Noise- A Pain in the Ears!
Published: 10th Oct 2004
Despite the existence of Noise at Work regulations since 1989, the problem of understanding noise remains among some industrial users.
"How do we measure it?" is a common question noise professionals hear. Of course a short article cannot answer this fully, but perhaps some light can be shed on what seems a complex subject.
Noise of course is simply the effect of changes in air pressure impinging on the ear. The basis of the regulations is that if this pressure change is too great over a period, damage will result. However, the range of pressures a human can hear is from 0,001 Pa (Pascal) up to about 200 Pa. Such a range is hard to deal with so we first 'A-weight' the sound to mimic a notional ears response, then take the logarithm of the pressure and scale the results from 0 to 140dB and call the result "Sound Level"; very simple in concept but it makes calculations hard.
What we measure
In industry we usually measure noise at work by taking two noise metrics, one effectively representing the energy of the noise as an average level over a working day and another representing the highest pressure occurring, called the PEAK. The 'energy' metric in the UK from 2006 is to be in terms of LEX, the average level over an 8-hour daily period. This is currently called LEP,d but only the name is being changed. Today there are two ACTION LEVELS or limits at which you have to take action and these are 85 and 90 dBA, but from Feb 2006 these will be 80 and 85 dBA with an EXPOSURE LIMIT VALUE of 87dBA.
Today there is only one PEAK VALUE of 140 dB above which no worker must be exposed (200Pa in pressure), but from Feb 2006, there will be two action levels of 135dBC, 137dBC and the same 'limit value' of 140dBC peak.
How to protect the workers and meet the regulations
First you have to measure the noise levels and there are two basic ways of doing this: by measuring the actual EXPOSURE of each worker with a Noise Dosemeter, usually called a Personal Sound Exposure Meter (PSEM), or measuring the area noise and deducing the exposure of each worker using a Sound Level Meter (SLM). Traditionally, professional acousticians have considered the SLM to be the more accurate method, mainly because it was considered that workers could modify dosemeter data and they thought that untrained users might make errors when using an SLM. Also, classes of SLM existed that were more accurate in the laboratory, even if in the field this accuracy showed little or no advantage.
Dosemeters or PSEM
USA industry has always favoured the use of dosemeters as they measure the noise right at the workers ear, in other words the actual noise affecting the worker. Today, a new generation of ultra-miniature dosemeter is available. Such new units can give huge improvements in accuracy and ease of use and allow the HSE regulations to be met at far lower cost. Shoulder mounted PSEMs are so small that they do not affect the worker in any practical way and allow many workers to be monitored relatively automatically. Some even provide full data logging of the noise that can be downloaded to a PC so the exact time at which excess noise occurs can be found.
Further, as these new badges have no user controls and no obvious read-out, the wearer is far less tempted to 'modify' the data; thus these new noise badges have revolutionised factory noise data collection. Traditional PSEM, which were belt mounted with a cable to a microphone on the shoulder, are still available and some older users prefer these, as they tend to have many more features, although few of the added features are needed to meet the EU Directive. One problem with classic instruments is that the thin microphone cable stretching from belt to shoulder can be caught on machinery and is also the most common source of failure.
Sound level meters
While for routine noise measurements, the PSEM is today the method of choice, there are still some measurements where an SLM is required as well. For example, if the overall level in dBA is too high, some form of hearing protection must be provided to bring the level at the ear ideally below 85dB. The chosen ear protection should match the SPECTRUM of the noise and to do this the frequency content of the noise must be measured. The two main ways are to divide the frequency spectrum into OCTAVE or THIRD OCTAVE bands.
There are typically 9 octaves to be measured and 27 third octaves, so octaves are easier to manage and for this application are usually adequate. An SLM that measures octaves exists in two forms, a REAL TIME ANALYSER or RTA, where each octave is measured simultaneously, or the older manual method where each octave is measured one at a time. Clearly as noise is changing, the manual method may not be measuring the same noise in each band, so with these instruments either an average must be taken or the noise must be very steady.
UK industry is fortunate in having British companies that have taken the trouble to have at least one of their RTA, SLM or PSEM tested by an EU national laboratory to obtain Pattern Approval. In other words, they can offer an instrument that is proven to meet the claimed International specification. As these instruments are intended for Health and Safety it may seem a sensible precaution to take such approvals into account when making a purchasing decision, especially as these companies are able to advise on methods of meeting the requirements. For current 'official' data see www.hse.gov.uk ?
Alan D. Wallis SMI
Noise - A Pain in the Ears!
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Published: 10th Oct 2004 in Health and Safety International