Sound Level Meter Working Principle
A sound level meter is a basic noise measuring instrument, which 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 can simulate the time characteristic of the human ear's response speed to the sound wave; the frequency characteristic with different sensitivity to high and low frequencies and the intensity characteristic of changing the frequency characteristic at different loudness. A sound level meter is a subjective electronic instrument.
How a sound level meter works?
The sound is converted into electrical signals 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 press). Level recorder), the value of the noise sound level is given on the indicator head.
In order to simulate the different sensitivities of human hearing at different frequencies (20Hz-20KHz), there is a network in the sound level meter that can simulate the auditory characteristics of the human ear and correct the electrical signal to a similar value to the sense of hearing. It's called a weighted network.
The sound pressure level measured by the weighting network is no longer the objective physical sound pressure level (called linear sound pressure level), but the sound pressure level corrected by the sense of hearing, which is called weighted sound level or noise level.
Since people have different perceptions of noise in each frequency band, they are sensitive to the intermediate frequency Z of about 3 kHz, but less to low frequency and high frequency. Therefore, the unweighted signal-to-noise ratio may not be as good as the human ear's subjective perception of the noise level. match. How to integrate the measured value with the subjective sense of hearing? So there is an equalization network, or a weighting network, which moderately attenuates both low and high frequencies, so that the middle frequency is more prominent.
This weighting network is connected between the equipment under test and the measuring instrument, so the influence of the IF noise of the equipment will be "amplified" by the network, in other words, the IF noise that has the greatest impact on the sense of hearing is given a higher weight, the measured signal-to-noise ratio at this time is called the weighted signal-to-noise ratio, which can more truly reflect people's subjective sense of hearing.
The structure of the sound level meter
The sound level meter consists of a microphone, an amplifier, an attenuator, a weighting network, a detector, an indicator head and a power supply.
It is a device that converts a sound pressure signal into a voltage signal, also known as a microphone, a sensor. Common microphones include crystal, electret, moving coil, and condenser.
The moving coil sensor is composed of a vibrating diaphragm, a movable coil, a magnet and a transformer. The vibrating diaphragm starts to vibrate after being subjected to sound wave pressure, and drives the movable coil installed with it to vibrate in the magnetic field to generate induced current. The current varies according to the magnitude of the acoustic pressure on the vibrating diaphragm. The greater the pressure, the greater the current generated; the lower the sound pressure, the smaller the current generated.
Capacitive sensors are mainly composed of metal diaphragms and metal electrodes that are close to each other, which are essentially one flat capacitor. The metal diaphragm and the metal electrodes constitute the two plates of the flat capacitor. When the diaphragm is subjected to sound pressure, the diaphragm is deformed, the distance between the two polar plates changes, and the capacitance also changes, thereby generating an alternating voltage whose waveform is related to the sound pressure level within the linear range of the microphone. The ratio is formed, and the function of converting the sound pressure signal into a voltage signal is realized.
Condenser microphone is an ideal microphone in acoustic measurement. It has the advantages of large dynamic range, flat frequency response, high sensitivity and good stability in general measurement environment, so it is widely used. Since the output impedance of the capacitive sensor is very high, it is necessary to perform impedance transformation through a preamplifier, which is installed inside the sound level meter near the part where the capacitive sensor is installed.
Amplifiers and Attenuators
At present, many popular domestic and imported amplifiers use two-stage amplifiers, namely input amplifiers and output amplifiers, whose function is to amplify weak electrical signals. The input attenuator and output attenuator are used to change the attenuation of the input signal and the attenuation of the output signal, so that the pointer of the meter head points to the appropriate position, and the attenuation of each gear is 10 decibels. The adjustment range of the attenuator used by the input amplifier is the bottom end of the measurement (such as 0 to 70 minutes), and the output is amplified.
The adjustment range of the attenuator used by the instrument is the high end of the measurement (70 to 120 points). The dials of the input and output attenuators are often made of different colors. Currently, black and transparent are often paired. Since the height and bottom of many sound level meters are limited by 70 decibels, it is necessary to prevent exceeding the limit when rotating to avoid damage to the device.
In order to simulate the different sensitivities of human hearing at different frequencies, there is a network that can simulate the auditory characteristics of the human ear and correct the electrical signal to be similar to the hearing. This network is called a weighting network. The sound pressure level measured by the weighting network is no longer the sound pressure level of the objective physical quantity (called the linear sound pressure level), but the sound pressure level corrected by the sense of hearing, which is called the weighted sound level or the noise level.
There are generally three types of weighted networks: A, B, and C. A-weighted sound level is to simulate the frequency characteristics of the human ear to low-intensity noise below 55 dB; B-weighted sound level is to simulate the frequency characteristics of medium-intensity noise of 55 to 85 dB; C-weighted sound level is to simulate high-intensity noise characteristic. The difference between the three is the attenuation of the low-frequency components of the noise. A attenuates Z more, B is the second, and CZ is less. A-weighted sound level is the most widely used noise measurement in the world because its characteristic curve is close to the hearing characteristics of the human ear. B and C are gradually not used. The noise level readings obtained from the sound level meter must indicate the measurement conditions.
Detector and indicator head
In order to display the amplified signal through the meter, a detector is also required to convert the rapidly changing voltage signal into a slower changing DC voltage signal. The magnitude of this DC voltage is proportional to the magnitude of the input signal. According to the needs of measurement, the detectors are divided into peak detectors, average detectors and RMS detectors. The peak detector can give the Zda value of a certain time interval, and the average detector can measure its average value in a certain time interval. Root-square detectors are used in most measurements, except for pulses like gunshots that need to measure their peaks.
The rms value detector can square, average and square the AC signal to obtain the rms value of the voltage, and then transmit the rms voltage signal to the indicating meter. The indicator head is an electric meter. As long as the scale is calibrated to a certain extent, the decibel value of the noise level can be directly read from the head. The damping of the sound level meter head generally has two blocks of "fast" and "slow". . The average time of "fast" gear is 0.27s, which is very close to the physiological average time of human auditory organs; the average time of "slow" gear is 1.05s. When the steady-state noise is measured or the sound level change process needs to be recorded, it is more appropriate to use the "fast" gear; when the fluctuation of the measured noise is relatively large, it is more suitable to use the "slow" gear.