Digital Noise Meter Basic Principles and Classification
A digital noise meter, also called a sound level meter, is the most basic instrument in noise measurement. A digital noise meter generally consists of a condenser microphone, a preamplifier, an attenuator, an amplifier, a frequency weighting network, and an RMS indicator head.
The working principle of a digital noise meter is as follows: the microphone converts the sound into an electrical signal, and then the preamplifier converts the impedance to match the microphone with the attenuator. Amplifier will be added to the output signal weighting network, the signal frequency weighting (or external filter), and then by the attenuator and amplifier will be amplified to a certain amplitude of the signal, sent to the effective value of the detector (or outside the level recorder), in the indication of the meter head to give the numerical value of the noise level.
The frequency weighting network in the sound level meter has three standard weighting networks, A, B and C. A network simulates the response of the human ear to the 40-square pure tones in the equal-loudness curve, which is opposite to the shape of the 340-square equal-loudness curve, thus making the middle and low-frequency bands of the electrical signals to have a larger attenuation.B network simulates the response of the human ear to the 70-square pure tones, which makes the low-frequency bands of the electrical signals to have a certain degree of attenuation. The C network simulates the response of the human ear to a 100-square pure tone, which has a nearly flat response over the entire frequency range of the sound. The sound pressure level measured by a sound level meter through a frequency weighting network is called the sound level, and according to the different weighting networks used, it is called the A sound level, B sound level and C sound level, and the units are recorded as dB(A), dB(B) and dB(C). At present, the measurement of noise with the sound level meter, the meter head response according to the sensitivity can be divided into four kinds:
(1) "Slow". (1) "Slow", with a head time constant of 1000 ms, is generally used to measure steady state noise, and the measured value is the RMS value.
(2) "Fast". The time constant of the meter head is 125 ms, which is generally used to measure the unstable noise with large fluctuation and transport noise. The fast gear is close to the human ear's response to sound.
(3) "Pulse or pulse hold". The rise time of the meter needle is 35ms, which is used to measure the impulse noise with longer duration, such as punching machine, pressing hammer, etc. The measured value is the maximum RMS value.
(4) "Peak hold". The rise time of the meter needle is less than 20ms. It is used to measure the impulse sound with short duration, such as gun, cannon and explosion sound, and the measured value is the peak value. The value measured is the peak value, i.e. the maximum value.
Sound level meter can be connected to external filters and recorders, to do spectrum analysis of noise. Domestic ND2 type precision sound level meter is equipped with an octave page range filter, easy to carry to the scene and make spectrum analysis.
Digital noise meter can be divided into precision sound level meter and common sound level meter according to the accuracy. The measurement error of precision sound level meter is about 1dB, and that of ordinary sound level meter is about 3dB. sound level meter can be divided into two categories according to its usage: one category is used to measure steady state noise, and the other category is used to measure unsteady state noise and impulse noise.
Integral 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 and is used primarily to measure noise exposure.
Impulse sound level meter is used to measure impulse noise, which is in accordance with the response of the human ear to impulse sound and the average time of the human ear response to impulse sound. It is a necessary instrument for places where we usually have a noisy environment.
