The design and operation of a sound level meter
A sound level meter generally consists of a microphone, amplifier, attenuator, weighting network, detector and indicator.
①Microphone: A transducer element that converts an acoustic signal (sound pressure) into an electrical signal (voltage). There are crystal microphones, electric microphones and electret microphones. Condenser microphones have the advantages of wide dynamic range, flat frequency response, small sensitivity change, long-term stability, etc., and are mostly used in precision sound level meters and standard sound level meters.
② Amplifier: Amplifies the weaker electrical signal. The amplifier used in the sound level meter requires high input impedance and low output impedance, reasonable dynamic range, small linear distortion and frequency range to meet the needs. Includes input amplifier and output amplifier.
③Attenuator: The range of the sound level meter is generally 25~130dB, but the detector and analog indicator do not have such a wide range, so the attenuator is usually used to attenuate the strong signal 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 file during general measurement, and the input attenuator should be adjusted to the minimum attenuation file on the premise that the input amplifier is not overloaded, so that the input signal and the electrical noise of the input amplifier have the same as large a difference as possible.
④ Weighting network: According to the regulations of IEC, select several curves close to the frequency response of human ear to sound, and design A. B． C． D Four standard weighting networks. The A-weighted network frequency response curve is roughly equivalent to the inversion curve of the 40phon equal-loudness curve, so that the middle and low frequency bands of the electrical signal are greatly attenuated, and the high frequency band is also attenuated to a certain extent. The B weighting network is approximately equivalent to the inversion curve of the 70phon equal loudness curve, which attenuates the electrical signal mainly in the low frequency band. The C-weighting network is equivalent to the inversion curve of the 100phon equal loudness curve, and has a nearly flat response in the entire audio frequency range, which is approximately equivalent to the human ear's response to high-frequency sounds. by A． B． C． The reading measured by the D-weighting network is called the sound level, and the sound level is the sound pressure level after frequency weighting, which should be distinguished from the sound pressure level.
The A-weighted frequency response is compatible with the sensitivity of the human ear to sounds with a wide range of frequencies, so it is most commonly used in actual measurements. D-weighting networks are commonly used to measure aviation noise.
