How exhaust muffler works?

This article contains the following:

• Working principle

• Structure

Working principle 

As a necessary part of automobile, automobile muffler is mainly used to reduce the noise produced when the engine of motor vehicle works. Because the difference of the length of the two pipes is equal to half the wavelength of the sound wave emitted by the car, the sound intensity is weakened by the interference of the two sound waves at the time of superposition, and the sound is reduced, thus the silencing effect is achieved. 

Exhaust mufflers mainly use porous sound-absorbing materials to reduce noise. The exhaust muffler is formed by fixing the sound-absorbing material on the inner wall of the air passage or arranging in a certain way in the pipe. When the sound wave enters the exhaust muffler, a part of the sound energy is rubbed in the pores of the porous material and the heat energy consumption is dissipated, which weakens the sound wave through the muffler. Exhaust muffler is like a pure electrical resistance circuit, sound-absorbing material is similar to resistance. The muffler is therefore called a exhaust muffler. Exhaust muffler has good effect on medium and low frequency silencing. 

the resistant muffler is composed of tubes and chambers of the mutant interface, like an acoustic filter, similar to an electrical filter. each small chamber with a tube is a mesh of the filter. the air quality in the tube is equivalent to the electrical inductance and resistance, called acoustic mass and acoustic resistance. The volume of air in the small chamber is equivalent to the electrical capacitance, known as sound cis. Like electrical filters, each small chamber with a tube has its own natural frequency. When a sound wave with various frequency components enters the first short tube, only certain frequencies near the natural frequency of the first mesh can reach the second short tube through the mesh hole, while the sound wave of the other frequencies cannot pass through the mesh hole. It can only be reflected back and forth in the small chamber, so we call this structure with filter function to the acoustic filter. Select the appropriate tube and chamber for combination. The noise of some frequency components can be filtered out to achieve the purpose of silencing.  Exhaust muffler is suitable for eliminating medium and low frequency noise. 

The impedance composite muffler is formed by combining the resistive structure and the resistive structure in a certain way. 

The micro perforated muffler is generally made of pure metal sheet with thickness less than 1mm, and perforated with a bit with aperture less than 1mm on the sheet with a perforation rate of 1% and 13%. Selecting different perforation rate and different cavity depth of plate thickness can control the spectrum performance of muffler and make it obtain good noise attenuation effect in the required frequency range. 

Structure

The structure of the hole muffler is a closed end of the straight pipe, the pipe wall drilled many holes. The principle of eyelet muffler is based on the spectrum of jet noise, if the total area of the nozzle is kept constant and replaced by many small vents, when the airflow passes through the hole, the spectrum of jet noise will move to high frequency or ultra-high frequency, so that the audible composition in the spectrum is obviously reduced, thus reducing the interference and injury to human beings. 

The basic principle of active muffler is to use electronic equipment to produce another sound wave with the same sound pressure size and opposite phase in the original sound field in a certain range to offset the original sound field. This muffler is a set of instrument devices, mainly composed of microphones, amplifiers, phase shift devices, power amplifiers and loudspeakers.