i The Sound Level Meter
DEVELOPMENTS IN SOUND MEASUREMENT INSTRUMENTATION
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 fi nally 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 fi lters, 1/1 octave or 1/3 octave being the most common.
The March of Digitisation
No-one today could have overlooked the fact that everything is going or has gone ‘digital’. The sound level meter was no different, and the process started at the back end of the chain – the display. By sampling the output of the detector, albeit at the slow sample rates (~1Hz) available at that time, the values could be displayed with greater precision on a digital display, to the nearest 0.1dB, and the limited dynamic range of the A/D converters could be improved by doing the log conversion in the detector before sampling.
The next step was to sample the detector output at a higher rate, which allowed some basic mathematics to be done - calculating the average value of the signal over a time period. The idea of the equivalent continuous sound pressure level, or Leq, gained a foothold, and this was easily estimated by sampling the output of a Fast time-weighted detector. The fi rst ‘integrating sound level meters’ had been born.
The 10th anniversary of the Control of Noise at Work
Regulations (2005) is a great opportunity to review what has happened in the development of sound level meters in recent years.
Figure 2 John Shelton, Managing Director, AcSoft Ltd and Svantek UK Ltd Svantek / AcSoft, 8B Wingbury Courtyard, Leighton Road, Wingrave, Aylesbury, HP22 4LW Tel: 01296 682040 • Email:
johnshelton@svantek.co.uk • Web:
www.svantek.co.uk or
www.acsoft.co.uk
The new family of digital sound level meters now followed the layout of Figure 2, with the output of the detector being sampled at 256 Hz.
At the same time, the concept of Short Leq emerged, where the digital detector emitted Leq values over short periods. This was ideal
www.envirotech-online.com IET January / February 2016
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