Noise dosimeter

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A noise dosimeter (American) or noise dosemeter (British) is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations such as the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard[1] or EU Directive 2003/10/EC.[2]

Measuring industrial noise

Basically, there are two different instruments to measure noise exposures: the sound level meter and the dosimeter. A sound level meter is a device that measures the intensity of sound at a given moment. Since sound level meters provide a measure of sound intensity at only one point in time, it is generally necessary to take a number of measurements at different times during the day to estimate noise exposure over a workday. If noise levels fluctuate, the amount of time noise remains at each of the various measured levels must be determined.

To estimate employee noise exposures with a sound level meter it is also generally necessary to take several measurements at different locations within the workplace. After appropriate sound level meter readings are obtained, people sometimes draw "maps" of the sound levels within different areas of the workplace. By using a sound level "map" and information on employee locations throughout the day, estimates of individual exposure levels can be developed. This measurement method is generally referred to as "area" noise monitoring.

A dosimeter is like a sound level meter except that it stores sound level measurements and integrates these measurements over time, providing an average noise exposure reading for a given period of time, such as an 8-hour workday. With a dosimeter, a microphone is attached to the employee's clothing and the exposure measurement is simply read at the end of the desired time period. A reader may be used to read-out the dosimeter's measurements. Since the dosimeter is worn by the employee, it measures noise levels in those locations in which the employee travels. A sound level meter can also be positioned within the immediate vicinity of the exposed worker to obtain an individual exposure estimate. Such procedures are generally referred to as "personal" noise monitoring.

Area monitoring can be used to estimate noise exposure when the noise levels are relatively constant and employees are not mobile. In workplaces where employees move about in different areas or where the noise intensity tends to fluctuate over time, noise exposure is generally more accurately estimated by the personal monitoring approach.

In situations where personal monitoring is appropriate, proper positioning of the microphone is necessary to obtain accurate measurements. With a dosimeter, the microphone is generally located on the shoulder and remains in that position for the entire workday. With a sound level meter, the microphone is stationed near the employee's head, and the instrument is usually held by an individual who follows the employee as he or she moves about. Manufacturer's instructions, contained in dosimeter and sound level meter operating manuals, should be followed for calibration and maintenance. To ensure accurate results, it is considered good professional practice to calibrate instruments before and after each use.[3]

Variations in legal requirements

This meant that the user had to specify under what legal regime he wanted to use his dosimeter, as each country could - and did have their own local laws for such measurements. In each region, sometimes down to regions with tiny populations such as Western Australia, only one combination was usually legally accepted and that could be very different to an adjoining region. While these decision may have seemed sensible at the time, users were in general not adequately informed of these problems and this resulted in many measurement errors.

The five main parameters that were commonly different in different political entities were:

  • Threshold
  • Criterion level
  • Exchange rate
  • Exponential or linear integration
  • 'Head-on' or random incidence calibration

There were of course others, such as the single event maximum level, the peak value etc.

To reduce the variations of instrument required to be manufactured, some commercial companies made complex "Universal" units where all these things could be selected by the user. As measuring noise was not well understood, such complexity clearly militated against accurate results and many anecdotal stories give examples of huge errors resulting; as very few unqualified users understood the complex issues involved.

Standardization

International standards

The international body that specifies the technical requirements of such instruments as sound level meters and dosimeters is the International Electro-technical Commission (IEC) based in Geneva; whereas the method of their use is normally given in an ISO publication. However, in any particular political region, local laws apply as the IEC and ISO publications only have the status of "recommendations" and so countries could - and did - have their own sets of rules - many of which were technically flawed and in some cases scientifically impossible. Every new regulation thus made the concept of "%dose" more meaningless. The "100%" dose was different in different countries, but many users did not understand this and would buy low cost USA built dosimeters where the American "100%" was not correct for their local regulations and usually very much underestimated the noise exposure.

Military Standards

  • MIL-S-1474[4] This standard establishes acoustical noise limits and prescribes testing requirements and measurement techniques for determining conformance to the noise limits specified herein. This standard applies to the acquisition and product improvement of all designed or purchased (non-developmental items) systems, subsystems, equipment, and facilities that emit acoustic noise. This standard is intended to address noise levels emitted during the full range of typical operational conditions.
  • TOP-1-2-608A[5] This Test Operations Procedure (TOP) describes procedures for measuring the sound levels transmitted through air of developmental and production materiel as a means of evaluating personnel safety, speech intelligibility, security from acoustic detection and recognition, and community annoyance. It covers tests for steady-state noise from military vehicles and general equipment, and impulse noise from weapon systems and explosive-ordnance materiel.

Permitted levels reduced

OSHA's 29 CFR 1910.95 standard requires the employer to supply, but not mandate, hearing protection at 90 dBA when averaged over an 8 hour Time Weighted Average (TWA). An exposure of 85 dBA when averaged over an eight hour work period, the employer is mandated to implement a hearing conservation program, provide audiograms to employees, provide hearing protection, and training for the use of hearing protection. Other requirements are also required by the employer and are listed in the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard[1] During the 1980s and 1990s many workers determined that the '90dBA for 8 hours' limit was far too high and an unacceptable number of workers would be damaged at these levels, so a level of 85 dB(A) for 8 hours was felt to be a better criterion. Even later the EU reduced the limits still further to the 80 dB(A) we have today, as given in the UK's "The Control of Noise at Work Regulations 2005.". These regulations follow closely the EU Directive 2003/10/EC,[2] normally called the Physical Agents Directive.

A further complication for the sound level meter designer was that it was realised that a single very high noise peak could instantaneously damage hearing, so a limit was originally set by the then European Community so that no worker should ever be exposed to an rms acoustic pressure of more than 200 Pa - equating to 140 dB re 20 μPa - and that this should be measured using an instrument with no frequency weighting. This while a good idea, was patent nonsense as 200 Pa could be generated by trains going through tunnels, closing a door, in fact many everyday things could cause such a pressure wave below the frequencies that could be heard or could cause hearing damage. Accordingly C-frequency-weighting was specified to measure the peak level as this has a flat frequency response between 31 Hz and 8 kHz. However, this missed a significant amount of important energy and a new frequency weighting of 'Z' (zero) weighting was specified in IEC 61672 : 2003 that has a flat response from at least 20 Hz to10 kHz.

PSEM arrival

To try and simplify the situation, in the 1980s Working Group 4 of IEC Technical Committee 1 started to write a new standard for a dosimeter, but decided that for many good reasons, a new name was called for and the long - but more correct name - of Personal Sound Exposure Meter (PSEM) was used. WG4 was mainly made up of design engineers from the International Sound Level meter companies together with scientists from various national acoustic laboratories and a few academics. The result of their efforts became IEC 61252 :1993, the current PSEM standard. This has tolerance limits based on a Class 2 - what was a Type 2 - sound level meter, but because it is intended to be worn on the body, it has relaxed directional characteristics.

The favoured metric for many scientists was simply the sound exposure in pressure-squared-time, for example Pa2h and this was used for the PSEM standard and at a stroke this removed all the various options for measurement. However for health and safety legislation in Europe for legal purposes the metric chosen was the daily personal noise exposure level, LEP,d, which corresponds to LEX,8h as defined in international standard ISO 1999: 1990 clause 3.6, and is expressed in decibels A-frequency-weighted [dB(A)]. In simple terms this is the normalised sound exposure expressed in decibels.

Use of dosimeters

Fig 2 : A classic belt worn unit

The original dosimeters were designed to be belt worn with a microphone connected to the body of the dosimeter and mounted on the shoulder as near to the ear as practicable. These devices were worn for the full work shift and at the end would give a readout initially in percentage dose, or in some other exposure metric. These were the most common way of making measurements to meet legislation in the USA, but in Europe the conventional sound level meter was favoured. There were many reasons for this, but in general in Europe the dosimeter was distrusted for several reasons, some being.

  • The cable was considered dangerous as it could catch on rotating machinery
  • The dosimeter could tell you the level had been exceeded, but it did not say when this happened
  • Workers could falsify the data very easily
  • The device was big enough to affect the work pattern

In the USA - where most of the early devices were manufactured, these reasons did not seem to matter so much.

To remove these European objections, dosimeters became smaller and started to include a data store where the Time History of the noise, usually in the form of Short Leq was stored. This data could be transferred to a personal computer and the exact pattern of the noise exposure minute by minute plotted. The usual method used was to store data in the form of Short Leq, a French concept that helped to bring computers into acoustics. As well, dosimeters started to incorporate a second C-frequency-weighted channel that allowed the true peak to be indicated. By the time the PSEM standard was published, many major sound level meter companies - in both Europe and the USA had a dosimeter in their range. A typical noise dosimeter is shown in figure 2

21st-century devices

Fig 3 : A 3 and 5 dB doubling dosebadge with time history data storage

The next technology breakthrough came when in the 1990s the United Kingdom Department of Trade and Industry awarded a SMART grant to Cirrus Research to design an ultra-miniature dosimeter.[6] It was to be non-intrusive and not to affect the worker's ability to perform their responsibilities. The resulting device (Fig. 3) was a twin channel device able to meet all the requirements of the European Directive and also the market need for data storage. An important design criteria was that the device had to have no internal display nor any controls, so workers would not be tempted to try to 'modify' or affect the readings; instead the acquired data was transmitted and the device was controlled by an infra-red link. Normally, the acquired exposure data from several badges is transferred to a reader unit where it can be read and stored. As well, most manufacturers offer software to transfer the reader's data to a computer where it can be archived and as well put into a database to allow full and complete reports to be generated.

These new devices are split into two groups; those that have no display on the body-worn acquisition unit, and those that include such a display. While it would seem that a display may be advantageous, many professionals regard it as another potential error source as the user may be tempted to put the device near to a noise source. Their motivation is thought to be either to watch the display move or to try to modify the results, but in either case an additional uncertainty has been introduced into the measurement. An advantage of an internal display however, is that if only a single unit is in use, the exposure can be read without external devices.

Companies

The following major manufacturers are among those who offer current technology PSEM, claimed to comply with IEC 61252 : 1993. The engineer who was the member of the working group at the time follows.

References

  1. ↑ 1.0 1.1 "OSHA 29 CFR 1910.95 Occupational Noise Exposure Standard". Occupational Heath and Safety Administration. 2011-03-03. Retrieved 2012-09-10. 
  2. ↑ 2.0 2.1 "EU Directive 2003/10/EC (normally called the Physical Agents Directive)". European Union. 2013-02-15. Retrieved 2013-08-26. 
  3. ↑ "OSHA 29 CFR 1910.95 Appendix G Occupational Noise Exposure Standard". Occupational Heath and Safety Administration. 2011-03-03. Retrieved 2012-09-10. 
  4. ↑ "Department of Defense Design Criteria, Noise Limits". Department of Defense. 12 February 1997. Retrieved 2012-09-18. 
  5. ↑ "Test Operations Procedure:Sound Level Measurements". US Department of Defense. 1 January 2011. Retrieved 2012-09-18. 
  6. ↑ "Environmental Expert". Environmental Expert. 2012-10-23. Retrieved 2012-10-23. 
  • Occupational Safety and Health Act. Code of Federal Regs, Title 29, Chapter XVII, Part 1910.
  • IEC 61252 :1993 - Acoustics - Personal Sound Exposure Meter
  • Wallis A.D. & Krug R.W. "A data storing dosimeter" Proc IOA Vol 11 part 9 101-106 Nov 1989.
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