound intensity difference effect

If the sound heard by the left ear is louder than that by the right ear, then the listener will feel that the sound comes from the left direction, and vice versa. This phenomenon is called the sound intensity difference between the left and right ears.

The sound intensity difference effect is also one of the important basis for our hearing to distinguish the direction of the sound source. Its principle is: if a sound comes from the central axis directly in front of the listener, then the sound reaches both ears. The sound size is the same, so the listener feels that the sound is in front; if the sound comes from the left side of the listener, the listener feels that the sound source is to the left.

The combination of time difference and sound intensity difference. Both time difference and sound intensity difference can have an independent effect on the sense of direction of the sound source. When they are combined with each other, a comprehensive effect occurs.

The time difference effect and sound intensity difference effect between the two ears are the most important basis for the human ear to distinguish the direction of the sound source, collectively called the binaural effect. The working principle of artificial stereo is simply based on the binaural effect.

According to this principle, when producing stereo programs, electroacoustic technology is used to make a sound signal reach both ears at the same time or with the same sound intensity, then the listener must feel that the sound source is directly in front of him; if If the time for the sound signal to reach the left ear is delayed (causing a time difference between the two ears), or the sound intensity of the signal reaching the left ear is weakened (causing a difference in sound intensity between the two ears), then the perception of the sound source direction must be biased towards the right direction.

Using the theory of stereo image positioning, the following sound image positioning effect can be obtained by reconstructing the sound image system with two channels or multi-channels:

——Equidistant directly in front of the listener There is one speaker (speaker) in each of the two examples. If the playback time of the two speakers is the same, the listener will feel that the sound is in the center between the two speakers; if the playback time of the left speaker is advanced, the listener will feel that the sound is in the middle. Like on the left and vice versa.

——Place two speakers (speakers) equidistantly directly in front of the listener. If the sound intensity of the two speakers is the same during playback, the listener will feel that the audio image is located in the center of the two speakers. ; If you gradually increase the volume of the left speaker (or gradually weaken the volume of the right speaker), the listener will feel that the audio image gradually moves to the left, and vice versa.

The old stereo positioning technology uses many microphones (microphones) to record the natural stereo effect of the scene, and strives to reproduce the stereo effect of the original sound field (such as a symphony orchestra performing in a theater) in the program. Therefore, the traditional Stereo technology is called reproduction stereo image technology. Modern stereo positioning technology allows the sound engineer (sound director) to rearrange the sound source layout directly according to artistic intentions. In layman's terms, it uses artificial methods to rearrange the direction of sound emission. Therefore, modern stereo technology is also called reconstructed stereo image technology.

3. Auditory sense of orientation and stereo sound

Stereo refers to sound with a sense of space. Stereo technology uses the auditory sense of orientation to reproduce various sound sources during playback. direction and relative position technology.

(1) Binaural effect

People listen to sounds with two ears at the same time. When the distance from a certain sound source to the two ears is different, although the two ears are What is heard is the same sound wave, but there are time differences (phase differences) and intensity differences (sound level differences). They become an important objective basis for the auditory system to judge the direction of low-frequency sound sources. For sounds with higher frequencies, the diffraction performance of sound waves must also be considered. Due to the blocking effect of the head and ear shell on the propagation of sound waves, there will also be a difference in sound intensity and timbre between the two ears. In short, due to the difference in the state of the sound waves reaching the two ears, the sense of direction and depth of hearing are caused. This is often called the "binaural effect." Sound sources in different directions will produce different (but specific) sound wave states at the two ears, allowing people to judge the direction and location of the sound source. If people try to deliberately create the same sound wave state at both ears as the actual sound source can produce, they should be able to create a corresponding illusion (sound image) of the sound source in a certain direction. This is exactly the stereo sound Physiological basis of technology.

(2) Stereo system

The two-channel stereo system is the most basic system that can create a stereo image for both ears. In a two-channel stereo system, in order to correctly reproduce the direction of the real sound source, two paired microphones must be used during recording. The signal transmission channels must also be independently separated, and the amplification rate and frequency response characteristics of each channel must be Likewise, any difference will significantly change the position of the sound image and affect the stereo effect.

If the phases of the two speakers are connected in an anti-phase relationship (that is, the phase difference is 180°) when connecting to the playback equipment, then the sound image will run to the outside of the speakers. This phenomenon is called "outside stereo." During normal playback, care should be taken to prevent phase reversal. However, the out-of-bounds stereo phenomenon also has practical value. In a portable stereo device, due to the close distance between the two speakers, which affects the stereo effect, you can use the outside stereo principle to add a stereo broadening circuit to take out a part of the signals from the left and right channels, and use a certain phase shift and delay to Then, they are input to another channel crosswise, so as to obtain a wider sound effect than the distance between the two speakers. The stereo widening switch (Stereo Wide) on the recorder is for people to control the opening and closing of the widening circuit according to needs.

In addition to two-channel sound systems, there are also four-channel sound systems (two speakers are also placed behind the listener). This type of stereo sound system is commonly known as surround sound. In stereo movies, five channels and seven channels are also used. Generally speaking, the greater the number of channels, the stronger the sense of immersion when listening.