Three key factors for voice chip programming

Due to the complexity of the human hearing system and the specificity of its physiological structure, it is impossible to get a perfect interpretation from the perspective of physiological anatomy. The range of human hearing of different intensities and frequencies is collectively called the sound domain. The subjective feeling of human hearing is roughly derived from the three characteristics of loudness, pitch and timbre, also known as the "three elements" of sound. This is also the main component of audio positioning when burning general voice chips.

Loudness

Loudness, also known as sound intensity or volume, indicates the strength of sound energy, which is mainly determined by the amplitude of the sound wave. The loudness of sound is generally measured by sound pressure (dyne/square centimeter) or sound intensity (watt/square centimeter). The unit of sound pressure is Pascal (Pa). The logarithm of its ratio to the reference sound pressure is called sound pressure level, and the unit is decibel (dB). The psychological feeling of loudness is generally measured in the unit of sones, and the loudness of a pure tone of 1kHz and 40dB is defined as 1 sone.

The relative amount of loudness is called loudness level, which represents the logarithm of the ratio of a certain loudness to a reference loudness, and the unit is phon. That is, when the human ear feels that a certain sound is as loud as a pure tone of a single frequency of 1kHz, the decibel number of the sound pressure level of the sound is its loudness level. It can be seen that the concepts of these two units are completely different, both objectively and subjectively. Except for a pure tone of 1kHz, the value of the sound pressure level is generally not equal to the value of the loudness level, so please pay attention to it when using it. Loudness is the basis of hearing.

The intensity range of normal human hearing is 0dB-140dB (some people think it is -5dB-130dB). Of course, even if the sound is louder, the human ear cannot hear it (i.e. the loudness is zero) if it is beyond the audible frequency range (i.e. frequency domain) of the human ear. However, within the audible frequency domain of the human ear, if the sound is too weak or too strong, the human ear cannot hear it either. When the sound is weakened to the point where the human ear can just hear it, the sound intensity at this time is called the "hearing threshold".

Generally, the measurement is based on 1kHz pure tone. The sound pressure that the human ear can just hear is 0dB (usually greater than 0.3dB is felt) and the sound intensity is 10-16W/cm2, which is defined as the loudness level of 0. When the sound is enhanced to the point where the human ear feels pain, this threshold is called the "pain threshold". The measurement is still based on 1kHz pure tone. The sound pressure level that makes the human ear feel pain is about 140dB.

pitch

Pitch is also called tone, which refers to the subjective feeling of the human ear about the pitch of a sound. Objectively, the pitch is mainly determined by the fundamental frequency of the sound wave. The higher the frequency, the higher the pitch, and vice versa. The unit is Hertz (Hz). The unit of subjective pitch is "mel", and the pitch of a 1kHz pure tone with a loudness of 40 fang is usually defined as 1000 mel. Hertz and "mel" are also two different concepts of pitch, but they are related units.

The human ear's perception of loudness has a range from the threshold of hearing to the threshold of pain. The human ear's perception of frequency also has a range from low audible frequency 20Hz to high audible frequency 20kHz. The measurement of loudness is based on a 1kHz pure tone. Similarly, the measurement of pitch is based on a pure tone with a sound intensity of 40dB.

Experiments have shown that the change between pitch and frequency is not a linear relationship. In addition to frequency, pitch is also related to the loudness and waveform of the sound. The change in pitch is proportional to the logarithm of the relative change of the two frequencies. Regardless of the original frequency, as long as the two 40dB pure tone frequencies increase by 1 octave (i.e. 1 times), the pitch change felt by the human ear is the same. In musical acoustics, the continuous change of pitch is called glissando, and 1 octave is equivalent to the musical sound being raised by one octave. According to the actual perception of the human ear to pitch, the frequency range of human speech can be relaxed to 80Hz--12kHz, the musical sound is wider, and the effect sound is even wider.

Timbre

Tone, also known as timbre, is determined by the harmonic spectrum and envelope of the sound waveform. The clearly audible sound produced by the fundamental frequency of the sound waveform is called the fundamental tone, and the sound produced by the tiny vibrations of each harmonic is called the overtone. A single frequency sound is called a pure tone, and a sound with harmonics is called a polyphonic tone. Each fundamental tone has an inherent frequency and overtones of different loudness, which can be used to distinguish other sounds with the same loudness and pitch.

The proportion of each harmonic of the sound waveform and the decay over time determine the timbre characteristics of various sound sources. The envelope is the line between the peaks of each cycle. The steepness of the envelope affects the transient characteristics of the sound intensity. The timbre of sound is colorful and ever-changing. The goal of high-fidelity (Hi-Fi) audio is to transmit, restore and reconstruct all the characteristics of the original sound field as accurately as possible, so that people can actually feel the stereo surround sound effects of various immersive listening sensations such as sound source positioning, spatial envelopment, layer thickness, etc.