The Use of Sound Level Meters in the Automotive Industry
The structure and working principle of a sound level meter
A sound level meter is an instrument that can approximately measure the noise level of industrial noise, household noise, and traffic noise according to the auditory characteristics of the human ear. Noise level refers to the sound pressure level (dB) or loudness level (phon) measured by a sound level meter and corrected for auditory perception. According to the accuracy of sound level meters in measuring 1000Hz pure tones under standard conditions, in the 1960s, sound level meters were internationally divided into two categories: precision sound level meters and ordinary sound level meters. Our country also adopts this method. Since the 1970s, some countries have introduced four classification methods, 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 for sound level meters, they can also be divided into two types: AC type and DC type with dry batteries, and the latter can also become portable. Portable has the advantages of small size, light weight, and convenient on-site use.
(1) Microphone
It is a device that converts sound pressure signals into voltage signals, also known as a microphone, and is a sensor. Common types of microphones include crystal type, electret type, dynamic coil type, and capacitive type.
The dynamic coil sensor consists of a vibrating diaphragm, a movable coil, a * * magnet, and a transformer. After being subjected to sound pressure, the vibrating diaphragm begins to vibrate and drives the movable coil installed with it to vibrate in the magnetic field, generating induced current. The current varies according to the amount of acoustic pressure applied to the vibrating diaphragm. The higher the sound pressure, the greater the generated current; The lower the sound pressure, the smaller the generated current.
A capacitive sensor mainly consists of a metal diaphragm and a metal electrode that is very close to it, essentially a flat capacitor. The metal membrane and metal electrode form the two plates of the flat capacitor. When the diaphragm is subjected to sound pressure, it undergoes deformation, causing a change in the distance between the two plates and the capacitance, resulting in an alternating voltage. Its waveform is proportional to the sound pressure level within the linear range of the microphone, achieving the function of converting the sound pressure signal into an electrical pressure signal.
Capacitive microphones are ideal microphones in acoustic measurement, with advantages such as large dynamic range, flat frequency response, high sensitivity, and good stability in general measurement environments, making them widely used. Due to the high output impedance of capacitive sensors, impedance transformation needs to be carried out through a preamplifier, which is installed inside the sound level meter near the location where the capacitive sensor is installed.
(2) Amplifiers and Attenuators
Many popular domestic and imported amplifiers currently use two-stage amplifiers in amplification circuits, namely input amplifiers and output amplifiers, which 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 is in the appropriate position, and the attenuation of each gear is 10 decibels. The adjustment range of the attenuator used in the input amplifier is the measurement bottom (such as 0-70 decibels), and the adjustment range of the attenuator used in the output amplifier is the measurement * * (70-120 decibels). The dials of the input and output attenuators are often made in different colors, and currently they are mostly paired with black and transparent. Due to the fact that many sound level meters have a high and low limit of 70 decibels, it is necessary to prevent exceeding the limit during rotation to avoid damaging the device.
