The structure, working principle and use of sound level meters
A sound level meter is an instrument that can approximately measure the noise levels of industrial noise, domestic noise, traffic noise, etc. according to the hearing 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 sound level meters in measuring 1000Hz pure tone under standard conditions, in the 1960s, sound level meters were divided into two categories internationally, one was called precision sound level meter, and the other was called ordinary. Our country also adopts this method. Since the 1970s, some countries have introduced four categories of classification, namely type 0, type 1, type 2 and type 3. Their accuracy is ±0.4dB, ±0.7dB, ±1.0dB and ±1.5dB respectively. According to the different power sources used by sound level meters, they can also be divided into two types: AC sound level meters and DC sound level meters using dry batteries. The latter can also be made portable. Portable has the advantages of small size, light weight and easy on-site use.
It is generally composed of a microphone, amplifier, attenuator, weighting network, detector, indicator meter and power supply.
Microphone It is a device that converts sound pressure signals into voltage signals. It is also called a microphone and is a sensor. Common microphones include crystal, electret, moving coil and condenser types.
The moving coil sensor consists of a vibrating diaphragm, a movable coil, a permanent magnet and a transformer. 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 induced current. This current changes according to the magnitude of the acoustic pressure on the vibrating diaphragm. The greater the sound pressure, the greater the current generated; the lower the sound pressure, the smaller the current generated.
The capacitive sensor is mainly composed of a metal diaphragm and closely spaced metal electrodes. It is essentially a flat capacitor. The metal diaphragm and the metal electrode constitute the two plates of the flat capacitor. When the diaphragm is affected by sound pressure, the diaphragm deforms, causing the distance between the two plates to change and the capacitance to change, thereby generating an alternating voltage, the waveform of which is consistent with the sound pressure level within the linear range of the microphone. 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 environments, so it is widely used. Since the output impedance of the capacitive sensor is very high, impedance conversion needs to be performed through a preamplifier. The preamplifier is installed inside the sound level meter close to where the capacitive sensor is installed.
