Last time, we learned what sound is: vibrations travel in elastic media (gases, solids, and liquids) in the form of waves, which are called sound waves. Sound waves in a certain frequency range act on the human ear to produce the sensation of sound. Today we're going to learn about sound measurement.

Sound waves are density waves, which make air denser and thinner. As the air becomes denser, the pressure increases; As the air becomes thinner, the pressure decreases. In this way, the atmospheric pressure fluctuates rapidly due to the presence of sound waves. This fluctuation is called sound pressure. The higher the sound pressure, the stronger the sound; The lower the sound pressure, the weaker the sound. Therefore, people use sound pressure as a measure of sound size. Sound pressure is usually expressed as P and is measured in newtons per square meter, or microbars, where 1 microbar =0.1 Newtons per square meter.

The sound pressure of the sound just heard by the normal ear is called threshold sound pressure; The sound pressure of the sound that can make the ear feel pain is called the pain threshold sound pressure; From hearing threshold to pain threshold, the absolute value of sound pressure varies by a million times. Therefore, it is very inconvenient to use the absolute value of sound pressure to represent the intensity of sound. For the sake of convenience, as in the case of wind and earthquake, the magnitude of the sound is denoted by a multiply-ratio relation, the magnitude, which is the sound pressure level.

The sound pressure level is expressed as Lp, and its unit is the decibel (dB) we often hear. The mathematical e­xpression is as follows:

With the concept of sound pressure level, it is possible to change the range of millions of times expressed by the absolute value of sound pressure to the range of 0-160 decibels. Even the intense noise of a jet with a sound pressure of hundreds of newtons per square meter is only 140-150 decibels.

It can be seen from the formula that every change of 20 dB is equivalent to a 10-fold change in the sound pressure value, every change of 40 dB is equivalent to a 100-fold change in the sound pressure value, and every change of 60 dB is equivalent to a 1000-fold change in the sound pressure value. So a 20 or 40 decibel increase or decrease in sound is quite a change.

Perhaps you do not have a clear concept of decibels, we give the following set of numbers for your reference:

0dB is the faintest sound a person can hear;

30dB ~ 40dB is the ideal quiet environment;

More than 50dB can affect rest and sleep

More than 70dB will affect learning and work;

More than 90dB will affect hearing;

If suddenly exposed to noise of up to 150db, the eardrum will rupture and bleed, and there will be a complete loss of hearing in both ears.

Now you should have a preliminary understanding of the volume of sound in our daily life and work.