Audio Frequency and the Unit of Volume

Human ear can distinguish the sound from 20 Hz to 20,000 Hz, which is called “the human ear sound range”. Human can not hear infrasonic wave which is lower than 20 Hz and supersonic wave which is higher than 20 KHz. The sound that we can hear includes many frequencies, which can be divided into 8 or 24 “frequency bands”. We call the 8 frequency bands octave and the 24 frequency bands 1/3 octave. There are different sound intensity and sound pressure in different frequency bands of sound of voice.
We usually use SPL (sound pressure level) to describe sound. SPL is the sum up of all octaves or 1/3 octaves. The sensitivity of human ear is decided by the frequency of sound. Human ear is most sensitive to the sound from 2500 Hz to 3000 Hz, and is less sensitive to low frequency sound. Therefore, the sum up of all octaves or 1/3 octaves can not effectively reflect the frequency nonlinearity reaction of human ear.
Take “A” weighting sound level as an example, before summing up the sound level of low frequency and high frequency, sound level would be weaken. The sum up of all figures in DB is widely used for it can more accurately reflect the reaction of human ear to sound frequency. The measuring device of sound level usually has weighting network, so as to provide numerical reading of DB. In addition, DB is the unit of volume. The higher the DB is, the bigger the sound will be. When calculating DB, for each 10 additional DB, the sound will be 10 times bigger, which means that 20 DB is 10 times more than 10 DB in power, and 30 DB is 10 times more than 20 DB in power, and so on. The power of ten 10 DB sound sources is equal to 20 DB’s power.
However, it is theoretical that human ear can distinguish sound from 20 Hz to 20,000 Hz. In reality, it is not true. And there are no speaker boxes which can cover the frequency range of 20HZ----20KHZ. One’s hearing ability is closely related to his/her living environment and physiological conditions. The human threshold of feeling is about 1 DB. Well-fitting living environment should be between 15 DB to 45 DB. To an ordinary people,  0－2 0 DB is so quite that they hardly feel; 2 0－4 0 DB is like soft voice;  4 0－6 0 DB is medium sound; 6 0－7 0 DB indoor will be noisy and harm for nerve;  7 0－9 0 DB is pretty noisy and harm for nerve cell; 9 0－1 0 0 DB is upgrading the noise and weakening hearing; 1 0 0－1 2 0 DB is hard to bear and will be deaf in one minute.