What are the three standard weighting networks for noise meters?

　　The A network simulates the human ear's response to the 40-square pure tone in the equal loudness curve. Its curve shape is opposite to the 340-square equal loudness curve, resulting in a greater attenuation of the mid- and low-frequency bands of the electrical signal.

　　The B network simulates the human ear's response to 70° pure tone, which causes a certain attenuation in the low-frequency band of the electrical signal.

　　The C network simulates the human ear's response to 100-pole pure tone and has a nearly flat response over the entire audio frequency range.

　　The sound pressure level measured by the sound level meter through the frequency weighting network is called the sound level. Depending on the weighting network used, it is called A sound level, B sound level and C sound level respectively, and the units are dB(A), dB(B) and dB(C). At present, the sound level meters used to measure noise can be divided into three types according to the sensitivity of the meter head response:

　　(1) “Slow”. The meter time constant is 1000 ms. It is generally used to measure steady-state noise. The measured value is the effective value.

　　(2) "Fast". The meter time constant is 125ms, which is generally used to measure unstable noise with large fluctuations and traffic noise. The fast gear is close to the human ear's response to sound.

　　(3) "Pulse or pulse hold". The needle rise time is 35ms. It is used to measure pulse noise with a long duration, such as punching machines, hammers, etc. The measured value is the maximum effective value. (4) "Peak hold". The needle rise time is less than 20ms. It is used to measure pulse sounds with a very short duration, such as guns, cannons and explosions. The measured value is the peak value, that is, the maximum value.