(1) Main causes of exhaust noise

Exhaust noise is the main component of engine noise. Its noise level is generally 10~15dB(A) higher than that of the engine itself. Exhaust noise is the part of engine noise with the greatest energy and the most components. Its fundamental frequency is the engine firing frequency, and the overall exhaust noise spectrum should show the extension of the fundamental frequency and its higher harmonics.

The main components of exhaust noise are as follows:

1) Periodic exhaust noise. The frequency of low-frequency pulsation noise caused by exhaust is generally 63~125Hz, with noise levels as high as 105~125dB(A).

2) Resonance noise of the gas column inside the exhaust duct.

3) Resonance noise of the cylinder.

4) Jet noise generated when high-speed airflow passes through the exhaust valve annulus and bent ducts.

5) Vortex noise, together with regenerative noise generated in the exhaust system under the excitation of pressure waves inside the pipe, forms a continuous high-frequency noise spectrum, with frequencies all above 1000Hz. As airflow velocity increases, the frequency rises significantly.

(2) Methods for controlling exhaust noise

Silencers are the basic method for controlling exhaust noise. Proper selection of a silencer (or silencer combination) can reduce exhaust noise by more than 30~40dB(A). According to the silencing principle, silencer structures can be divided into two main categories: absorptive silencers and reactive silencers:

1) Absorptive silencer

An absorptive silencer, also known as an industrial-type silencer, uses porous sound-absorbing materials arranged in a certain way inside the duct. When airflow passes through the absorptive silencer, the sound waves cause the air and fine fibers inside the pores of the sound-absorbing material to vibrate. Due to friction and viscous resistance, sound energy is converted into heat energy and absorbed, thereby achieving the silencing effect.

2) Reactive silencer

A reactive silencer, also known as a residential-type silencer, uses an appropriate combination of ducts of different shapes and resonance chambers. Through reflection and interference caused by acoustic impedance mismatch resulting from changes in the duct cross-section and shape, it achieves noise attenuation. Its silencing effect is related to duct shape, size, and structure. It is generally highly selective and suitable for reducing narrow-band noise and low- to mid-frequency noise.

Typically, a combination of a corrugated vibration isolator, an industrial-type silencer, and a residential-type silencer is used to effectively isolate the transmission of exhaust vibration and exhaust noise.

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