How does a sound level meter work?
The sound level meter is a basic noise measuring instrument. It is an electronic instrument, but it is different from objective electronic instruments such as voltmeters.
When converting the acoustic signal into an electrical signal, it can simulate the time characteristics of the human ear's response speed to the sound wave;
A frequency characteristic that has different sensitivity to high and low frequencies and an intensity characteristic that changes the frequency characteristic at different loudness. Therefore, the sound level meter is a subjective electronic instrument.
The working principle of the sound level meter is:
The sound is converted into an electrical signal by the microphone, and then the impedance is transformed by the preamplifier to match the microphone with the attenuator. The amplifier adds the output signal to the weighting network, and performs frequency weighting on the signal (or an external filter);
Then the signal is amplified to a certain amplitude through the attenuator and amplifier, sent to the effective value detector (or external press level recorder), and the value of the noise level is given on the indicator meter.
1) A microphone is a device that converts a sound pressure signal into a voltage signal, also known as a microphone, which is a sensor of a sound level meter. Common microphones are crystal, electret, moving coil and condenser.
A moving coil microphone is composed 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 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, and the smaller the sound pressure, the smaller the current generated.
The condenser microphone is mainly composed of a metal diaphragm and metal electrodes that are close together, which is essentially a flat plate capacitor.
The metal diaphragm and the metal electrode constitute the two plates of the flat capacitor. When the diaphragm is subjected to sound pressure, the diaphragm deforms and the distance between the two plates changes;
Therefore, the capacitance is changed, and the voltage in the bit measurement circuit also changes, realizing 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 condenser microphone is very high, it is necessary to perform impedance transformation through the preamplifier. The preamplifier is installed inside the sound level meter near the place where the condenser microphone is installed.
2) amplifier
Generally, a two-stage amplifier is used, that is, an input amplifier and an output amplifier, and its 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 meter can be pointed at the proper position.
The adjustment range of the attenuator used in the input amplifier is to measure the low end, and the adjustment range of the attenuator used in the output amplifier is measured to be excellent.
The high and low ends of many sound level meters are bounded by 70dB.
3) Weighting network
In order to simulate the different sensitivities of human hearing at different frequencies, a sound level meter is equipped with a network that can simulate the auditory characteristics of the human ear and correct the electrical signal to an approximate value of the sense of hearing. This network is called a weighting 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.
