What are Microperforations?

Micro-perforated plate muffler is a new type of muffler made of micro-perforated plate sound absorption structure. Do not use any porous sound-absorbing material, only drill a lot of micro holes in thin metal plates with a thickness of less than 1 mm. The diameter of these micro holes is generally about 0.5 to 1 mm, and the perforation rate is between 1% and 3%. With a certain cavity, it becomes a sound-absorbing structure of a micro-perforated plate. Micro-perforated plate is a sound absorbing element with high sound resistance and low sound quality.

The acoustic structure of micro-perforated plate has a wide range of applications in the field of sound attenuation technology. Micro-perforated plate muffler is made of metal perforated sheet. Common micro-perforated plate can be steel plate (tube), stainless steel plate (tube), alloy plate (tube), etc. Material production, because the air layer behind the micro-perforated plate can be filled with porous rock wool material, that is, the principle of resistive sound absorption of sound-absorbing materials is used to further achieve the purpose of noise reduction. Its sound absorption coefficient is high, the absorption frequency band is small, the pressure loss is small, the airflow regeneration noise is low, and it is easy to control. In order to obtain a wide band and high absorption effect, a double-layer micro-perforated plate structure is generally used. The size of the cavity between the micro-perforated plate and the shell and between the micro-perforated plate varies according to the frequency band that needs to be absorbed. When absorbing low frequency, intermediate frequency and high frequency, the cavity size is 150-200mm, 80-120mm, and 30 in order. 50mm, the depth of the front cavity of the double-layer structure should generally be smaller than the back cavity, and the ratio of the depth of the front cavity to the front cavity is not greater than 1: 3. For the influence of sound transmission, a transverse partition can be added every 500mm in the cavity of the micro-perforated plate muffler.

If a thin metal plate is used, it has the unique advantages of high temperature resistance, moisture resistance, fire resistance and corrosion resistance, strong applicability and also suitable for use in high-speed air flow fields, such as venting exhaust systems, internal combustion engines, air compressors and other exhaust systems. Minimal pressure loss.

Micro-perforated plate muffler has the following characteristics:

1. Not only has the characteristics of impedance composite muffler, but also has a wider muffler frequency band;

2. Metal structure can be used instead of sound-absorbing material, and it also has the characteristics of high temperature resistance, oil resistance, water resistance, fire resistance and corrosion resistance;

3. The micro-perforated plate has a low perforation rate, a smooth surface, and a small resistance to the air flow, which is suitable for equipment with low resistance requirements;

4. Low airflow regeneration noise, suitable for high-speed airflow, especially for venting exhaust;

5. It can be used in medicine, food and other industries with strict requirements on sanitary environment.

The simplest form of micro-perforated plate muffler is a single-layer tube muffler, which is a resonant sound-absorbing structure. For low-frequency sound, when the wavelength of the sound wave is larger than the size of the resonance cavity (cavity), the amount of sound attenuation can be calculated by the calculation formula of the resonance muffler, that is:

In the formula:

,

;

rrelative sound resistance;

S-channel cross-sectional area, m ² ;

Vthe volume of the cavity behind the plate, m ³ ;

cvelocity of sound in air, m / s;

ffrequency of incident sound wave, Hz;

f _r resonant frequency of micro-perforated plate, Hz; it can be calculated by:

Where: t '= t + 0.8d + 1 / 3PD;

tmicro-perforated plate thickness;

Pperforation rate;

Dthe depth of the cavity behind the plate;

dperforation diameter.

For IF muffler, the amount of muffler can be calculated by using the resistive muffler formula;

For high-frequency noise, the amount of noise cancellation can be calculated using the following empirical formula:

In the formula: vair velocity, m / s; its applicable range is 20 ~ 120 m / s.

Therefore, the amount of noise reduction is related to the flow velocity, the flow velocity increases, and the noise reduction performance deteriorates. The metal micro-perforated plate muffler can withstand the impact of higher airflow. When the flow rate is 70 m / s, there is still 10 dB of muffler.

Micro-perforated plate mufflers often use double-layer micro-perforated plates in series, which can widen the sound absorption band. For low-frequency noise, when the resonance frequency is reduced by D ₁ / (D ₁ + D ₂ ) times (D _{1 and} D ₂ are the depths of the front cavity and the back cavity of the double-layer micro-perforated plate, respectively), the absorption frequency is extended to the low frequency by 3.5 times . ^[2]