People often say that the sound recognition is that after hearing the sound, people can distinguish which direction the sound is transmitted from, and the sound is different in different environments. This is the effect of the human ear on the sense of direction of the sound. .
Sound source orientation is another sensory element of the auditory organ in addition to the pitch, intensity, timbre and length of the sound. It involves complex physiological and psychological aspects. At the same time, the sound source orientation is also the theoretical basis of stereo technology.
First, the time difference, phase difference and sound level difference, tone color difference binaural effect by positioning principle is time difference, phase difference, sound level difference, acoustic color difference.
(1) Time difference and phase difference The time difference mainly refers to the difference between the sounds and the moments of the ears. The speed at which sound waves propagate at normal temperature is 344 m/s. When the sound source deviates from the central axis directly in front of the listener, the distance between the ear A and the ear B is different, so that the sound reaches the ear A and the ear B. The time difference between.
The time difference is used as the sound source localization mechanism, and the sound source positioning accuracy is high on the front side and the two sides, and the error is large in the positioning of the sound source from the back. The reason is not very clear. Perhaps because the sound comes from the back side, the ear shell shadowing effect is caused by the left or right ear, so that the sound varies due to diffraction.
Because the human ear is adaptable to the sound, when the sound reaches the basement membrane, the hair cells are excited and sensitive. When the sound continues to stimulate, the hair cell response is relatively slow. Therefore, the sound source positioning accuracy of the sudden sound and the transient sound is high.
A rapidly flowing sound source will attract auditory attention. Therefore, the voice with a constantly changing orientation has less error in the recognition of the orientation of the human ear. This is the reason for the sound shift in modern stereo programs.
A continuous sound, although there is a time difference between reaching the ears, but because the subsequent sound reaching the same ear masks the front sound, the time difference becomes inconspicuous.
The high frequency sound is consistent with the low frequency sound propagation speed, so the time difference is independent of the frequency of the sound source. However, the phase difference is related to the frequency of the sound source. When a sound reaches both ears, a time difference occurs between the ears, and a phase difference necessarily occurs. In a certain frequency range, the phase difference is one of the information of the sound source orientation.
The phase difference positioning mechanism is more effective at lower frequencies. For example, at a normal temperature, the wavelength of the 20 Hz sound is 17 m, and the 200 Hz is 1.7 m, and the phase difference formed by the time difference can be perceived by the human ear. When the sound source is in the high frequency region, for example, the wavelength of 85 kHz at 10 kHz, 20 kHz is 42.5 px, and the phase difference caused by the time difference is even more than 360°, which is equal to starting another wavelength. The phase difference at this time has no effect as positioning information because it is impossible to distinguish that the phase belongs to hysteresis or lead. Therefore, the high frequency sound belongs to the "chaotic phase difference" information.
(2) Sound level difference and timbre difference The sound level difference means that the sound wave reaches different sound intensity at both ears. The main cause of the formation of the sound level difference is the shadowing effect. A shadowing effect occurs when an advancing sound wave encounters an obstacle having a geometric size equal to or greater than the acoustic wavelength. The principle is that: when the high frequency sound encounters an obstacle, it cannot form an acoustic shadow area behind the obstacle because the obstacle cannot pass over the obstacle; the low frequency sound wavelength is larger than the obstacle and the sound diffraction area is formed behind the obstacle. High-frequency sound plays an important role in the difference of sound level. Because high-frequency sound waves cannot bypass the listener's head, the ear in the sound shadow area is different from the ear that can hear the direct sound. The higher the frequency, the larger the sound source deviates from the front center axis, and the sound level difference becomes more apparent.
From the perspective of the diffraction effect, the low frequency sound will of course also form a sound level difference. However, since the diameter of the head is about 500px, when the low-frequency sound is diffracted, the distance of walking is limited, and the energy lost by diffraction is small, so the low-frequency sound deviating from the central axis, the sound level difference reaching the two ears is almost zero, Sound source localization is not obvious.
While the shadowing effect has an effect on the sound level difference, it also has an effect on the chromatic aberration. We know that the main components that make up the tone are the fundamental tones and the components of the harmonics above them. For example, a composite wave point sound source with a fundamental frequency of 200 Hz and an incident angle of 45°, then its fundamental and lower harmonics produce a diffraction effect after encountering a head obstacle, and its higher harmonics are headed. The part is shaded and a high frequency sound shadow area appears. At this time, the sound reaching the one ear is the direct sound (the original sound), and the sound reaching the other side of the ear changes the sound due to the high frequency loss. The cerebral cortex recognizes the sound source orientation based on the timbre of the two ears. It can be seen that the timbre difference is another reflection of the sound level difference of the high frequency signal.
It should be noted that the formation of timbre is mainly a composite sound source whose fundamental frequency is above 60 Hz. Since the sound harmonic wavelength below 60 Hz is large, the obstacle that encounters the head size (about 500 px in diameter) does not cause a shadowing effect. For example, a fundamental frequency of 30 Hz sound, the 16th harmonic is 480 Hz, the wavelength is 0.716 m, the wavelength is much larger than the head diameter, and no obvious timbre difference is formed between the ears, and the 17th, 18th, and 19th harmonics are The intensity is very weak and does not make much sense to the tone. Therefore, the sound quality below 60 Hz is lower than that of the medium frequency and high frequency sound source.
From the difference between the intensity difference and the timbre difference to the binaural effect, it can be inferred that the pure tone is difficult to locate than the composite tone because the pure tone is a sine wave (single wave) and the timbre cannot be constructed.
(3) Sound source depth sense The sound source depth sense is the distance between the listener and the sound source, so the sound source depth sense is also called the sound source distance location.
Sound source depth perception is often associated with a digital pattern. When we hear a sound, we feel the approximate distance of the sound in addition to the general orientation of the sound. To accurately sense the depth of the sound source, familiarize yourself with the sound field environment, familiarize yourself with the sound source, or directly measure the distance between the sound source and yourself. This shows that the sound source depth sense is formed and can be trained.
Depth positioning is mainly determined by the degree of sound attenuation. In the process of radiation, the energy is lost with the distance of propagation. Firstly, the first attenuation of the amplitude of the higher harmonics is smaller, forming a timbre change. After the human ear hears the acoustic signal, it compares with the acoustic signal stored in the brain to determine the depth of the acoustic signal source.
Another way to feel the depth is the sound source comparison method. When there are several sound sources (array sources) of different distances, the human ear can estimate the depth of other sound sources by the nearby point source. A plurality of sound sources formed by point sources of different distances and incident angles cause the sense of sound to have a sense of width and envelopment. Repeat one more sentence: the sound source depth sense is usually parallel with the vision, the experience is formed by vision, and the visual aid is used to accurately locate.
(4) The difference between the time difference and the sound level difference binaural effect can affect the sound source orientation. When they are combined with each other, they have a combined effect. If they do the opposite (which rarely happens under normal conditions), they do not cancel each other out. The practice of modern stereo technology proves that the combination of time difference and sound level difference has obvious effect on the sound source orientation. Experiments show that under certain conditions, the 1ms time difference is equivalent to a sound level difference of 5?12dB, and the relationship is interchangeable.
In a hall where the reverberation time exceeds the normal acoustic requirements, the reflected and reverberant sound levels of the sound source greatly exceed the direct sound. At this time, the human ear is most sensitive to the stimulation of the first wavefront of the sound source. If the reflected sound and the reverberation are 40 60 ms for the direct sound delay, the human ear may grasp the sound source orientation. If the delay exceeds this range, the human ear cannot distinguish the time difference and the sound level difference between the original sound and the two ears, and a sense of direction of separation or a sense of direction of confusion may occur. This is why in a hall with heavy echoes, people often do not easily grasp the direction of the sound source and need to use eye positioning.
Second, the sound source orientation mechanism Classical psychoacoustics believes that people's perception of sound source mainly depends on the difference between binaural listening, called binaural effect. Just as the two eyes observe the scene to produce a sense of perspective and a sense of three-dimensionality, it is possible to judge the sound coming and produce a stereo sense by the feeling of the difference in sound intensity between the ears. Until modern times, the binaural effect is still the main theoretical basis for the sense of orientation of the sound source. However, in recent years, experts have found that people with hearing loss in one ear still have the ability to judge the sound source. Therefore, the new theory of ear shell effect is proposed, which makes the theory of sound source orientation more perfect.
The mechanism of sound source orientation is very complicated. The principle of the binaural effect is that if the position of the ears is on the head, if the sound source is in the central axis directly in front of the person, the time at which the sound reaches the ears, the sound intensity level and the phase are the same; if the sound source deviates The central axis of the listener directly in front of the sound, the distance of the sound reaching the ears is not equal, therefore, the sound arrives at both ears and there will be a time difference and a phase difference. At the same time, due to the shadowing effect on one side of the ear, there is a difference in sound level and timbre between the two ears.
Sound source orientation is a physiological function that is inherently innate. However, the width, depth and all the digital-related sensations of the sound source are related to one's acquired experience.
Third, the ear shell effect As early as more than one hundred years ago, some people found that a single ear deaf, still have the ability to identify the sound source orientation, and proposed the idea of ​​the ear shell effect, but not for people's attention. It was not until the 1960s that when stereo technology developed rapidly, it was recognized that the binaural effect was difficult to interpret for certain sound source orientations, and the ear shell effect was re-recognized.
When we were a child, we might have done some interesting experiments, such as pulling our ears out into a pocket, then we feel that the outside voice suddenly becomes bigger and clearer; if we press the ear backwards to stick to the skull, You will find that the sound is weakened. As mentioned earlier, the ear shell has the function of reflecting and collecting sound. At the same time, due to the unevenness of the ear shell, the reflected sound generated by different parts of the ear shell enters the eardrum slightly later than the direct sound, forming a repeating sound that is shorter than the direct sound, and repeating the sound than the direct sound. It varies depending on the angle of incidence.
The ear shell effect is also effective in judging the sound from behind the listener. When a sound comes from behind, the ear shell will block the high-frequency overtone of the sound, so that there is a noticeable timbre difference compared with the front source; at the same time, due to the shielding effect of the ear shell, the sound from behind will Does not produce reloading. The auditory area of ​​the brain compares this information with the signals that have been mastered in the past, so that the sound source comes from behind.
Fourth, the single ear effect The single ear effect refers to the single ear listening and positioning function within the scope of the binaural effect principle. There is no doubt that a normal hearing person with both ears listens to the direction of the sound source mainly by listening to both ears. However, an interesting fact tells us that people don't use both ears to listen to the sound on average, but use one side of the ear.
For example, when a point source appears near the left side of the listener by 35° from the center axis, the listener hears the sound and simultaneously measures the general direction of the sound source; if the listener is the source of the sound source If the sound is attracted, then the head will be turned to the left 35° to align its central axis with the sound source, so that the binaural ear (using both eyes) can be used to identify the exact position of the sound source; if the sound source is still not accurately measured When the attraction is further strengthened, the listener will turn the head to the right with the left ear toward the sound source and move closer to the sound source until the exact position of the sound source is found. This process, that is, the three-steps of judgment → correction → seeking, reflects the systemic coordination mechanism of auditory localization, and explains the importance of single ear listening in positioning.
Fifth, the principle of bone conduction mechanism The effects of binaural effect, monaural effect and ear shell effect provide the main basis for auditory localization. It seems that the mechanism of sound source orientation of auditory organs has been fully answered. However, after careful consideration, some details in the mechanism of auditory localization have not been fully explained. For example, from the sound source behind the listener, how the human ear is identified. According to the principle of binaural effect and ear shell effect, the recognition of the sound from behind is mainly based on the shadow mask effect of the ear shell, which forms the timbre difference between the back sound and the front sound, and the feeling of the timbre is from the contrast of the brain hearing. . Tests have shown that a three-month baby can tell the sound from behind, and an adult can judge a sound from behind and never heard. It can be seen that the positioning of the sound source behind the human ear cannot be attributed only to the timbre.
Experts believe that there must be an auditory function to assist the human ear to judge the direction of the sound source from behind. This function is the bone conduction positioning mechanism.
The possibility of bone conduction positioning has always been controversial. The focus of the controversy is not whether the skull can conduct sound waves, but how much of the external sound waves are converted from air conduction to bone transmission. Some scholars believe that in the process of sound wave, when a medium enters another medium, reflection and incidence will occur on the boundary between the two media. According to Newton's second law of motion, the incidence of sound waves in the skull can be calculated. The intensity of the reflection. After calculation, the intensity of the reflected wave is 99.89% dB, and the incident wave is only 0.00003% dB, which indicates that almost all the external acoustic energy is reflected off. Therefore, it is determined that the bone conduction positioning function does not actually exist.
The author believes that the key problem is that there is no incident wave. If the existence of the incident wave is recognized, the energy specific gravity is extremely small, and above the critical limit, the human ear can feel it.
Since the hair cells on the basement membrane of the cochlea can sense the vibration of 1/10 angstroms (1 angstrom equivalent to the diameter of the nitrogen atom), the human ear can feel the micro-acoustic information introduced by the bone conduction.
We know that the ear is at a depth of about 114.99999999999999px on both sides of the skull, where the distance from the various parts of the skull is different. The outer surface of the skull is forming a bone conduction time difference between the sound waves entering the cochlea of ​​the sound source, and between the ears. There is also a bone conduction time difference. The speed of sound waves in the skull is 3013m / s, the air conduction velocity is 344m / s, the distance between the various parts of the skull and the ears is about 4.6 ~ 385px, for this reason, there is a time difference between the direct conduction of the bone conduction and the direct sound of the air conduction, the brain The auditory area quickly determines the sound source orientation based on the time difference between the bone conduction and the time difference between the bone conduction and the air conduction.
In theory, the positioning function of the bone conduction is omnidirectional. Therefore, the bone conduction positioning mechanism has an effect on the sound waves from the front, the side, the back and the top of the head. However, the bone conduction positioning function is auxiliary and secondary, which is complementary to the binaural effect and the single ear effect and the ear shell effect.
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