An introduction to the sound level meter's operation
The sound level meter is an instrument that can measure the noise level of industrial noise, domestic noise, traffic noise, etc. according to the auditory characteristics of the human ear. Noise level refers to the sound pressure level (dB) or loudness level (phon) measured with a sound level meter and corrected for hearing.
According to the accuracy of the sound level meter measuring 1000Hz pure tone under standard conditions, the international sound level meter was divided into two categories in the 1960s, one was called precision sound level meter, and the other was called ordinary sound level meter. Our country also adopts this method.
How Sound Level Meters Work
The sound is converted into an electrical signal by the microphone, and then the impedance is transformed by the preamplifier to match the microphone with 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 effective value detector (or external press level recorder), and gives the value of the noise level on the indicator head.
microphone
A microphone is a device that converts a sound pressure signal into a voltage signal, also known as a microphone, and it is a sensor of a sound level meter. Common microphones are crystal, electret, moving coil and condenser.
1. The moving coil microphone is composed of a vibrating diaphragm, a moving coil, a magnet and a transformer, etc.
sound level meter
The vibrating diaphragm begins 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 an induced current. The current varies according to the magnitude of the acoustic pressure on the vibrating diaphragm. The greater the sound pressure, the greater 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 metal electrodes that are close together, which is essentially a flat plate capacitor.
The metal diaphragm and the metal electrode constitute the two plates of the plate capacitor. When the diaphragm is subjected to sound pressure, the diaphragm deforms, which changes the distance between the two plates, thus changing the capacitance and the voltage in the position measurement circuit, realizing the function of converting the sound pressure signal into a voltage signal.
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 condenser microphone is very high, it is necessary to perform impedance transformation through the preamplifier. The preamplifier is installed inside the sound level meter near the place where the condenser microphone is installed.
amplifier
Generally, a two-stage amplifier is used, that is, an input amplifier and an output amplifier, and its function is to amplify weak electrical signals.
The input attenuator and the 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 can be pointed at the proper position.
The adjustment range of the attenuator used by the input amplifier is to measure the low end, and the adjustment range of the attenuator used by the output amplifier is to measure the high end. The high and low ends of many sound level meters are bounded by 70dB.
weighting network
In order to simulate the different sensitivities of the human ear at different frequencies, a sound level meter is equipped with a network that can simulate the auditory characteristics of the human ear and correct the electrical signal to an approximate value of the sense of hearing. This network is called a weighting network.
The sound pressure level measured through the weighting network is no longer the sound pressure level of the objective physical quantity (called linear sound pressure level), but the sound pressure level corrected by the sense of hearing, called the weighted sound level or noise level.
Maintenance
⑴Keep the exterior of the instrument clean;
(2) The microphone should be kept dry when not in use;
(3) The microphone diaphragm should be kept clean and must not be touched by hand;
⑷ When the instrument is not used for a long time, it should be powered on for 2 hours a month, and it should be powered on for 2 hours a week during the mildew rain season;
(5) The battery should be taken out in time after the instrument is used;
⑹Regularly sent to the metrology department for verification;
Since the 1970s, some countries have introduced a four-category method, which is divided into Type 0, Type 1, Type 2 and Type 3. Their accuracies are ±0.4dB, ±0.7dB, ±1.0dB, and ±1.5dB, respectively. According to the different power sources used by the sound level meter, it can also be divided into AC type and DC type sound level meter with dry batteries, and the latter can also be portable. Portable has the advantages of small size, light weight and convenient on-site use.
