The principle and accuracy of measuring noise with a sound level meter
A sound level meter generally consists of a capacitive microphone, preamplifier, attenuator, amplifier, frequency weighting network, and effective value indicator head.
The working principle of a sound level meter is that the microphone converts sound into electrical signals, and then the preamplifier transforms impedance to match the microphone with the attenuator. The amplifier adds the output signal to the weighting network, performs frequency weighting (or external filter) on the signal, and then amplifies the signal to a certain amplitude through an attenuator and amplifier, and sends it to the effective value detector (or external level recorder). The noise level value is given on the indicator head.
There are three standard weighting networks in sound level meters: A, B, and C.
The A network simulates the response of the human ear to a 40 square pure tone in the equivalent response curve. Its curve shape is opposite to the 340 square equivalent response curve, resulting in significant attenuation in the middle and low frequency bands of the electrical signal. The B-network simulates the response of the human ear to 70 square pure tones, which attenuates the low-frequency range of electrical signals to a certain extent.
The C network simulates the response of the human ear to 100 square pure tones, with a nearly flat response throughout the entire audio frequency range. The sound pressure level measured by the sound level meter through a frequency weighting network is called the sound level. Depending on the weighting network used, it is called the A sound level, B sound level, and C sound level, with units recorded as dB (A), dB (B), and dB (C).
At present, the sound level meters used for measuring noise can be divided into four types based on sensitivity:
(1) Slow. The time constant of the meter head is 1000 ms, usually used for measuring steady-state noise, and the measured value is the effective value.
(2) Quick. The time constant of the meter head is 125ms, which is generally used to measure unstable noise and transportation noise with significant fluctuations. The fast gear approaches the human ear's response to sound.
(3) Pulse or pulse hold. The rise time of the watch needle is 35ms, which is used to measure pulse noise with a longer duration, such as punching machines, press hammers, etc. The measured value is * significant value.
(4) Peak Hold. The rise time of the watch needle is less than 20ms. It is used to measure pulse sounds with a short duration, such as guns, artillery, and explosive sounds. The measured value is the peak value, which is the maximum value. The sound level meter can be connected to an external filter and recorder for spectral analysis of noise.
The accuracy of sound level meters are: Level I accuracy ± 0.7dB, Level II accuracy ± 1dB, Level III accuracy ± 1.5dB, and ordinary sound level meters ± 2dB
Sound level meters can be divided into two categories according to their purpose: one is used to measure steady-state noise, and the other is used to measure unstable noise and pulse noise.
