The inherent irregular motion and mutual repulsion of air molecules creates a static force, and this pressure is the atmospheric pressure. Sound is the vibration of air molecules, vibration of air molecules on the cross-section through which it will produce additional pressure, this additional pressure is called sound pressure. Sound pressure is much smaller than the atmospheric pressure, the general use of sound pressure level to describe the size of the sound, that is, a very small sound pressure p0=2 х10-5 Pa as a reference sound pressure, the measurement of the sound pressure p and reference sound pressure p0 ratio to take the common logarithm of the value obtained by multiplying by 20 is called the sound pressure level, the unit is the decibel (db ). Decibel (dB) is named after Bell, the American inventor of the telephone, because the unit of shellfish is too large, so the decibel is used, representing 1/10 shellfish. The operation of dB is not linearly proportional, but logarithmically proportional, and the frequency needs to be given at the same time when using dB to describe sound.