When it comes to sound waves, we need to explain the mechanism behind their generation: we need to give a brief introduction to sound, and Shuifu
Hanyang Machinery Technology Co., Ltd. will guide you through it.
The three major elements of sound are: sound source, propagation, and reception. Communication relies on media such as air, water, and steel pipes;
The sound source is an object that excites the vibration of the medium, for example, the exhaust sound wave is generated by the vibration of the
exhaust gas emitted by the engine in the exhaust pipe; And the human ear is a receiving device.
The vibration of a medium produces sparse and dense regions of molecules, which are commonly referred to as sound waves (similar to water waves).
The distance between adjacent densely populated areas (or sparse areas, which can be felt by placing your hand on the outlet section of the muffler to
experience intermittent increases and decreases in pressure, with the larger being the dense area and the smaller being the sparse area) determines
the frequency of the sound wave. The longer this distance, the lower the sound frequency (bass), and the shorter this distance, the higher the sound
frequency (treble). In GT analysis, the frequency of exhaust noise generally does not exceed 1000 Hz (the accuracy of GT will decrease if the noise
frequency exceeds 1000 Hz). The part below 100 Hz can be considered bass, 100 to 300 Hz can be considered mid-range, and above 300 Hz belongs to
treble, which is different from the normal classification of low, medium, and high frequencies in music. The size of sound is expressed in decibels. The
normal sound of a person speaking is about 60-70 decibels, while the sound of an F1 racing car can reach 100-110 decibels.
Reasons for noise generation
Firstly, let's understand how the engine works. A common four stroke engine has four working stages: "1-intake", "2-compression", "3-ignition", and "4-
exhaust". There are three other working stages between adjacent exhaust processes, so the gas pressure in the exhaust pipe is discontinuous. The
discontinuous gas pressure produces sparse and dense areas of gas molecules, which is similar to the sound generation mechanism mentioned
earlier. The exhaust sound wave is generated by the periodic exhaust of the engine exhaust gas.
Characteristics of exhaust sound waves
The exhaust sound is divided into two parts: one part is related to the periodic rotation of the engine, called step sound; The other part is the hissing
sound generated by the flow of exhaust gas and its emission into the air, called the airflow sound. The charming sound waves pursued by modified
exhaust are all step sounds. In order to highlight the order sound, it is necessary to reduce the airflow sound. The usual methods are to use sound-
absorbing cotton to absorb sound and thicken the exhaust tailpipe to reduce the flow velocity of the exhaust gas, thereby reducing the airflow sound.
The infrasound can be divided into many parts, and the names of infrasound vary depending on the number of cylinders in the engine. For example, 4-
cylinder engines correspond to 2nd, 4th, and 6th order, while 6-cylinder engines are divided into 3rd, 6th, and 9th order. Professional exhaust sound
design usually records and analyzes the exhaust sound through professional equipment to obtain a color spectrum.