A brief introduction to noise meters

A noise meter, also known as a sound level meter, is a fundamental instrument in noise measurement. A sound level meter generally consists of a condenser microphone, preamplifier, attenuator, amplifier, frequency meter network, and effective value indicator head. It simulates the time characteristics of the human ear's response speed to sound waves by converting sound signals into electrical signals; The frequency characteristics with different sensitivities to high and low frequencies, as well as the intensity characteristics that change the frequency characteristics at different loudness levels.
working principle
The sound is converted into an electrical signal by a microphone, and then the impedance is transformed by a preamplifier to match the microphone with an attenuator. The amplifier adds the output signal to the weighting network, performs frequency weighting on the signal (or an external filter), and then amplifies the signal to a certain amplitude through an attenuator and amplifier, and sends it to the effective value detector (or an external level recorder). The noise level value is displayed on the indicator head.
Weighted network
In order to simulate the different sensitivities of human auditory perception at different frequencies, a network is installed in the sound level meter that can simulate the auditory characteristics of the human ear and correct the electrical signal to approximate the auditory perception. This network is called a weighted network. The sound pressure level measured through a weighted network is no longer an objective physical quantity of sound pressure level (called linear sound pressure level), but a sound pressure level corrected for auditory perception, called weighted sound level or noise level.
Weighted parameters, also known as weighted parameters, are parameters measured after weighting the frequency response curve to distinguish them from unweighted parameters in a flat frequency response state.
Weighted networks attenuate both low and high frequencies moderately, making the mid frequency more prominent. By connecting this weighted network between the tested equipment and the measuring instrument, the influence of intermediate frequency noise in the equipment will be "amplified" by the network. In other words, intermediate frequency noise that has a greater impact on listening sensation Z will be given higher weight. The measured signal-to-noise ratio at this time is called weighted signal-to-noise ratio, which can more accurately reflect human subjective listening sensation.
Detector and indicator head
The function of a detector is to convert rapidly changing voltage signals into slower changing DC voltage signals. The magnitude of this DC voltage should be proportional to the magnitude of the input signal. According to the measurement requirements, detectors can be divided into peak detectors, average detectors, and root mean square detectors. The peak detector can provide the maximum value of Z within a certain time interval, while the average detector can measure its average value within a certain time interval. Pulse sound requires measurement of its peak value, and in most noise measurements, a root mean square detector is used.
The root mean square detector can square, average, and square root of the AC signal to obtain the root mean square value of the voltage, and then transmit the root mean square voltage signal to the indicator head. At present, the sound level meter used for measuring noise can be divided into four types of meter responses based on sensitivity:
⑴ "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.
⑵ "Quickly". The time constant of the meter head is 125ms, which is generally used to measure unstable noise and transportation noise with large fluctuations. Fast gear approaches the human ear's response to sound.
Pulse or pulse hold. The rise time of the gauge needle is 35ms, used to measure pulse noise with a longer duration, such as punch presses, hammers, etc. The measured value is the effective value of Z.
⑷ "Peak Maintenance". The rise time of the meter needle is less than 20ms. It is used to measure pulse sounds with short duration, such as gun, cannon, and explosion sounds. The measured value is the peak value, which is the Z-value.
Classification of Sound Level Meters
According to the sensitivity of the sound level meter, there are two types of classification methods for sound level meters: one is ordinary sound level meters, and the other is precision sound level meters. Their microphones require wide frequency response, high sensitivity, good long-term stability, and can be used in conjunction with various bandpass filters. The amplifier output can be directly connected to level recorders and recorders to display or store noise signals.
Some people classify sound levels into four categories, namely 0, 1, 2, and 3. Their accuracies are ± 0.4 decibels, ± 0.7 decibels, ± 1.0 decibels, and ± 1.5 decibels, respectively.
There are three standard weighting networks for frequency 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 an acoustic curve, and its curve shape is opposite to the 340 square acoustic curve, resulting in significant attenuation in the middle and low frequency bands of the electrical signal. B network simulates the response of the human ear to 70 square pure tones, which causes a certain attenuation in the low frequency range of electrical signals. The C network simulates the response of the human ear to 100 square tones, with a nearly flat response across the entire audio frequency range. The sound pressure level measured by a sound level meter through a frequency weighting network is called sound level. Depending on the weighting network used, it is referred to as A sound level, B sound level, and C sound level, with units denoted as dB (A), dB (B), and dB (C).
Integral sound level meter is used to measure the equivalent sound level of unstable noise over a period of time. A noise dosimeter is also an integral sound level meter, mainly used to measure noise exposure.
Pulse type sound level meter is used to measure pulse noise, which conforms to the response of the human ear to pulse sound and the average time of the human ear's response to pulse sound.
1. Selection of operating environment for sound level meter: Select a representative testing location, and the sound level meter should be kept away from the ground and walls to reduce the additional impact of reflected sound from the ground and walls.
2. The weather conditions require that the sound level meter should keep the microphone diaphragm clean during the time of no rain or snow. A wind cover must be added when the wind speed is above level three (to avoid wind noise interference), and measurements should be stopped when the wind speed is above level five.
3. Open the carrying case of the sound level meter, take out the sound level meter, and put on the sensor.
4. Place the sound level meter in state A, test the battery, and then calibrate the sound level meter.
5. Refer to the table (commonly used environmental sound level reference) and adjust the measurement range.
6. Below, various functions can be used for measurement, including fast (measuring instantaneous values in environments with large changes in sound pressure levels), slow (measuring average values in environments with small changes in sound pressure levels), pulse (measuring pulse sound sources), and filter (measuring sound levels in frequency bands).
7. Record data as needed, and also connect to a printer or other computer terminal for automatic collection. Organize equipment and put it back in place.
Sound level meters can be widely used for industrial noise measurement and environmental noise measurement of various machines, vehicles, ships, electrical appliances, etc. They are suitable for sound testing fields in factories, enterprises, building design, environmental protection, labor hygiene, transportation, teaching, medical and health, scientific research and other departments.
Precautions
1) Before use, read the instruction manual to understand the usage and precautions of the instrument.
2) The instrument should be avoided from being placed in places with high temperature, humidity, sewage, dust, and air or chemical gases containing high levels of hydrochloric acid and alkali.
3) Pay attention to the polarity when installing batteries or external power sources, and do not reverse the connection. If not in use for a long time, the battery should be removed to prevent leakage and damage to the instrument.
4) Do not disassemble the microphone to prevent it from being thrown or dropped, and place it properly when not in use.
5) Do not disassemble the instrument without authorization. If the instrument is not functioning properly, it can be sent to a repair unit or factory for maintenance.
6）. During use, if there is an undervoltage alarm in the LCD, the battery should be replaced in a timely manner.
7) Before measuring the sound level meter, it can be turned on and preheated for 2 minutes, and preheated for 5 to 10 minutes on humid days.
Sensitivity calibration
To ensure measurement accuracy, calibration should be performed before and after use.
Connect the sound level calibrator to the microphone, turn on the calibration power, read the value, adjust the sensitivity potentiometer of the noise meter, and complete the calibration.