The sound level meter is the most basic noise measuring instrument. It is an electronic instrument, but it is different from objective electronic instruments such as voltmeters. When converting an acoustic signal into an electrical signal, it is possible to simulate the temporal characteristics of the human ear's response to sound waves; the frequency characteristics with different sensitivities for high and low frequencies and the intensity characteristics for changing the frequency characteristics for different loudnesses. Therefore, the sound level meter is a subjective electronic instrument.
working principle
The sound is converted into an electrical signal by the microphone, and the impedance is converted by the preamplifier to match the microphone to 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 the attenuator and the amplifier, and sends it to the RMS detector (or externally pressed) Flat recorder), the value of the noise level is given on the indicator head
microphone
A microphone is a device that converts a sound pressure signal into a voltage signal, also called a microphone, which is a sensor of a sound level meter. Common microphones are available in crystal, electret, moving coil and capacitive.
1. The moving coil microphone consists of a vibrating diaphragm, a movable coil, a permanent magnet and a transformer.
Sound level meter
Sound level meter
. The vibrating diaphragm starts to vibrate after being subjected to the sound wave pressure, and drives the movable coil mounted with it to vibrate in the magnetic field to generate an induced current. This current varies depending on the magnitude of the sound wave pressure of the diaphragm. The larger the sound pressure, the larger the current generated, and the smaller the sound pressure, the smaller the current generated.
2. The condenser microphone is mainly composed of a metal diaphragm and a metal electrode which is close to each other, and is essentially a plate capacitor. The metal diaphragm and the metal electrode constitute two plates of the plate capacitor. When the diaphragm is subjected to the sound pressure, the diaphragm is deformed, and the distance between the two plates is changed, thus changing the capacitance. The voltage in the bit measurement circuit also changes, realizing the effect of converting the sound pressure signal into a voltage signal. Capacitive microphones are ideal microphones for acoustic measurement. They have wide dynamic range, flat frequency response, high sensitivity and good stability in general measurement environments. Since the output impedance of the condenser microphone is very high, the impedance conversion by the preamplifier is required, and the preamplifier is installed inside the sound level meter near the portion where the condenser microphone is mounted.
Detector and indicator head
The function of the detector is to convert a rapidly changing voltage signal into a slower changing DC voltage signal. This DC voltage is proportional to the size of the input signal. Depending on the needs of the measurement, the detector has a peak detector, an average detector and a rms detector. The peak detector can give the maximum value over a certain time interval, and the average detector can measure its absolute average value over a certain time interval. The pulse sound needs to measure its peak value, and the rms detector is used in most noise measurements.
The rms detector can square, average, and square the AC signal to obtain the rms value of the voltage, and finally deliver the rms voltage signal to the indicator head. At present, the sound level meter for measuring noise can be divided into four types according to the sensitivity:
(1) "Slow". The head time constant is 1000 ms, which is generally used to measure steady-state noise, and the measured value is an effective value.
(2) "Fast". The time constant of the meter is 125ms, which is generally used to measure unsteady noise and traffic noise with large fluctuations. The fast gear is close to the human ear's reaction to the sound.
(3) "Pulse or pulse hold". The needle rise time is 35ms, which is used to measure impulse noise with a long duration, such as punching, hammering, etc., and 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 short duration, such as guns, guns and explosions. The measured value is the peak value, ie the maximum value.
Classification of sound level meters
According to the sensitivity of the sound level meter, there are two types of sound level meter classification: one is the ordinary sound level meter, which is not very demanding on the microphone. The dynamic range and the frequency response are relatively narrow, and generally do not have a band-pass filter connected; the other is a precision sound level meter, which requires a wide frequency response, high sensitivity, long-term stability, and A bandpass filter is used together, and the amplifier output can be directly connected to the level recorder and the recorder to display or store the noise signal. If the microphone of the precision sound level meter is removed, and the input converter is connected to the accelerometer, the vibrometer can be used for vibration measurement.
In recent years, sound level meters have been classified into four categories, namely, type 0, type 1, type 2, and type 3. Their accuracy is ±0.4 decibels, ±0.7 decibels, ±1.0 decibels, and ±1.5 decibels, respectively.
The 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 a RMS indicator head. The sound level meter works by converting the sound into an electrical signal by a microphone, and then transforming the impedance by a preamplifier to match the microphone to 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 the attenuator and the amplifier, and sends it to the RMS detector (or externally pressed) Flat recorder), the value of the noise 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 is a response to the 40-square pure tone of the human ear's equal-acquisition curve. Its curved shape is opposite to the 340-square equal-cord curve, so that the middle and low frequency bands of the electrical signal have a large attenuation. The B network is a simulation of the human ear's response to the 70-square pure tone, which causes a certain attenuation of the low frequency band of the electrical signal. The C network simulates the human ear's response to 100-square pure tone, with a nearly flat response over the entire audio range. The sound level measured by the sound level meter through the frequency weighting network is called the sound level. According to the weighting network used, it is called A sound level, B sound level and C sound level, and the unit is recorded as dB(A). , dB(B) and dB(C).
At present, the sound level meter for measuring noise can be divided into four types according to the sensitivity:
(1) "Slow". The head time constant is 1000 ms, which is generally used to measure steady-state noise, and the measured value is a valid value.
(2) "Fast". The time constant of the meter is 125ms, which is generally used to measure unsteady noise and traffic noise with large fluctuations. The fast gear is close to the human ear's reaction to the sound.
(3) "Pulse or pulse hold". The needle rise time is 35ms, which is used to measure the impulse noise with long duration, such as punching, pressing hammer, etc., and 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 short durations, such as guns, guns and explosions. The measured value is the peak value, ie the maximum value.
The sound level meter can be connected to a filter and a recorder to perform spectrum analysis on noise. The domestic ND2 precision sound level meter is equipped with a multiplier page path filter, which is easy to carry to the scene and for spectrum analysis.
Sound level meters can be divided into precision sound level meters and ordinary sound level meters according to accuracy. The measurement error of the precision sound level meter is about 1 dB of soil, and the ordinary sound level meter is about 3 dB of soil. Sound level meters can be divided into two categories according to their use: one for measuring steady-state noise and the other for measuring unsteady noise and impulse noise.
Classification introduction
The sound level meter is the most basic instrument in measuring noise. It belongs to an electronic instrument.
The sound level meter is generally composed of a condenser microphone, a preamplifier, an attenuator, an amplifier, a frequency weighting network, and a RMS indicator head. According to the sensitivity of the sound level meter, it can be divided into two categories, one is the ordinary sound level meter; the other is the precision sound level meter, which can be divided into two categories according to the use of the sound level meter: one type is used for measuring stability. State noise; one class is used to measure unsteady noise and impulse noise.
The sound level meter uses advanced digital detection technology to greatly improve the stability and reliability of the instrument. The sound level meter has the advantages of simple operation and convenient use; it has the characteristics of large range dynamic range, large-screen liquid crystal digital display, automatic measurement and storage of various data.
Sound level meter can be widely used in industrial noise measurement and environmental noise measurement of various machines, vehicles, ships, electrical appliances, etc. It is suitable for factories, building design, environmental protection, labor hygiene, transportation, teaching, medical and health, scientific research and other departments. The field of sound testing.
maintenance
(1) Keep the outside of the instrument clean;
(2) The microphone should be stored dry when not in use;
(3) The microphone diaphragm should be kept clean and should not be touched by hand;
(4) When the instrument is not used for a long time, it should be energized for 2 hours per month, and the mildew rain season should be energized for 2 hours per week;
(5) When the instrument is used, the battery should be taken out in time;
(6) Regularly sent to the metrology department for verification;
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