OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 16 Noise

Section 219 Noise exposure assessment

The first step in a noise management program or in efforts to control noise is to assess existing workplace noise levels. This section requires employers to conduct an assessment where workers are or may be exposed to noise levels in excess of the occupational exposure limits shown in Table 1 of Schedule 3 and exceed a noise level of 85 dBA Lex.

A number of factors should be considered when analyzing the extent of the hazard to which workers may be exposed:

(a) sound from a source can travel by more than one path to the location where it becomes a hazard;
(b) many industrial sound sources are directional i.e. sound sources such as intake and exhaust vents radiate more sound in one direction than another;
(c) sound from equipment may be transmitted by vibration;
(d) the frequency of the noise has a large impact on how far it travels from the source as well as the measures needed to control it. In addition, a person’s perception of noise is related to the frequency of the noise, with human hearing being best at frequencies between 500 and 5000 Hz. Noise at frequencies below 500 Hz and above 5000 Hz can still cause hearing damage, even though these sounds are not perceived to be as loud. Noise measurements made with an instrument equipped with an A-weighted network will discount the contribution of low frequency components to the overall noise measurement;
(e) if noise exposure changes due to seasonal or product variations, noise measurements need to be repeated; and
(f) where workers rotate irregularly between different jobs and activities, it may be more useful to determine noise exposure based on the job and worker noise exposure based on the time a worker spends at each job or activity.

An initial noise assessment should be performed in any work area where workers must significantly raise their voices to be heard over background noise. The assessment should include work areas that are indoors, outdoors and in mobile equipment. If the results of the initial assessment indicate that no workers are exposed to noise levels exceeding the exposure limits, then periodic assessments should be performed thereafter to make sure that conditions have not changed over time. Periodic assessments should be done on an annual basis and when

(a) new equipment or work processes that generate noise are introduced to the work site,
(b) noise levels change due to equipment deterioration,
(c) work practices or work procedures change, or
(d) workers complain of ringing in the ears, temporary changes in hearing or increased levels of noise in their work area.

The noise assessment should be done by an industrial hygienist, audiologist or professional with appropriate training and the noise must be measured in accordance with the CSA Standard Z107.56-06, Procedures for the Measurement of Occupational Noise. Workers should be permitted and encouraged to observe and participate in monitoring activities, as long as doing so does not interfere with the monitoring. Worker participation helps ensure valid results as workers can identify noise sources, indicate periods when noise exposure may differ, and recognize whether noise levels are typical or atypical. Workers can explain how different operating modes affect equipment sound levels and can describe their tasks and working positions.

Performing noise measurements

There are many ways of measuring occupational noise and a variety of instruments for doing so. The choice of a particular method or instrument depends on many factors, including the purpose of the measurement and the environment in which the measurement is made. Monitoring procedures need to be thoroughly defined to ensure consistency from one measurement to the next. Instrumentation, calibration, measurement parameters and methods for data analysis must be clearly described.

This section requires that noise measurements be performed in accordance with CSA Standard Z107.56-06, Procedures for the Measurement of Occupational Noise Exposure. Using this Standard ensures that a consistent procedure is followed when making noise measurements. The procedure determines a worker’s long term noise exposure using measurements of equivalent sound level in the workplace. Procedures for measuring all types of noise – continuous, pure tones, impulse – are provided in the Standard. All types of noise are measured in terms of equal energy. The Standard can be used to measure the noise exposure of individuals or groups and measurements are taken using a 3 dB exchange rate. The Standard provides procedures for measuring noise and reporting the results.

While the CSA Standard does not address the frequency of the noise being measured, this is an important factor when determining noise exposure. In environments where the noise is mostly low frequency, measurements taken with an instrument equipped with an A-weighted filter will underestimate noise exposure. It is useful therefore to conduct a frequency analysis of the noise using a sound level meter with an octave band filter or, if this type of equipment is not available, to measure the noise with both C-weighted and A-weighted filters and compare the results.

If the results vary widely, the noise may have a large low frequency component to it. This will need to be taken into account when measuring noise exposure (correction may be applied to the A-weighted values) and when determining noise levels for the purpose of implementing control measures. Table 16.3 shows the variations among readings taken with A-, B- and C-weighted filters.

Table 16.3 Selected relative response for A-, B- and C-weighted filters

Frequency
(Hz)

A-weighting
(dB)

B-weighting
(dB)

C-weighting
(dB)

20

-50.5

-24.2

-6.2

50

-30.2

-11.3

-1.3

100

-19.1

-5.6

-0.3

500

-3.2

-0.3

0

1000

0

0

0

2000

+1.2

-0.1

-0.2

5000

+0.5

-1.2

-1.3

10000

-2.5

-4.3

-4.4

20000

-9.3

-11.1

-11.2

Source: Earshen, John J. (1986) Sound Measurement: Instrumentation and Noise Descriptors, Noise and Hearing Conservation Manual 4th Edition, American Industrial Hygiene Association, pp 51

Notes:
(1) Although the B-weighted filter is rarely used in noise exposure measurements, its responses are presented for comparison purposes.
(2) How to use the table: A sound with a frequency of 1000 Hz has an intensity of 90dB. At this frequency, the readings in dBA, dBB and dBC are all the same (90). At 500 Hz, the reading corresponds to 86.8 dBA, 89.7 dBB and 90 dBC. At 50 Hz, the reading corresponds to 59.8 dBA, 78.4 dBB and 88.7 dBC.

Competent person

A competent person, otherwise known as a competent worker, performs the noise measurements. This person must be able to correctly use the instrumentation and be able to understand and interpret the measurement results.

Updating measurements

Noise measurements must be updated when equipment or processes change that could affect the noise levels or the duration of worker exposure.

Instrumentation

Four different types of instrumentation can be used to measure noise exposure:

(1) a sound level meter;
(2) a noise dosimeter;
(3) an integrating sound level meter; or
(4) other equipment acceptable to a Director of Occupational Hygiene.

The fourth option involves applying for an acceptance, a process described in section 10 of the OHS Regulation.

Sound level meter

This is the basic measuring instrument for noise. It consists of a microphone that converts sound pressure variations into electrical signals, a frequency selective amplifier, a level range control, frequency weighting to shape the frequency response of the instrument, and an indicator. For each particular application, the measurement technique must be carefully chosen and controlled to obtain valid and consistent results.

ANSI Standard S1.41983 (R2006), Specification for Sound Level Meters, provides three frequency weighting scales (A, B and C, although only A-weighted measurements are used in the OHS Code) and two exponential time averaging characteristics, slow and fast. In most industrial settings, the meter fluctuates less if measurements are made with the slow response setting. The fast response setting is normally used to measure how noise fluctuates over time rather than noise exposure.

The Standard specifies three grades of instruments:

(1) Type 0 – intended for use in a laboratory as a reference standard;
(2) Type 1 – intended for precision measurements of sound in the field and laboratory; and
(3) Type 2 – designated for general field use to measure typical environmental sounds where high frequencies do no dominate.

The accuracy of a sound level meter is dependant on the type of meter, the frequency being measured, the orientation of the sound relative to the microphone and the time variation of the sound pressure. Selection of a particular sound level meter should be made following a review of the need for accuracy, the frequency and other characteristics of the sound being measured.

Correct use of the microphone is very important to obtaining accurate measurements. Microphones are designed for use in a particular environment across a specific range of sound pressure levels and frequencies. They also differ in directionality as some must be pointed directly at the sound source and others at an angle to the sound source. Equipment users need to follow the manufacturer’s instructions regarding the type and size of microphone and how it is to be used.

Noise dosimeter

A noise dosimeter is a sound level meter worn by the worker. It measures and stores sound levels during an exposure period and computes the exposure as a percentage of a criterion level such as an occupational exposure limit. The noise must be measured using an A-weighted filter with a 3 dBA exchange rate in order to compare the measured results to the exposure limits specified in the OHS Code. ANSI Standard S1.25-1991 (R1997), Specification for Personal Noise Dosimeters, provides acceptable characteristics for noise dosimeters.

In noise dosimetry, the microphone is attached to the worker whose noise exposure is being measured. Placement of the microphone is important in estimating exposure. The microphone is usually mounted on a shoulder, at the chest, or in the ear.

When noise levels are continuous and the worker remains essentially in one work area during the work shift, measuring noise exposure with a sound level meter is relatively straightforward. However, a noise dosimeter is preferred for measuring worker exposure when noise levels vary or are intermittent, when they contain components of impulse noise or when the worker frequently moves around during the work shift.

Integrating sound level meter

This instrument is a sound level meter with properties similar to those of a dosimeter. Like a noise dosimeter, it can be used to measure varying or intermittent noise and impulse noise and the worker can move around while wearing the instrument.

Typical applications for integrating sound level meters are identical to those for standard sound level meters. Integrating sound level meters can however be used to measure the average sound pressure level around noisy equipment or other sound sources where the integrating capacity can be used to determine the average sound level in space as well as time. The two main differences between sound level meters and integrating sound level meters are:

(1) averaging durations for an integrating meter are usually much longer than those for a standard sound level meter, extending to minutes or hours; and
(2) the integrating meter gives equal emphasis to all sounds that occur during the selected averaging period, while the standard sound level meter gives more emphasis to recently occurring sounds.

Instrument operation

To ensure that measurements are accurate, sound measuring equipment must be calibrated, maintained and operated according to the manufacturer’s specifications. Calibrations are often required annually.

For more information
Noise in the Workplace
Bulletin HS003