What Is Psychoacoustics?

Psychoacoustics is a marginal subject that studies the relationship between sound and the hearing it causes. The term psychoacoustics seems puzzling, but it's actually very simple. It means "the way the human brain interprets sound." All forms of compressed audio use powerful algorithms to remove audio information that we cannot hear. For example, if I shout with a throat and step on my foot at the same time, you will hear my shout, but you may not hear my stepping sound. By removing the footsteps, the amount of information is reduced and the file size is reduced, but it sounds the same.

Psychoacoustics

Psychoacoustic model is a mathematical expression model of the statistical properties of human hearing, and it explains the physiological principles of human hearing.

The psychoacoustic model can greatly reduce the bandwidth of digital audio signal transmission under the condition that the subjective hearing sense is not much deteriorated. It is mainly based on the physiological structure and perceptual mode of the human auditory organ. Through corresponding processing of digital audio signals, it removes inaudible signal components and introduces inaudible distortion to achieve a compression ratio that cannot be achieved by ordinary entropy coding.

Due to the complexity of the human ear auditory system, humans have so far not been able to fully explain some of its problems with its mechanism and auditory characteristics from the perspective of physiological anatomy. Therefore, the study of the auditory characteristics of the human ear is limited to psychoacoustics and speech acoustics. The hearing range of the human ear for sounds of different intensities and frequencies is called the sound field. Within the range of the human ear, the subjective perception of sound auditory psychology mainly includes characteristics such as loudness, pitch, and timbre, masking effects, and high-frequency localization. Among them, loudness, tone, and timbre can be used to subjectively describe any complex sound with three physical amplitudes, frequencies, and phases. Therefore, it is also called the "three elements" of sound. Masking effects and other characteristics are particularly important, which are the basis of psychoacoustics.

A fringe discipline that studies the relationship between sound and the hearing it causes. It is both a branch of acoustics and a branch of psychophysics. Psychoacoustics could have included compound sounds such as speech and music and their perceptions. These visible speech acoustics, musical acoustics, and other articles are limited to the more basic and simple psychoacoustic phenomena, that is, sounds that can just cause hearing-hearing threshold; sounds determined by the parameters of sound intensity, frequency, frequency spectrum and duration Subjective attributes of ---- loudness, pitch, timbre and pitch; some special psychoacoustic effects related to composite sounds-aftertaste, masking, non-linear, binaural effects.

The branch of optics corresponding to psychoacoustics is photometry and

Listening threshold is divided into intensity threshold and difference threshold. The sound is not strong enough to cause hearing. The minimum sound pressure level that can cause hearing 50% of the time is called the intensity threshold (also called the hearing threshold). There are individual differences in the hearing threshold, so the so-called normal hearing threshold can only be a statistical average of the hearing threshold of some young people with normal hearing. The hearing threshold varies with frequency. The threshold between 500 and 4000 Hz is the lowest, and the thresholds of high-frequency and low-frequency sounds above and below them are higher. For example, the threshold of 20Hz pure tone is about 70dB higher than the threshold of 1000Hz pure tone, and the threshold of 10000Hz pure tone is also higher than the threshold of 1000Hz pure tone. About 10dB higher. The most sensitive frequency is about 3000Hz, and the vibration amplitude of air molecules can be heard at 10-11m, which is only one tenth of the diameter of hydrogen molecules. The hearing threshold increases with age, especially in the high-frequency part, which is manifested as elderly deafness, such as 70-year-olds, the hearing threshold of 5000Hz pure tone is increased by about 45dB. The concept of hearing threshold also includes the difference threshold, that is, the smallest perceptible difference between two sounds that cause a difference in hearing. As far as frequency is concerned, the human ear with experience around 63Hz can distinguish the difference between two pure tones with a difference of 0.5Hz, but this threshold must be increased to 1.4Hz at 1000Hz. The higher the frequency, the larger the difference threshold. The human ear can distinguish the smallest intensity difference of 0.25dB (1000-4000Hz, more than 70dB). When the intensity is low or the frequency is higher or lower, the intensity difference threshold is larger. In the whole hearing range, there are about 340,000 distinguishable sounds.

Psychoacoustic aftertones

For pure tones, the pitch of the sound is mainly determined by the frequency, and for composite sounds composed of fundamental waves and harmonics, since H.von Helmholtz, it is generally believed that the pitch of the composite sound is determined by the frequency of the fundamental wave, because the fundamental wave The amplitude is dominant in the spectrum, and it feels so. However, experiments show that if the fundamental frequency of the composite sound is very weak, or even completely filtered out, its pitch remains the same as that of the fundamental frequency. This tone that loses the fundamental frequency is called a residual tone. There is also a residual effect in daily life. People's speech is based on the low frequency sound from the vocal cords. In the phone, although it was filtered, the tone of the speech was not affected. The phenomenon of aftertones is noticed because it involves a basic auditory theory, namely whether the pitch depends on frequency or periodicity. Current research is inconclusive.

Psychoacoustic acoustic masking

If an acoustic signal appears with a kind of noise at the same time, it will become weak or completely inaudible, which means that the signal's hearing threshold is increased. This is the masking effect, and noise masks the signal. The magnitude of the masking effect depends on the relationship between noise and signal frequency. Generally speaking, the closer the frequency of the signal and noise, the greater the masking, and the masking of low-frequency noise by high-frequency signals is often greater than the masking of low-frequency signals by high-frequency noise. Through masking experiments on pure tone signals with variable bandwidth noise, it is found that when the noise with 1000Hz as the center frequency increases the bandwidth, its masking effect on 1000Hz pure tone signals also increases. However, after the bandwidth is increased to 100 Hz, further increase will not affect the masking change. That is, the masking effect of this noise is limited to this frequency band, and sounds outside it have no effect. This 100 Hz band is called the critical band. It widens as the frequency increases.

Psychoacoustic nonlinearity

The transmission characteristics of the human ear, like other transducers, have some non-linear characteristics. Its product is the so-called "harmony" feeling. Consonants include two types: difference and harmony. Example of difference: When two pure tones are emitted at the same time at 400Hz and 500Hz, it sounds carefully that there are also sounds whose frequency is the difference (100) and its harmonic frequency (200,300). This is the difference. The frequency of the harmony is the sum of the original two pure audio frequencies, which in this example is 900Hz. It is similar to the difference, but it is weaker and has a higher pitch. In addition, there is an appropriate frequency and intensity relationship that can suppress or reduce the response (perception) of another tone. These phenomena are generally explained by the non-linear response of the cochlea.

Binaural effect Many hearing effects are determined by people having two ears. The main factors for sound source localization are the time difference and intensity difference between the two ears (see Bioacoustics). Due to the resonance and reflection of the head, auricle, external auditory canal, etc., the spectrum of the sound heard is modulated. The sound from the right reaches the right ear first, and the intensity is stronger than that received by the left ear. The direction of the sound source is often determined by the rotation of the head. The localization of composite sound is easier than pure sound. Pure sound, especially 2000-3000Hz pure sound, is particularly difficult to locate. For example, the snoring sound of chirp is like this, although it is not strictly pure. In the audible range, the directivity of the auricle is not significant, but it still has an effect on positioning. At low frequencies, there is not much difference in the intensity of the two ears, and the positioning is mainly based on the phase or time factor. At high frequencies, the phase changes are complex and the intensity difference is more important. At IF, positioning is more dependent on the combined effects of time and intensity.