THE PRINCIPLES OF
NOISE MEASUREMENT
James Jerome
Sr Occupational Audiologist
Tom Lloyd
Noise Control Engineer
The Principles of Noise Measurement
Agenda topics
Acoustical instrumentation
Types of noise surveys
Noise exposure criteria
Details of the hearing conservation survey
Overview of the principles of noise control
Federal Regulations
Occupational Safety & Health Act (OSHA)29 CFR 1910.95 (effective March 8, 1983)
Mine Safety & Health Act (MSHA)30 CFR Part 62 (effective September 13, 2000)
Federal Railroad Administration (FRA)49 CFR 227 & 229 (effective February 26, 2007)
Acoustical Instrumentation
The basic tools for noise measurement include
the sound level meter, personal noise dosimeter,
and acoustical calibrator.
Sound Level Meters
The sound level meter (SLM) measures sound
pressure level (SPL).
SLM Components
Regulations require use of the following microphone
types:
Type 2 (±2dB accuracy)
General survey measurements
Lower cost than precision SLM
May be used with octave-band filters
Type 1 (±1dB accuracy)
Diagnostic measurements
Commonly used with octave-band filters
Frequency Analysis
Real-Time Analyzer (RTA) or Octave Band
Analyzer (OBA)
Serial-band filters divide the audible frequency
range into separate bands.
Octave-Band (1/1): nine filter bands cover the
audible range
Third-Octave Band 1/3): each octave is split into
three narrow bands for more resolution
Acoustical Calibrators
An acoustical calibrator is a device that produces
a fixed SPL at a fixed frequency.
Instrument Settings
Measurement parameters must be set to match the
process and survey objectives.
Range: measured signal is near the middle of
range limits
Integration: average signal over time; set by
user with run/stop key
Instrument Settings
Weighting
Three weighting networks are commonly used
for industrial surveys:
Linear: used for octave-band analyzers (OBAs)
A-weighting: used for regulatory compliance,
employee exposure, and noise control evaluation
C-weighting: used for quick evaluation of lowfrequency noise without full spectrum
Instrument Settings
Weighting
As per the federal regulations, the A-weighted
level must be used when conducting a sound
survey for hearing conservation purposes.
A-weighted Sound Level
The A-weighted sound level provides a
single number rating that correlates
reasonably well with human perception of
the loudness of sound and hearing-damage
risk due to exposure to continuous sound.
The results are expressed in dBA.
Used world-wide.
A-weighted Correction Factors
Instrument Settings
Response
Four settings are typically available:
Slow: 1-second time constant
Fast: 1/8th-second time constant
Peak: measures instantaneous peak level
Impulse: for special conditions
Instrument Settings
Response
As per the federal regulations, the Slow
response setting must be used when
conducting a sound survey for hearing
conservation purposes.
Using the Sound Level Meter
Several factors affect the
measurement results:
Instrument position
Environmental variables
Human factors
Using the SLM
Instrument position
Microphone orientation:
Hold at 70-degrees for pressure microphone
(random incidence)
Point SLM at source for free-field microphone
Windscreen:
Use outdoors when wind speed above 10mph
Use indoors to protect mic from bumps and
moisture
Vibration:
Never set SLM on vibrating surface
Using the SLM
Environmental variables
Wind speed and direction:
Sound level increases downwind from source
Barometric pressure and temperature:
Small effect on sound level readings
Calibrate SLM at site elevation
Humidity:
Water can damage microphone diaphragm
Using the SLM
Human Factors
Body reflections:
Hold SLM away from body
Position body perpendicular to noise source
Reflecting surfaces:
Choose test location away from walls or large
machine panels.
Background noise:
Does background add to machine noise?
Using the SLM
Human Factors
Study machine/source function:
Match instrument settings to machine process
Use Leq to average over fluctuating work cycle
Use Lmax to identify compressed-air burst
Use Lpeak to measure peak levels due events such
as gun fire
Identify cyclical operations to capture loudest parts of
total work cycle
Using the SLM
Perform these tasks for each survey period:
Understand settings of SLM
Check batteries; replace if necessary
Inspect SLM for damage
Pre-survey calibration
Verify instrument settings for survey
Take written notes of machine operation
Post-survey calibration check
Personal Noise Dosimeters
A noise dosimeter is basically an SLM designed to
measures a worker’s noise exposure over the entire
work shift, and it is worn by the worker. Almost all
dosimeters are Type 2 instruments.
Personal Noise Dosimeters
Miniature microphone positioned at the employee's
mid-shoulder with a wire running to a microprocessor
unit, which, in turn, is clipped to the wearer's belt or
placed in a pocket.
Newer units are one piece "badges" that are placed
mid-shoulder. Very compact and easy to wear.
Continuously measure and process A-weighted sound
levels obtained from person's "hearing zone". Then
produces a dBA TWA or equivalent per cent dose
based on the length of the sampling period.
Types of Noise Surveys
Preliminary
General Purpose
Noise Control
Types of Noise Surveys
Preliminary
Basic needs assessment.
Involves a walk through of each plant area, using a
sound level meter to identify areas at or above 80 dBA,
which would then be identified for follow-up under the
General Purpose - Hearing Conservation Survey.
Quite often the Preliminary survey is bypassed and a
General Purpose survey is conducted in all areas.
Include a memo in the “noise file” of all low-noise areas
(<80 dBA).
Types of Noise Surveys
General Purpose
This is the Hearing Conservation Survey
The objectives are:
Determine employee daily noise exposures
Measure and document the sound levels
throughout an area
Identify equipment for potential noise control
evaluation
The Hearing Conservation Survey
Who typically does this service?
Industrial Hygienists, Safety Professionals, and
Audiologists trained in noise measurement
What does it cost?
Roughly $500-$1500 per day - depending on the
scope
The Hearing Conservation Survey
When should a survey be conducted?
After the initial survey, repeat surveys are required
when changes in production or equipment occur that
may affect the noise exposures. Recommend
updated surveys every 2-3 years.
Where should a survey be conducted?
In all areas where sound levels are 80 dBA or above
Hearing Conservation Requirements
The OSHA Regulatory Criteria are:
Inclusion in the HCP when the dailyaverage noise exposure equals or exceeds
85 dBA, or an equivalent 50% dose. This is
termed the Action Level.
The Permissible Exposure Limit (PEL) is a
daily-average noise exposure of 90 dBA, or
an equivalent 100% dose.
Action Level
Point at which an effective hearing conservation
program must be implemented (≥85 dBA 8-hr TWA).
At minimum, program consists of:
Conducting annual hearing testing
Conducting annual hearing conservation training
Making hearing protection available (mandatory when
TWA is >90 dBA or if worker has an STS when working
between 85-89 dBA of noise)
Hearing Conservation Requirements
The OSHA Regulatory Criteria are:
Engineering noise controls are required to be
investigated when worker noise exposures
exceed the PEL of 90 dBA (100% dose)
Noise controls must be implemented when
determined to be feasible
The feasibility assessment must take into account
both technical and economic factors
Hearing Conservation Requirements
The OSHA Regulatory Criteria are:
Hearing protection must be made available for all
workers with a daily-average noise exposure equal
to or above 85 dBA (≥50% dose),
Use of hearing protection is mandatory for all
workers exposed above 90 dBA (≥100% dose),
unless they have a standard-threshold shift (STS) in
hearing at which point mandatory protection is
required for exposures at or above of 85 dBA.
Hearing Conservation Requirements
Best-Practice Criteria:
The hearing conservation program action
level (inclusion), permissible noise
exposure limit, mandatory hearing
protection, and noise control assessment
are recommended when the daily-average
noise exposures equal or exceed 85 dBA.
Determining Worker Noise Exposure
One objective of the hearing conservation
survey is to determine the average daily
noise exposure of workers.
To complete this objective the federal
regulations first require determination of
the workers’ Noise Dose.
Determining Worker Noise Exposure
Noise Dose is a percentage (%) of the
maximum allowable noise that a worker may
be exposed per day.
Determining Worker Noise Exposure
Noise Dose (%) is calculated using the following
variables:
Criterion Level (CL) = 90 dBA
Threshold Level (TL) = 80 dBA and 90 dBA
Exchange Rate (q) = 5 dBA
Determining Worker Noise Exposure
Criterion Level (CL) is set to 90 dBA and represents the
daily limit of accumulated sound energy a person may be
exposed to in an 8-hour period. So a Noise Dose of
100% would be equal to 90 dBA.
Note: for shorter or longer work shifts, the Noise Dose
calculation should be normalized to 8 hours, which allows
comparison of the results to the regulation criteria.
Determining Worker Noise Exposure
The Threshold Level (TL) is the cutoff level
below which no sound energy is added into
the dose calculation.
The federal regulations require the use of two
separate cutoff or threshold levels, which are
80 dBA and 90 dBA.
Determining Worker Noise Exposure
What is the purpose of two threshold levels?
LTL = 80 dBA; referred to as the “low” threshold
level and is used to determine whether or not
worker noise exposures warrant inclusion in
the hearing conservation program.
HTL = 90 dBA; referred to as the “high” threshold
level and is used to determine compliance with
the PEL of 90 dBA.
Determining Worker Noise Exposure
The Exchange Rate (q) is the trade-off
relationship between an increase (or decrease) in
sound level and the corresponding change in
allowable exposure time.
(see Table in next slide)
Regulatory
As the sound level
increases by 5 dBA, the
allowable reference
duration one is permitted
to be exposed to that
level is cut in half.
Conversely, as the
sound level decreases
by 5 dBA, the allowable
time of exposure to that
level doubles.
TABLE G-16a (OSHA) or
TABLE 62-1 (MSHA)
Sound level, dBA
Reference duration, T (hours)
80
32
81
27.9
82
24.3
83
21.1
84
18.4
85
16
86
13.9
Time is
87
12.1
q = 5 dBA
doubled or
88
10.6
halved
89
9.2
90
8
91
7.0
92
6.1
93
5.3
94
4.6
95
4
96
3.5
97
3.0
98
2.6
99
2.3
100
2
101
1.7
Table continues for higher sound levels
Table 1. TLVs for Noise (ACGIH)
Best Practice
ACGIH – as the sound
level increases by 3
dBA, the allowable
reference duration one
is permitted to be
exposed to at that level
is cut in half.
Conversely, as the
sound level decreases
by 3 dBA, the allowable
time of exposure to that
level doubles.
Sound Level, dBA
Reference Duration, T (hours)
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
24
20.2
16
12.7
10.1
8
6.35
5
4
3.2
2.5
2
1.6
1.3
1
0.8
0.63
0.5
0.4
0.3
0.25
0.2
q = 3 dBA
Time is
doubled or
halved
-Table continues for higher sound levels-
AllowableTim
Time,e,Hrs
Hrs
Allowable
HLPP Inclusion Criteria
16
16
14
12
10
8
6
4
22
00
85
85
90
90
95
95
100
100
105
105
Noise Level, dBA
110
110
NIOSH
NIOSH
115
115
OSHA
Slide courtesy of Rick Neitzel
OSHA = 5 dB exchange rate; NIOSH = 3 dB exchange rate
Determining Worker Noise Exposure
Calculation of Noise Dose (D),%:
D = 100 (C1/T1 + C2/T2 + ... + Cn/Tn)
Cn = Total time of exposure at a certain noise
level,
Tn = Reference duration for that level, as given
by in the appropriate federal regulations
Determining Worker Noise Exposure
As you can see in the previous equation,
Noise Dose is a function of the noise level
and duration of exposure to each level a
worker experiences throughout the
workday.
Determining Worker Noise Exposure
Noise Dose (D), %
Action Level (AL) is a dose of 50% or more,
Permissible Exposure Limit (PEL) is 100%
Time-weighted average (TWA), dBA
AL is a TWA of 85 dBA,
PEL is a TWA of 90 dBA.
Determining Worker Noise Exposure
Time-Weighted Average (TWA), dBA
The average noise level a worker is exposed to
throughout the day normalized to an 8-hour
period. Once the TWA is determined it may be
compared to the regulation criteria for
compliance with the PEL and inclusion in a
hearing conservation program.
Determining Worker Noise Exposure
Time-Weighted Average (TWA), dBA
TWA = 16.61 Log(D/100) + 90
If you know the dose (D), the TWA can be found
by looking in Table A-1 of the OSHA standard or
Table 62-2 of the MSHA regulation. (See
Example in next slide)
Determining Worker Noise Exposure
Example:
During a typical 8-hour shift, an employee spends:
4 hours at 87 dBA
3 hours at 92 dBA
30 minutes at 95 dBA
30 minutes at 84 dBA.
What is this employee’s TWA?
TABLE G-16a (OSHA) or
TABLE 62-1 (MSHA)
Sound level, dBA Reference duration, T (hours)
80
32
81
27.9
82
24.3
83
21.1
84
18.4
85
16
86
13.9
87
12.1
88
10.6
89
9.2
90
8
91
7.0
92
6.1
93
5.3
94
4.6
95
4
96
3.5
97
3.0
98
2.6
99
2.3
100
2
101
1.7
Table continues for higher sound levels
Determining Worker Noise Exposure
Example:
L1 = 87 dBA
C1 = 4 hours
T1 = 12.1 hours
L2 = 92 dBA
C2 = 3 hours
T2 = 6.1 hours
L3 = 95 dBA
C3 = 30 min.
T3 = 4 hours
L4 = 84 dBA
C4 = 30 min.
T4 = 18.4 hours
Determining Worker Noise Exposure
Example:
D = 100 (C1/T1 + C2/T2 + ... + Cn/Tn)
D = 100 x (4/12.1 + 3/6.1 + 0.5/4 + 0.5/18.4)
D = 100 x (0.975)
D = 97.5%
TWA = 89.8 dBA (from formula or table)
Who Needs to be Monitored?
Determining who to monitor needs to be
carefully planned.
The goal is to determine the daily-average
noise exposure per worker. But the level of
accuracy needs to be decided upon:
Are you just trying to identify the potential for a
worker to be exposed at or above 85 dBA?
Or, do you prefer more precise exposure results?
Who Needs to be Monitored?
If the survey objective is to identify the potential for a
worker to be exposed at or above 85 dBA, then
considerably less sampling is required than a precise
exposure determination. In the former situation, it is
a “go” or “no go” decision as far as inclusion in the
HCP.
If you prefer more precise exposure results, then it is
very likely significantly more sampling is required to
establish a more accurate and representative
exposure level for each worker.
Measuring Equipment and Area
Sound Levels
The second objective
for the hearing
conservation survey is
to measure and
document equipment
and/or area sound
levels.
Measuring Equipment and Area
Sound Levels
This survey is conducted with at least a
Type 2 sound level meter (±2 dB), as
per ANSI requirements.
Typically, the A-weighted sound level is
measured approximately 3 feet or one
meter away from each piece of
equipment in the area of concern.
Measuring Equipment and Area
Sound Levels
These data may be documented in a separate
table report, as a point measurement on a
floor plan or equipment layout, or be used to
generate a series of noise contour maps.
An example of a noise contour is shown in the
next slide.
Measuring Equipment and Area
Sound Levels
Noise contour maps may be used to illustrate to
workers the degree of noise hazard in their work
areas, as an easy-to-understand educational
tool, to illustrate hearing protection required
areas, or to identify dominant noise sources that
may be further investigated for potential noise
control measures.
Types of Noise Surveys
Noise Control Survey
The objectives are:
Identify and prioritize dominant noise sources,
Determine source contributors to excessive
worker noise exposure, and
Obtain frequency data for the selection of the
appropriate control measures and/or
acoustical materials.
Hierarchy of Noise Reduction
Elimination
Safe
Workplace
Substitution
Engineering Controls
Administrative Controls
Personal Protective
Equipment (PPE)
Safe
Worker
Engineering Controls
Can be accomplished by:
Reduction at source: mufflers, enclosures
Interrupt noise path: barriers, curtains,
sound absorbing materials
Reduction at receiver: enclosures
Comparison of HC Regulations
“The Modern Evolution of Hearing
Conservation Regulations”
Theresa Schulz, PhD
http://www.cdc.gov/niosh/mining/
userfiles/works/pdfs/tmeoh.pdf
OSHA
29 CFR 1910.95
Issue
MSHA
30 CFR Part 62
FRA
49 CFR 227 & 229
Exposure limit
PEL = 90 dBA TWA.
Similar to OSHA, except
integration range is explicit
in regulation (62.101), and
is for all sounds from 90 to
at least 140 dBA.
Same as OSHA
Action level
85 dBA TWA.
Similar to OSHA, except
integration is for all sounds
from 80 to at least 130
dBA.
Same as OSHA
Exchange rate
5 dB
Same as OSHA.
Same as OSHA
Impulse noise
Should not exceed 140 dB
peak SPL; to be integrated
with measurements of all
other noises
Integrate with
measurements of all other
noise.
Same as OSHA
Issue
OSHA
29 CFR 1910.95
MSHA
30 CFR Part 62
FRA
49 CFR 227 & 229
Ceiling
No exposures > 115 dBA,
interpreted as no
unprotected exposures;
give credit for HCP, HPDs
and engineering controls.
No exposures > 115 dBA;
no adjustment for use of
hearing protection; “P”
code issued where the
miner is still over-exposed
even though feasible
engineering and
administrative controls are
in place.
No exposures > 115 dBA,
except continuous > 115
dBA and < 120 dBA are
permissible, provided total
daily exposures < 5
seconds.
Monitoring
noise exposure
Once to determine risk and
HCP inclusion; then as
conditions change resulting
in more potential exposure.
Mine operator must
establish system to
evaluate each miner’s
exposure sufficiently to
determine continuing
compliance with rule.
Same as OSHA;
measurement artifacts may
be removed.
Issue
OSHA
29 CFR 1910.95
MSHA
30 CFR Part 62
FRA
49 CFR 227 & 229
Noise control
Feasible controls required
where TW A > 90 dBA,
compliance policy (OSHA
can change/revoke any
time) permits proven
effective HCP in lieu of
engineering where TWA <
100 dBA.
Feasible engineering and
administrative controls
required for TW A > 90
dBA; even if controls do not
reduce exposure to PEL,
they are required if feasible
(i.e. > 3-dBA reduction);
administrative controls
must be provided to the
miner in writing and posted.
FRA describes the specific
actions that railroads and
manufacturers must take
when designing, building,
and maintaining
locomotives (instead of
engineering controls);
“noise operational controls”
(administrative controls);
hearing protection (same);
“FRA has no hierarchy of
noise controls.”
Administrative
controls / noise
operational
controls
Feasible administrative
controls required where
TWA > 90 dBA
Administrative controls
must be provided to miner
in writing and posted.
FRA does not require the
use of noise operational
controls but makes them
operational.
The Principles of Noise Control
Identify sources of noise and their relative importance.
List noise control options as they apply to the source,
path and receiver.
Identify contributions from direct and reflected sound.
Distinguish between sound absorption and sound
isolation materials.
Identify and evaluate significance of flanking paths.
Identify and evaluate significance of structure-borne
noise.
The Principles of
Noise Measurement
Questions/Discussion?