How is the stereoscopic feeling of listening formed?

1 Stereo concept

Stereo is a geometric concept and refers to things occupying a position in a three-dimensional space. Is the sound three-dimensional? In analogy, the answer can be affirmative. Because the sound source has a definite spatial location, the sound has a definite source of azimuth, and people's hearing has the ability to distinguish the direction of the sound source; especially when there are multiple sound sources simultaneously, people can perceive the distribution of sound groups in space by hearing. The situation can therefore be said to be "stereo". However, it would be more appropriate to say: "The original utterance is three-dimensional" because when the sound is recorded, amplified, etc. and then replayed, all the sound may come from a speaker. Putting it out, this kind of replay sound is not stereoscopic. Since various sounds are emitted from the same speaker, the original sense of space - especially the spatial distribution of the sound group - disappears. This reproduction sound is called "Mono (Mono). If the playback system can restore to a certain extent the sense of space of the original sound, then this playback sound is called "stereo" (stereo). Since the original sound is self-evident, it is " The word "stereo" refers specifically to the kind of playback sound that has a sense of space (or position).

2 binaural effect

In order to restore the sense of space in the replay sound, it is first necessary to understand why the human hearing system has the ability to discern the source location. The study found that this is mainly because people have two ears and not just one ear.

Ears grow on both sides of the head. Not only do they have a spatial distance but they are also blocked by the skull. Therefore, the sound received by the ears may be different. It is mainly based on these differences that people can distinguish the location of sound sources in space. These differences are mainly the following:

(1) The time difference between the arrival of the sound on both ears

Because there is a certain distance between the left and right ears, the sound from other directions arrives at both ears in addition to the sound coming from the front and the back, resulting in a time difference. If the sound source is to the right, the sound must reach the right ear and then the left ear; otherwise, it must reach the left ear and then the right ear sound source is more biased to one side, the greater the time difference, the experiment proved that if artificially caused by two ears Listening to the time difference can produce illusions of source bias. When the difference was around 0.6ms, it was felt that the sound was completely from one side.

(2) The sound level difference of the sound reaching both ears

Although the two ears are not far apart, due to the blocking effect of the skull on the sound, the sound level of the sound reaching the two ears may be different. The sound level on the side closer to the sound source is larger and the other side is smaller. Experiments have shown that the maximum sound level difference is about 25 dB.

(3) The phase difference between the two ears of the sound

　　It is known that sound travels in the form of waves, and the phase of the sound waves in different positions in space is different (unless it is exactly one wavelength apart). Since there is a spatial distance between the ears, there may be a difference in the phase of sound waves reaching both ears. The tympanic membrane in the ear vibrates with sound waves. The phase difference of this vibration is also a factor in our judgment of the sound source's position. Experiments have shown that even if the sound level and time are the same when the sound arrives at both ears, only the change is the same, and only the phase is changed. We will also feel that the sound source position is very different.　　（4）Difference in timbre when the sound arrives at both ears

If sound waves come from a certain direction on the right side, some parts of the head must be bypassed to reach the left ear. The diffraction ability of the known wave is related to the ratio between the wavelength and the scale of the obstacle. The diameter of the human head is approximately 20 centimeters, which is equivalent to the wavelength of the 1,700-Hz sound wave in the air, so the human head has masked the sound components above a thousand hertz. effect. In other words, the ability of each component in the same sound to bypass the head varies, and the higher the frequency, the greater the attenuation of the component. Then the tone heard by the left ear is different from that heard by the right ear. As long as the sound does not come from the positive direction, the timbre heard by the two ears will be different, thus becoming a basis for people to determine the direction of the sound source.

(5) Differences between direct sound and frontier sound groups

The sound from the sound source, in addition to the direct sound directly to our ears, but also through the surrounding obstacles once or multiple reflections to form a sound reflection group, one after another to reach people's ears. Therefore, the difference between direct sound and reflected sound groups will also provide information on the spatial distribution of sound sources.

(6) Differences caused by auricle

The auricle is forward, obviously making people distinguish between before and after. On the other hand, the shape of the auricle is very subtle, and the sound in different directions will have complex effects in it, and it will definitely provide certain position information.

It has been proved by practice that the differences in sound levels, time differences, and phase differences have the greatest impact on auditory orientation. However, their role is not the same under different conditions. In general, the phase difference plays a large role in the low and middle frequency bands of audio. The role of the sound level difference in the medium and high frequency bands is mainly for the vocalization of the vocalization, and the effect of the time difference is particularly significant. In the vertical positioning, the role of the auricle is more important. Actually, the binaural effect is comprehensive, and the human auditory system should The direction of the sound source is judged based on the combined effect.

By the way, people's hearing system has many other effects besides loudness, timbre, and orientation. One of them, which is closely related to our future lectures, is called the "priority effect" (also known as the "Haas effect"). It is known from experiments that when two identical sounds, one of which is delayed and arrives at the ears of a person in succession, if the delay time is within 30 milliseconds, people will not feel the existence of a delayed democratic delay. Can feel the change of tone and loudness. However, if the delay is too long, the situation will be different. Everyone already knows that when two consecutive sounds arrive at -60ms (50mm), which is equivalent to a sound path difference of more than 17m, the listener can feel it.