Use of Noise Meters in the Automotive Industry

    A sound level meter is an instrument that can measure the noise level of industrial noise, living noise, traffic noise, etc. approximately according to the hearing 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 by the sense of hearing. According to the accuracy of the sound level meter in measuring 1000Hz pure tone under standard conditions, sound level meters were divided into two categories in the 1960s, one called precision sound level meter and the other called ordinary sound level meter. my country also adopts this method. Since the 1970s, some countries have introduced a four-category classification method, namely 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 two categories: AC sound level meter and DC sound level meter using dry batteries. The latter can also be portable. Portable sound level meters have the advantages of small size, light weight and convenient on-site use.

It is generally composed of a microphone, amplifier, attenuator, weighting network, detector, indicator meter and power supply.

(1) 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, dynamic 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 the pressure of the sound wave, 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 size of the sound wave 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 a metal diaphragm and metal electrodes that are very close to each other. In essence, they are a flat plate capacitor. The metal diaphragm and the metal electrodes form the two plates of the flat plate capacitor. When the diaphragm is subjected to sound pressure, the diaphragm deforms, causing the distance between the two plates to change, and the capacitance also changes, thereby generating an alternating voltage, whose waveform is proportional to the sound pressure level within the linear range of the microphone, thus achieving the function of converting the sound pressure signal into a voltage signal.

Capacitor microphones are ideal microphones for acoustic measurement, with advantages such as large dynamic range, flat frequency response, high sensitivity and good stability in general measurement environments, so they are widely used. Since the output impedance of the capacitive sensor is very high, it is necessary to use a preamplifier for impedance transformation. The preamplifier 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 use two-stage amplifiers in the amplification circuit, namely input amplifiers and output amplifiers, which are used 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 meter pointer points to the appropriate position. The attenuation of each gear is 10 decibels. The attenuator adjustment range used by the input amplifier is the bottom end of the measurement (such as 0~70 decibels), and the attenuator adjustment range used by the output amplifier is the high end of the measurement (70~120 decibels). The dials of the input and output attenuators are often made of different colors, and currently black and transparent are mostly paired. Since the high and low limits of many sound level meters are 70 decibels, it is necessary to prevent exceeding the limit when rotating to avoid damaging the device.

(3) Weighting network? In order to simulate the different sensitivities of human hearing at different frequencies, a network that can simulate the hearing characteristics of the human ear and correct the electrical signal to be close to hearing is set up inside. This network is called a weighting network. The sound pressure level measured by the weighting network is no longer the sound pressure level of an objective physical quantity (called linear sound pressure level), but the sound pressure level corrected by the sense of hearing, which is called the weighted sound level or noise level.

There are generally three types of weighting networks: A, B, and C. A-weighted sound level simulates the frequency characteristics of low-intensity noise below 55 decibels to the human ear; B-weighted sound level simulates the frequency characteristics of medium-intensity noise between 55 and 85 decibels; and C-weighted sound level simulates the characteristics of high-intensity noise. The difference between the three is the degree of attenuation of the low-frequency components of noise, with A attenuating the most, B the second, and C the least. A-weighted sound level is currently the most widely used type of noise measurement in the world because its characteristic curve is close to the hearing characteristics of the human ear. B and C are gradually no longer used.

Noise level readings from a sound level meter must state the measurement conditions.

(4) Detector and indicator? In order to display the amplified signal through the indicator, a detector is also needed to convert the rapidly changing voltage signal into a slowly changing DC voltage signal. The magnitude of this DC voltage should be proportional to the magnitude of the input signal. According to the measurement needs, detectors are divided into peak detectors, average detectors and root mean square detectors. The peak detector can give the maximum value in a certain time interval, and the average detector can measure its absolute average value in a certain time interval. In addition to pulse sounds such as gunshots, which need to measure their peak values, root mean square detectors are used in most measurements.

The RMS detector can square, average and square the AC signal to obtain the RMS value of the voltage, and finally transmit the RMS voltage signal to the indicating meter. The indicating meter is an electric meter. As long as its scale is calibrated to a certain extent, the decibel value of the noise level can be directly read from the meter. The damping of the sound level meter meter generally has two gears: "fast" and "slow". The average time of the "fast" gear is 0.27s, which is very close to the physiological average time of the human ear's auditory organ; the average time of the "slow" gear is 1.05s. When measuring steady-state noise or needing to record the change process of the sound level, it is more appropriate to use the "fast" gear; when the fluctuation of the measured noise is relatively large, it is more appropriate to use the "slow" gear.

In order to meet the needs of the measurement site, the sound level meter generally has a tripod bracket so that it can be fixed on the tripod bracket as needed.

There are usually some jacks on the panel. If these jacks are connected to a portable octave band filter, they can form a small and simple spectrum analysis system for on-site use; if combined with a recorder, the on-site noise can be recorded on tape and stored for more detailed research later; if combined with an oscilloscope, the waveform of the sound pressure change can be observed, and the waveform can be photographed with a camera; analyzers, recorders and other instruments can also be combined and used in conjunction with sound level meters, which depends on the test conditions and test requirements.