Noise propagation is generally divided into three stages: noise source, transmission route and receiver. The propagation path includes the travel process of sound waves in the form of reflection and diffraction. The principle of noise control, is before the noise to reach the eardrum, damping, vibration isolation, sound absorption, sound insulation, muffler, personal protection and seven measures such as construction layout, and try our best to weaken or reduce the vibration of the sound source, or the ability to absorb the propagation of sound energy, or barriers, make the voice of all or part of reflection, abate the noise in the eardrum, so can achieve the goal of control noise.

According to the three stages of noise propagation, three different ways can be used to control the noise.

(1) reduce noise from sound source

This is the most fundamental aspect, including the development and use of low-noise equipment and processing technology and other measures. The main causes of noise are the vibration of airflow, the vibration caused by solid collision and friction, and the vibration caused by magnetostriction of iron core. The aerodynamic noise of fans, jet aircraft and automobiless can be controlled by smooth airflow passage and reduced airflow speed. Mechanical noise of lathes, looms and riveting and forging machines can be controlled by reducing friction or impact with lubricating or damping materials. Electromagnetic noise of motors and transformers is mostly reduced by muffler. Others, such as using silent hydraulic pressure instead of high-noise hammering, welding instead of riveting, improve the design of fan blades and improve the machining precision of transmission parts such as gears, adjust the balance of moving parts to reduce the vibration caused by eccentricity, etc., can greatly reduce the noise from the sound source.

(2) control the noise on the transmission path

This is the use of acoustic processing methods, such as sound absorption, sound insulation, vibration isolation and damping to reduce the noise. Since noise is transmitted by air or by equipment or by the building itself, this method can also be effectively controlled. The use of glass wool, felt, foam plastic and sound absorption brick and other sound absorption materials, as well as resonant sound absorption and micro-perforated plate sound absorption structure, can reduce the reflection of indoor noise, noise can be reduced by 10 ~ 15 db. Sound insulation and sound insulation machine cover used by the sound insulation material requirements of large weight, so generally more brick, reinforced concrete, steel plate and plank, in addition to the structural seal, no holes. Sealing cover can reduce in general, high noise 10-35 db, double layer structure than single layer structure in weight can be reduced by about 70%, it is more economical, but to pay attention to prevent the middle air layer resonance. The soft acoustic absorbing material can not only absorb sound but also reduce the vibration propagation. With metal plate when the hood, the lining for sound absorption treatment, should also be coated damping materials, the outer wall outside the connected to the machine components (supporting device) and viscous resistance (energy consumption equipment), commonly used have shear rubber, metal spring vibration isolation material, cork, mineral cotton, glass fiber and air cushion, etc., widely used in domestic at present two before, the latter is the development direction. Generally, the low frequency noise of vibration isolation pad can be reduced by about 10 db. Damping material is a few materials with big internal friction loss, wait like pitch, soft rubber and other high polymer coating, they can consume the vibration energy of metal plate and become heat energy dispersive lose, restrain the bending vibration of metal plate thereby, make radiate do not give noise.

A thorough understanding of the sound mechanism can reduce noise economically and effectively.

(3) stop the noise at the receiving point

In case of failure of the above two control methods, personal protective measures such as earplugs, earmuffs, acoustic wax cotton and protective masks should be adopted. These protective devices are required to be airtight for sound insulation, but are sometimes designed to allow some low-frequency or low-intensity sound to be absorbed, so that noise can be stopped without interfering with the conversation.

However, the most effective and economic way to control urban noise is not technical measures, but publicity and education, the source of noise, harm and control measures to explain to the public, widely mobilize the public to control the noise. Secondly, such as strong administrative management, among which the most important is the national standards and norms to control noise and the implementation of the necessary rules and regulations. Regional planning should reasonably keep residential areas away from sound sources to reduce the impact of traffic and industrial noise. Others, such as limiting the use of loud loudspeakers, changing the tram to run underground or to trackless, and strengthening the management of traffic vehicles, construction machinery, household noise, factory noise and so on, can achieve great results.