Smaller analysers big on benefits
May 1st 2009

There have been huge innovations in the design of noise monitoring equipment since the late 1960s when the first sound level meters were produced.Tim Turney, product manager Casella CEL, comments

Although they have evolved considerably, noise monitoring equipment still comprises two basic types. Firstly, bodily-worn noise dosimeters ensure that workers' personal exposure to noise is measured over a full working shift, providing a reading at the end of that time.

They are useful for mobile workers, such as forklift truck or crane drivers, where it is not always practical to measure with a hand-held device such as the second type of monitoring solution – the sound level meter. These are useful for individuals who work at fixed locations, where measurements are taken at the ear, pointing the instrument at the source of the noise for comparatively short periods of time, compared with a personal noise dosimeter.

Both types of instrument help employers and employees to prevent or reduce the risk of hearing damage from exposure to noise at work, and comply with legislation, which in the UK is the Control of Noise at Work Regulations (2005), which replaced the less stringent Noise at Work Regulations (1989).

By law, employers must assess and identify measures to eliminate or reduce risks from exposure to noise to protect the hearing of their employees, and provide them with hearing protection where risks cannot be eliminated by engineering controls. They must ensure that legal exposure limits are not exceeded.

Employers must take specific action at certain levels of employee exposure averaged over a working day, or week in some circumstances, together with the maximum noise (peak sound pressure) to which employees are exposed. So-called lower and upper exposure action values are daily/weekly exposure of 80 & 85 decibels (dB) and peak sound pressure of 135 & 137dB respectively.

So why is noise monitoring so important? High noise levels at work can permanently damage workers' hearing, and may be illegal.

Damage may be caused immediately by sudden, typically impulsive, loud noises, or may be a gradual process, resulting from prolonged exposure to noise.Monitoring is vital so that the risks to both employees and non-employees can be managed.

The noise measuring industry has moved on a long way since the use of outdated analogue techniques. A significant change is in the design of Casella's instruments; they now use all-digital technology. This fact is not always recognised, since it is often assumed that, if an instrument has a digital screen, it must use digital technology, which is not necessarily the case.

A microphone is common to all noise measurement techniques. It picks up changes in sound pressure and converts them into small electrical signals that vary widely in both level and time according to the pattern of the sound.

These electrical signals are analogue in nature, which historically have been processed using analogue circuitry and devices such as operational amplifiers and capacitor/resistor filter networks.

However, analogue devices are susceptible to the problems of electrical noise superimposition, temperature and humidity drift, and component ageing. Casella now offers instruments that use Digital Signal Processing (DSP), which converts analogue signals immediately into a series of digital numbers processed mathematically, giving accurate, precise, reliable and repeatable measurements that do not 'drift' when, for example, a worker goes outside from an indoor environment while wearing a dosimeter. DSP based instruments are programmable and upgradeable and offer the versatility to implement functions beyond the possibilities of an analogue circuit.

Another advance made possible by the latest digital technology is that both sound level meters and noise dosimeters are now considerably smaller than earlier generations. The original dosimeters were designed to be belt-worn, with a microphone connected by cable to the dosimeter and mounted on the wearer's shoulder close to the ear. Instruments have gradually become smaller with the advent of the badge-style dosimeter, such as Casella's dBadge. This is cable-free, small and lightweight yet still capable of measuring all required occupational noise parameters, as well as the time history of an individual's noise exposure throughout the day.

Digital technology is also used in sound level meters and DSP take up less 'real estate' than their equivalent analogue components.

Casella's CEL-600 series, for example, is believed to be the smallest colourscreen real-time analyser on the market.

So all in all, in terms of clarity, quality, and reliability, comparing old noise monitoring equipment with today's instruments is a bit like comparing a '78' gramophone record with an MP3 player.

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