Sound Level Meter Overview
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. 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.
Generally, it is composed of microphone, amplifier, attenuator, weighting network, detector, indicating meter and power supply.
(1) Microphone It is a device that converts a sound pressure signal into a voltage signal, also known as a microphone, and is an excellent sensor. Common microphones are crystal, electret, moving coil, and condenser. The moving coil sensor consists of a vibrating diaphragm, a moving coil, a permanent 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 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; the smaller the sound pressure, the smaller the current generated
Capacitive sensors are mainly composed of metal diaphragms and metal electrodes that are close together, which is essentially a flat plate capacitor. The metal diaphragm and metal electrodes constitute the two plates of the flat capacitor. When the diaphragm is subjected to sound pressure, the diaphragm deforms, the distance between the two plates changes, and the capacitance also changes, thereby generating an alternating voltage whose waveform is within the linear range of the microphone and the sound pressure level Forming a ratio realizes 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 capacitive sensor is very high, it is necessary to perform impedance transformation through the preamplifier. The preamplifier is installed inside the sound level meter close to the part where the capacitive sensor is installed.
(2) Amplifiers and attenuators Many domestic and imported amplifiers that are popular at present use two-stage amplifiers in the amplification circuit, that is, the input amplifier and the output amplifier, and their 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 gauge 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 to measure the bottom end (such as 0~70 decibels) and the attenuator adjustment range used by the output amplifier is to measure the high end (70~120 decibels). The dials of the input and output attenuators are often made of different colors, and currently black and transparent are often paired. Since the high and low of many sound level meters are limited by 70 decibels, it is necessary to prevent the limit from being exceeded when rotating, so as not to damage the device.
(3) In order to simulate the different sensitivities of human hearing at different frequencies, the weighting network has a built-in auditory characteristic that can simulate the human ear, and correct the electrical signal to a network that is similar to the hearing. This network is called a counting network. right 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.
