What are the structures of sound level meters
It consists of a microphone, amplifier, attenuator, weight network, detector, indicator head, and power supply.
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 permanent 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 at the measurement bottom (such as 0-70 decibels), while the adjustment range of the attenuator used in the output amplifier is at the measurement high end (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.
3. Weighting network
In order to simulate the sensitivity of human auditory perception at different frequencies, there is a network inside that can simulate the auditory characteristics of the human ear. The electrical signal is corrected to a network that is similar to auditory perception, which is called a weighted network. The sound pressure level measured through a weighted network is no longer an objective physical quantity (called linear sound pressure level), but a sound pressure level corrected by auditory perception, called weighted sound level or noise level.
There are generally three types of weighted networks: A, B, and C. A-weighted sound level is a frequency characteristic that simulates the human ear's response to low intensity noise below 55 decibels; The B-weighted sound level simulates the frequency characteristics of moderate intensity noise ranging from 55 to 85 decibels; The C-weighted sound level is a characteristic of simulating high intensity noise. The difference between the three is the degree of attenuation of the low-frequency components of noise, with A having the most attenuation, B taking the second place, and C having the least. A-weighted sound level is currently the most widely used type of noise measurement in the world due to its characteristic curve close to the auditory characteristics of the human ear, while B and C are gradually not used. The noise level reading obtained from the sound level meter must indicate the measurement conditions.
4. Sensors and indicator heads
In order for the amplified signal to be displayed through the meter head, a detector is also needed to convert the rapidly changing voltage signal into a slower changing DC voltage signal. The magnitude of this DC voltage is proportional to the size of the input signal. According to the needs of measurement, there are two types of detectors: peak detector and average detector, and black root mean square detector. The peak detector can provide the maximum value at a certain time interval, while the average detector can measure its absolute average value at a certain time interval. Except for pulse sounds like gunfire that require measurement of their peak, root square detectors are used in most measurements.
