Sound Level Meters

Any noise assessment, whatever its purpose, relies on monitoring or measuring noise levels. 

There are many types of measuring instrument.  This segment considers the basic elements of a sound level meter and then discusses in more detail some of the properties that affect the performance of the instruments.

The range of equipment available is constantly expanding because of the advances made in microelectronics.  Perhaps the more obvious changes are in the miniaturisation of equipment; sound level meters are now much lighter and more portable than they were, and often include a build-in data logger.  The advances have also meant that it is possible to make a wider range of measurements using reasonably cheap equipment than was possible 10 or 15 years ago.  In fact, the user can sometimes be swapped by so many readings and ways of expressing results!

Sound level meters in general

The hand-held sound level meter (SLM) is the basic tool to measure noise in the workplace.  In practice, the name sound level meter can be used to describe a wide range of instruments with may different features, from simple dB(A) meters to those which can apply A-, B-, C- and D- weightings or give a linear response and which can be used in conjunction with statistical analysers or with frequency analysers that measure the frequency spectrum m of a sound.

Many of the properties of SLMs described here are also applicable to more intricate sound level measuring instruments that need to be linked to a micro- or laptop computer for downloading and analysing results.  These instruments usually have much more sophisticated electronics that can sample the sound, provide statistical information, print out information or perform rapid, intricate signal processing to give frequency-based information.  The information may be stored and later recalled and displayed in a number of ways.  These instruments are, as a result, much more expensive but still rely on the same basic principles.  A disadvantage is that they may be too complex for one-off industrial noise measurements.

The main components start with the microphone, which converts the sound waves into electrical signals.  The signals, which vary with time, are increased or amplified by the amplifiers.  The signals may be modified at different frequencies by weighting networks or filters.  The varying signals are then averaged in the detector (over a longer or shorter time interval, depending on whether a ‘slow’ or ‘fast’ response is selected) and finally shown on the display.

Apart from sophisticated types of signal processing, the complexity of the basic instruments is dependent on the dB range of sound levels to be measured and the frequency range of the sounds.

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