In 1831 Victor Hugo wrote the Hunchback of Notre Dame; within it he casts the unlikely protagonist “Quasimodo” as someone who suffers from noise induced hearing loss. Quasimodo’s occupation of bell ringer, together with his residence in the bell tower of the Parisian Cathedral would undoubtedly have given rise to prolonged and repeated exposure to high levels of noise. Whilst a lot has changed in the 179 years since Hugo published his book, people continue to be exposed to noise and sadly they also continue to suffer from noise induced hearing loss.

The good news is that in the intervening years we have learnt a lot more about exposure to noise, which means we have been able to set safe levels of exposure. We also know how to measure noise and can use these measurements to plan and implement appropriate controls  which reduce worker’s exposure to noise.

“Subjective assessments have a place but to the uncalibrated ear they don’t go much beyond making a decision about whether to measure or not.”

Noise is unique in the world of Health and Safety in that it normally requires a quantitative assessment in the form of measurements rather than the more qualitative assessments we are used to. Subjective assessments have a place but to the un-calibrated ear they don’t go much beyond making a decision about whether to measure or not. As a general rule you’ll need to look at formal noise measurements, if any of the following criteria are to be met:

  • The noise sounds louder than busy city road traffic.
  • People have to raise their voice when trying to talk to someone one meter away.
  • People have to increase the volume on the TV or radio after they finish their shift.
  • In some circumstances noise measurement data from similar situations can be a helpful substitute, but in many cases there will be a real need to measure exposures.

The physics of noise and measurement units

Noise may simply be defined as any unwanted sound regardless of characteristic frequency or intensity. As sound is composed of vibrations in the air, it is measured as a pressure. In order to assess noise exposures we measure the Sound Pressure Level (SPL). Although sound is a pressure it is normally expressed in units of Decibels. The decibel scale is a sound pressure scale which has been converted to a logarithmic scale in order to make the numbers easier to comprehend. Instead of having a range of pressures from 0.00002 to 200 Pa, we measure noise over a range from 0 to 140 Decibels (dB). The table below shows some typical noise levels expressed both in terms of decibels and Pressure as Pascals.

SounddBPressure Pa

Threshold of hearing00.00002

Rustle of leaves in light breeze100.000063

Watch Ticking200.0002

Quiet rural area300.00063

Library/ whispered conversation400.002

Moderate Rainfall500.0063

Normal conversational speech at a distance of 1m600.02

Vacuum cleaner700.063

Kerbside of busy road,800.2

Use of angle grinder900.63

Fire engine siren at 1m1002

Fire engine siren at 2m1106.3

Pnuematic (jack) hammer12020

Threshold of pain13063.2

Jet aircraft taking off at a distance of 50m140200

Exposure to noise

If we consider a workers exposure to noise then it is unlikely to have been constant throughout the day. Differing types of work would give rise to a profile of noise over time, similar to the diagram below.

In terms of defining Quasimodo’s exposure, we are interested in the Equivalent Sound Level or Leq. The Leq is the noise level which is equivalent to the variable sound levels to which a person is exposed over the same time period. Scientific evidence has shown that hearing loss is affected by the total noise energy exposure. If a person is exposed to varying noise levels over an eight-hour work shift we can calculate an equivalent sound level which would equal the same total sound energy exposure. This would have the same effect on the person’s hearing as the variable exposure actually received.

The logarithmic nature of the decibel means that the calculation of this Leq is a little complex. The calculation process is known as integration and is normally undertaken by the electronics within the sound level meter.

“Scientific evidence has shown that hearing loss is affected by the total noise energy exposure.”

The nature of any sound is described by its intensity or loudness but also by the characteristic frequencies. Typically a noise is composed of a mixture of sounds at different frequencies, each at varying intensities. The human ear is not equally responsive to all frequencies; it is most sensitive around 4000 Hz and least sensitive in the low frequencies. The responses of sound level meters are automatically modified with frequency-weighting networks that represent some responses of the human ear. These weightings have been empirically derived. The A-scale, which approximates the ears response to moderate-level sounds, is the most commonly used in measuring noise to evaluate its effect on humans and has been incorporated in most occupational noise standards. Noise levels therefore tend to be deported as dB(A).

Decibel Weightings

As noise is a combination of sounds at various frequencies and intensities, the noise intensity can be either expressed as a spectrum or as a combination of all frequencies summed together in one value. As the human ear is more sensitive to certain frequencies than others, it is possible to make allowances for that in the electronic circuitry of a sound level meter. That is, certain frequencies are suppressed whilst others are enhanced in order to approximate to the response of the human ear. This technique is known as weighting and there are A, B, C and D weightings available for various purposes. The one that has been adopted for a workplace spectrum is given in dB(A). If the A-weighting is applied to a measurement in dB, the corresponding level in dB(A) is a good indication of loudness as perceived by the human ear.

Types of equipment

There is no standardized process for measuring exposure to occupational noise, whilst appropriate guidance exists, such as HSE’s ‘Controlling Noise at Work’ (L108) (ISBN 0 7176 6164 4) there is no standardized method of assessing noise exposure.

There have been tremendous advances in measurement equipment over the years. Like any electronic equipment, noise measurement instruments have become more powerful, smaller, lighter and cheaper over time. The Health and Safety Professional can choose from a variety of instruments to measure noise as well has having a range of instruments and software with which to analyze the data. Broadly speaking the instruments available can be split into the following three categories.

Simple Sound Level Meter

A simple sound level meter takes only instantaneous noise measurements. This kind of measurement may be appropriate where steady state noise levels occur in the workplace on where only a spot check is required. However, in most workplaces noise varies with time and contains impulses, intermittent and variable components. Under these circumstances, a simple meter should not be used. Although some simple sound level meters do not have any decibel weightings.

Integrating Sound Level Meter

The integrating sound level meter is the basic measuring instrument for noise exposures. It consists of a microphone, a frequency selective amplifier, and an indicator. The device automatically performs the integration of constantly varying noise to give the Leq.

Measurements are normally taken at arms length and at at the ear height for those exposed to the noise. The SLM must be calibrated before and after each use, whilst fluctuations were common with older equipment the new meters are much more stable.

An Integrating Sound Level Meter is also more likely to have a selection of noise weightings.

Noise Dosimeter

The noise dosimeter may be thought of as a personal sound level meter with an additional storage and computational function. It is worn by a worker and logs exposure to noise over time. This can produce a chart of noise level over time as well as an overall exposure value. The use of a noise dosimeter can reveal a large amount of information about the persons exposure. However, to analyse it effectively, it is necessary also determine what activities employees have been undertaken.

Pullout: to analyse effectively, it is necessary also to determine what activities employees have been undertaking

The Sound Level Meter and the Noise Dosimeter have overlapping roles when it comes to taking noise measurements, with each having distinct advantages over the other. Table 2 below gives some guidelines of which instrument is the most appropriate for the job.

Table 2 – Guidelines for Instrument Selection

Type of MeasurementAppropriate Instrument (in order of preference)Type of resultComments

Personal noise exposure1. DosimeterDose or equivalent sound levelMost accurate for personal noise exposures when worker is mobile.

2. Integrating Sound Level MeterEquivalent sound levelCan be useful when the work is divided into defined activities and/or areas.

Noise levels generated by a particular source1. Integrating Sound Level MeterEquivalent sound level Particularly useful if noise is highly variable; it can measure equivalent sound level over a short period of time (1 minute).

Noise survey1.  Integrating Sound Level MeterEquivalent sound level To produce noise map of an area; take measurements on a grid pattern.

2. DosimeterDose or equivalent sound levelFor highly variable noise.

Impulse noise1. Sound Level Meter with Impulse capabilityPeak pressure dB(A)To measure the peak of each impulse.

In addition to the Sound Level Meter and Dosimeter there are also a number of non standard, generally uncalibrated but cheap devices which can be used to help in undertaking semi- quantitative assessments as well as helping with risk and hazard communication.

For instance, it is possible to buy a key ring Sound Check Noise Level Indicator for under £10. The device uses three coloured LEDs to give an indication of noise levels between 55dB and 100 dB. It comes with appropriate disclaimers about its accuracy (+/- 3dB) but is being marketed as a device to be used by peripatetic workers. Whilst its usefulness as a tool for assessment purposes is limited, it undoubtedly has strengths in terms of hazard awareness and risk communication. The same is also true for a number of free and paid for applications for the Apple iPhone which use the phones internal microphone. Although caution should be advised when using these, since the impressive graphics and digital readout to one decimal place imply a level of both precision and accuracy that is way beyond its capability.

Picture taken from Sensorcom Web site – Permission not sought but I don’t doubt you’d get it.

Picture taken from  Permission not sought but I don’t doubt you’d get it.

In addition to this, a variety of companies sell noise alert signs which illuminate at preset noise levels indicating that exposure has reached a point where ear protection is required. Devices such as these may be useful in situations where work is varied in nature such as during maintenance activities.

Before taking any measurements it is important to understand their purpose. Whilst this may appear obvious I have seen lots of surveys fall short of their objectives because the purpose of the measurements wasn’t fully addressed at the start. Noise measurements can have a variety of purposes, each of which may require different strategies.

Assessment of Exposure – The most important purpose of noise measurements has to be the assessment of personal exposures. Making the decision as to whether exposures are acceptable or not influences the measurement strategy significantly. The assessment of exposure is best done using a mixture of Sound Level Meter readings as well as personal noise dosimetry results.

Identification of noise sources – Finding the sources of noise exposure is an important prerequisite to controlling the exposures. In many cases the sources of noise may seem obvious, but identifying their relative contribution to the overall exposure may not be as obvious. Identification is principally undertaken using a Sound Level Meter.

Specification of Ear Defenders or engineering controls – It is important to ensure that the control measures you introduce to reduce exposure are effective. In the case of ear defenders this normally involves the measurement of a frequency analysis. This involves the measurement of noise levels over a range of specific frequencies. Using this method the entire audible frequency range is divided into frequency windows of fixed width and noise level is measured in dB units at each of these frequency windows. Historically, the measurement of frequency analysis was time consuming and required repeated measurements at individual frequencies. Modern instruments can now measure all frequencies at the same time as well as being able to separate the analysis into a variety of different bands.

Gone too are the days of complicated nomograms; a variety of different, simple to use calculation tools are now readily available for calculating the efficacy of ear defenders. For instance the UK HSE produces an excel spreadsheet which will calculate efficacy using any of the three most common calculation methods.

Specification of Administrative controls – The production of noise maps, which illustrate how noise levels vary on a site, can be helpful when deciding where to allocate Ear Protection Zones.

Risk communication – Noise measurements can be used to illustrate the nature of hazard and risk under different circumstances. Measuring noise and showing the results to workers can help then understand the need for the use of PPE, engineering or administrative controls.

Demonstration of compliance with regulations – In theory this aim should be met if the measurements were undertaken in order to assess exposure. However some legislation gives specific requirements for measurements and reporting and it is important that these are considered.

Legislation (possible separate panel)

In the EU exposure to noise at work is covered by Directive 2003/10/EC of the European Parliament “The Physical Agents (Noise) Directive”. The Directive is implemented within member states through local legislation and places the following Duties on employers.

Assessment: The employer must assess the risk of exposure to noise.

Control: The employer should control exposure to noise through the use of engineering, administrative controls as well as Personal Protective Equipment.

Health Surveillance: Employers will need to provide audiometric testing for workers who are considered at risk.

Limits: The directive sets the following limits for noise.

•Lower Action Value (LAV) – 80dBA Leq,8hr and peak pressure Ppeak of 112 pascals.

•Upper Action Value (UAV) – 85dBA Leq,8hr and peak pressure, Ppeak of 140 pascals.

•Exposure Limit Value (ELV) – 87dBA Leq,8hr and peak pressure, Ppeak of 200 pascals


Like all things Health and Safety, competence is a central tenet when undertaking any form of noise measurements.  However this does not mean that noise measurements can only be undertaken by those with degrees in acoustics. A relatively small amount of training will enable rudimentary assessments to be undertaken. Whilst more detailed assessments will require a higher degree of training as well as experience a range of courses is available via professional bodies such as BOHS and IOSH. In addition to this the relatively new Occupational Hygiene Training Association (OHTA) administers international courses on noise.


Undertaking noise measurements can be a rewarding experience for the safety professional. The results are almost instantaneous and the conclusions are unambiguous. This makes the findings of any noise assessment straightforward and engaging to communicate to employees. Workers and managers alike find it easy to relate to numbers and simple limits. As well as using measurements for risk assessment and demonstrating compliance they are a useful tool when trying to implement control measures.

Adrian Hirst is an Occupational Hygienist based in the UK. He is also a senior lecturer in Occupational Hygiene at the University of Manchester.

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Published: 10th Apr 2010 in Health and Safety International