I. Sound Generation
Sound comes from the vibration of an object and is the sound wave produced by the vibration of an object.
In the research of acoustic technology, we call the object that can produce sound "sound source". Sound source is not necessarily non-solid vibration, liquid and gas vibration can also sound.
EXAMPLE: The exhaust noise of high pressure vessel during exhaust and emptying in industrial production is the result of air vibration caused by the interaction of high-speed airflow and surrounding static air.
2. Sound transmission
Sound energy can be transmitted through intermediate medium, which must be elastic medium. Air has both the natural characteristics of mass and elasticity. When sound propagates in the air, the air particle itself does not propagate with the sound wave, but vibrates reciprocally near its equilibrium position. Sound wave propagates in the air, which causes the vibration of air particles in the same direction as that of wave propagation. So the sound wave in the air is longitudinal wave, also known as the tight wave.
In air, at room temperature, the sound velocity is about 340 m/s; in water, it is about 1450 m/s; in steel, it is about 5000 m/s; in glass, it is about 6000 m/s; in rubber, it is about 40-150 m/s; the relationship between sound velocity, wavelength and frequency is as follows:
C = Twin. Lambda = 1/T (the number of complete vibrations in one second)
C - sound velocity, M / s; T - period, s;
Twin - Frequency, Hz;
Sound radiates from the sound source to the surrounding area, and its front area expands with the increase of propagation distance. Sound energy is dispersed, and the corresponding sound energy through unit area is small. The intensity of sound generally decreases with the increase of distance, because the sound energy emitted by the source is constant every second.
Like light waves, acoustic waves also produce reflection, refraction and diffraction phenomena in the process of propagation.
3. Physical Measurement of Sound
Sound Intensity: The sound energy passing through a unit area perpendicular to the direction of propagation in a unit time is called sound intensity. Sound intensity is a sign to measure the strength of sound, usually expressed in I, measured in watt/meter 2, W/m2.
Sound pressure: Sound wave in the process of propagation in the air, causing the vibration of air particles, resulting in changes in air density. At this time, air pressure in the vicinity of atmospheric pressure fluctuations rapidly, this part of pressure fluctuations is called sound pressure. Sound pressure is usually expressed in terms of P. The basic unit is Pa. It is sent to Newton/m, N/m2.
Sound power: The total energy emitted by a sound source in a unit time. Sound power is usually expressed in W, the unit name is Watt, and the unit symbol W.
Frequency doubling: The range of sound frequency that human ear can hear is generally between 20 and 20 000 Hz. In noise control engineering, in order to facilitate acoustics research and noise pollution control, the sound spectrum is divided into eight frequency doubling bands, that is, frequency doubling.
The central frequency is 63,125,250,500,1000,2000,4000,8000Hz.