Introduction to various conventional audio encodings (or formats) Part 3
Hearing model import: Experts have found that the human ear has a shadow effect through long-term acoustic research.
The sound signal is actually a type of energy wave, which propagates in air or other media. The most direct response of the human ear to the amount of sound energy, that is, the volume or pressure of the sound, is to hear the size of the sound. We call it the volume, which means the volume. The unit of energy is the decibel (dB). Even sounds of the same volume can be perceived by people as different in size due to their different frequencies. The 4000 Hz frequency is the easiest for the human ear to hear. It doesn’t matter if the frequency increases or decreases, even if the volume is the same, everyone will feel the sound becomes smaller. But when the volume drops to a certain level, the human ear cannot hear it, and each frequency has a different value.
You can see that this curve basically forms a V. When the frequency exceeds 15000 Hz, the human ear will feel that the sound is very small. Many people who are not very good at hearing cannot hear the frequency of 20000 Hz at all, no matter how loud it is… When the human ear hears two sounds with different frequencies and different volume at the same time, the one with the lower volume will also be ignored. For example, it is hard to hear the sound of the computer cooling fan during the day, but it becomes a noise source at night. According to this principle, the encoder can filter out many inaudible sounds to simplify information complexity and increase the compression ratio without significantly reducing sound quality. This shading is called the simultaneous shading effect. However, sound A is protected by sound B. If A is within the protection range centered on B, the protection will be more obvious. This range is called the critical bandwidth. The critical bandwidth of each frequency is different and the higher the frequency, the larger the critical bandwidth.
