The structure and working principle of a sound level meter
Sound level meters generally consist of microphones, amplifiers, attenuators, weighting networks, detectors and indicators.
①Microphone: A transducer element that converts acoustic signals (sound pressure) into electrical signals (voltage). There are crystal microphones, electric microphones and electret microphones. Condenser microphones have the advantages of wide dynamic range, flat frequency response, small sensitivity changes, and long-term stability. They are mostly used in precision sound level meters and standard sound level meters.
②Amplifier: Amplifies relatively weak electrical signals. The amplifier used on the sound level meter is required to have high input impedance and low output impedance, a reasonable dynamic range, small linear distortion and a frequency range that meets the needs. Includes input amplifier and output amplifier.
③Attenuator: The measuring range of sound level meters is generally 25 to 130dB. Detectors and analog indicators do not have such a wide measuring range. An attenuator is usually used to attenuate strong signals to avoid overloading the amplifier. Attenuators are divided into input attenuators and output attenuators. In order to improve the signal-to-noise ratio, the input attenuator is located before the input amplifier, and the output attenuator is connected between the input amplifier and the output amplifier. In order to improve the signal-to-noise ratio, the output attenuator should be adjusted to the maximum attenuation level during general measurement. Under the premise that the input amplifier is not overloaded, the input attenuator should be adjusted to the minimum attenuation level so that the input signal is consistent with the electrical noise of the input amplifier. The largest possible difference.
④ Weighting network: According to the provisions of IEC, several curves close to the human ear's frequency response to sound are selected, and A. B. C． D four standard weighting networks. The A-weighted network frequency response curve is approximately equivalent to the inverted curve of the 40phon equal loudness curve, which causes greater attenuation in the mid- and low-frequency bands of the electrical signal, and also attenuates the high-frequency band to a certain extent. The B-weighted network is approximately equivalent to the inverted curve of the 70phon equal loudness curve, which attenuates the electrical signal to a certain extent mainly in the low frequency band. The C-weighted network is equivalent to the inverted curve of the 100phon equal loudness curve. It has a nearly flat response in the entire sound frequency range, which is approximately equivalent to the response of the human ear to high-frequency sounds. By A． B. C． The reading measured by the D-weighted network is called the sound level. The sound level is the sound pressure level after frequency weighting. Please note that it is different from the sound pressure level.
The A-weighted frequency response is adapted to the sensitivity of the human ear to sound in a wide range of frequencies, so it is the most commonly used in actual measurements. D-weighting networks are often used to measure aviation noise.
