Sound is divided into pure tone, polyphony, noise and so on. According to the periodic characteristics of sound, it can be divided into periodic sound and aperiodic sound. Periodic sounds include pure tones and polyphones because their wave patterns are repetitive. Aperiodic sound is composed of many sounds with different frequencies, intensities and phases.
noise
The essence of sound is fluctuation, and the affected air will vibrate. When the vibration frequency is 20-20000Hz, the feeling produced by acting on the human eardrum is called sound. The sound source can be solid or fluid (liquid and gas) vibration. The medium of sound includes air, water and solid, which are called air sound, underwater sound and solid sound respectively. Noise monitoring mainly discusses air sound.
Human beings live in a sound environment, through which they talk, express their thoughts and feelings and carry out various activities. But some sounds can also bring harm to human beings. For example, the deafening sound of machines, the sound of planes whizzing by and so on.
These unnecessary sounds for people's life and work are called noise. From the physical phenomenon, all irregular or random sound signals are called noise. The judgment of noise is also related to people's subjective feelings and psychological factors, that is, all unwanted interference sounds are called noise. For example, in some cases, music may also be noise under certain emotional conditions.
There are four sources of environmental noise: traffic noise, including noise generated by cars, trains and planes; Factory noise, such as noise generated by blowers, steam turbines, looms and punches; Construction noise, such as the sound made by pile drivers, excavators and concrete mixers; Social noise, for example, excessive sound from speakers and tape recorders.