Engineering Horizons



October 01
16:22 2018


The Sound Level Meter

The fundamental design of the sound level meter has not really changed. We’re simply trying to make an objective and traceable measurement of the noise level, to allow us to assess environmental noise impact or potential damage to workers’ hearing.

Figure 1

The building blocks of our meter are shown in Figure 1. The starting point is the microphone, which transduces the acoustic pressure variation into a voltage analogue, which we can feed into our electronic circuits. Typically, we use a condenser type microphone, for its stability, linearity and ease of calibration. We need to polarise the capacitor, typically with 200 volts DC, and match its inconveniently high output impedance into something we can drive down the line. Now we have a signal to work with, and two types of ‘detector’ are commonly used to make a measurement of sound pressure level. The root mean square or RMS detector does what it says on the tin – backwards! Firstly the waveform is squared, making all the negative excursions positive, then this is averaged to estimate the power in the signal, and finally the square root is taken to get back to a number which is related to a pressure level.


The Peak detector simply measures the maximum excursion of the acoustic signal (either positive or negative) and this might be useful for estimating damage potential from the noise, such as from blasting or gun shots. If we wanted to assess the noise level and not just the sound level, then there would also be frequency weighting circuits prior to the

detector, A and C being the most popular, and for analysis of the frequency makeup of the signal, there may also be some filters, 1/1 octave or 1/3 octave being the most common.



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Engineering Horizons

Engineering Horizons

“Engineering Horizons” is the first & leading technical magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics field under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.

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