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Voice Wave | Most people know, only a few understand

When it comes to sound waves, we need to explain the mechanism of their generation: We need to give a brief introduction to sound.

The three major elements of sound are: sound source, transmission and reception. Transmission relies on media, such as substances like 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 produced by the vibration of the exhaust gas emitted by the engine in the exhaust pipe. And the human ear is a kind of receiving device.

The vibration of the medium generates sparse and dense regions of molecules, which is commonly known as sound waves (similar to water waves). The distance between adjacent molecular dense areas (or sparse areas; when you place your hand on the outlet section of the muffler, you will feel that the pressure increases and decreases intermittently; the larger ones are dense areas and the smaller ones are sparse areas) determines the frequency of the sound wave. The longer this distance, the lower the sound frequency (low frequency); the shorter this distance, the higher the sound frequency (high frequency). In GT analysis, the exhaust noise frequency generally does not exceed 1000 Hertz (the accuracy of GT will decline if the noise frequency exceeds 1000 Hertz). Among them, the part below 100 Hertz can be regarded as low frequency, 100 to 300 Hertz as medium frequency, and above 300 Hertz as high frequency. This is different from the normal division of low, medium and high frequencies in music. The volume of sound is expressed in decibels. The normal sound of a person speaking is approximately 60 to 70 decibels, while the sound of an F1 racing car can reach 100 to 110 decibels.

The causes of noise generation

First, let's understand how an 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 steps between two adjacent exhaust processes. Therefore, the gas pressure in the exhaust pipe is discontinuous. The generation of discontinuous gas pressure leads to sparse and dense areas of gas molecules, which is the same as the sound generation mechanism mentioned earlier. The exhaust sound is thus produced by the periodic discharge of engine exhaust.

The characteristics of the exhaust sound

The exhaust sound is divided into two parts: one part is related to the periodic rotation of the engine and is called the order sound; Another part is the hissing sound produced by the exhaust gas flowing and then being discharged into the air, which is called the airflow sound. The charming sound waves pursued by the modified exhaust are all octave sounds. To highlight the order sound, it is necessary to reduce the airflow sound. The common methods adopted are using sound-absorbing cotton for sound absorption and thickening the exhaust tailpipe to lower the flow velocity of the exhaust gas, thereby reducing the airflow sound.

The order sounds can be further divided into many parts. Depending on the number of engine cylinders, the names of the order sounds also vary. For instance, a 4-cylinder engine corresponds to 2nd, 4th, and 6th order sounds, while a 6-cylinder engine is further classified into 3rd, 6th, and 9th order sounds, etc. Professional exhaust sound design usually records the exhaust sound through professional equipment and analyzes it to obtain a color spectrum diagram. Take the color spectrum diagram of the exhaust sound of a certain 4-cylinder engine as an example: