Sound level meter is the most basic instrument in noise measurement. The sound level meter is generally composed of a condenser microphone, a preamplifier, an attenuator, an amplifier, a frequency weighting network, and an RMS indicator. The working principle of the sound level meter is: the sound is converted into an electrical signal by the microphone, and then the impedance is transformed by the preamplifier to match the microphone and the attenuator. The amplifier adds the output signal to the weighting network, performs frequency weighting on the signal (or external filter), and then amplifies the signal to a certain amplitude through the attenuator and amplifier, and sends it to the RMS detector (or external power supply). flat recorder), the numerical value of the noise sound level is given on the indicator head.
The frequency weighting network in the sound level meter has three standard weighting networks, A, B, and C. The A network simulates the response of the human ear to the 40-square pure tone in the equal-loudness curve. Its curve shape is opposite to the 340-square equal-loudness curve, so that the middle and low frequency bands of the electrical signal have greater attenuation. The B network simulates the response of the human ear to a 70-square pure tone, which makes the low frequency band of the electrical signal attenuated to a certain extent. The C network simulates the human ear's response to a 100-square pure tone, with 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. According to the different weighting network used, it is called the A sound level, the B sound level and the C sound level, and the unit is denoted as dB(A) , dB(B), and dB(C).
At present, the sound level meter used to measure noise, the meter response can be divided into four types according to the sensitivity:
(1) "Slow". The time constant of the meter head is 1000 ms, which is generally used to measure steady-state noise, and the measured value is the effective value.
(2) "Fast". The time constant of the meter head is 125ms, which is generally used to measure unstable noise and traffic noise with large fluctuations. Fast gear is close to the human ear's response to sound.
(3) "Pulse or pulse hold". The rise time of the needle is 35ms, which is used to measure pulse noise with a long duration, such as punches, hammers, etc. The measured value is the maximum effective value.
(4) "Peak Hold". The rising time of the needle is less than 20ms. Used to measure short-duration impulse sounds, such as guns, cannons and explosions, the measured value is the peak value. That is, the maximum value.
The sound level meter can be connected to an external filter and recorder to perform spectrum analysis on noise. The domestic ND2 type precision sound level meter is equipped with an octave page filter, which is easy to carry to the scene and perform spectrum analysis.
According to the accuracy, sound level meters can be divided into precision sound level meters and ordinary sound level meters. The measurement error of a precision sound level meter is about ±1dB, and that of an ordinary sound level meter is about ±3dB. Sound level meters can be divided into two categories according to their use: one is used to measure steady-state noise, and the other is used to measure unsteady noise and impulse noise.
The integrating sound level meter is used to measure the equivalent sound level of unsteady noise over a period of time. A noise dosimeter is also an integrating sound level meter, mainly used to measure noise exposure.
The impulse sound level meter is used to measure impulse noise. This sound level meter conforms to the response of the human ear to the impulse sound and the average time of the human ear to the impulse sound.
