MECH 482 – Noise Control
Week 3, Lecture 1
Noise Criteria
January 17, 2015
Page 1
Noise criteria
• An important part of any noise-control program is the
establishment of appropriate criteria for the determination of
an acceptable solution to noise problems.
• Thus, where the total elimination of noise is impossible,
appropriate criteria provide a guide for determining how
much noise is acceptable and/or how much reduction is
required.
• The required reduction in turn provides the means for
determining the feasibility of alternative proposals for
control, and finally the means for estimating the cost of
meeting the relevant criteria.
January 17, 2015
Page 2
Noise criteria
Occupational and environmental noise measures are
used to determine/define acceptable noise levels.
These criteria include:
• Hearing loss and hearing damage risk criteria
• Speech interference criteria
• Psychological effects of noise
• Ambient noise level specification
• Environmental noise criteria
January 17, 2015
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Noise criteria
The following criteria are based on:
• ISO (International Organization for Standardization)
standard 1999 (1990)
• OSHA (Occupational Safety and Health
Administration, USA)
• Many other standards exist, but will not be covered
in this course.
January 17, 2015
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Occupational Noise Measures
A-weighted equivalent continuous noise level, LAeq
The A-weighted equivalent continuous noise level is
obtained by first A-weighting the noise and then averaging
the sound pressure level over a period of time.
This type of averaging is often referred to as energy
averaging and is used as a descriptor of both occupational
and environmental noise .
LAeq ,T
January 17, 2015
 1 T LA (t )/10 
dt 
 10 log10   10
T 0

Page 5
Occupational Noise Measures
A-weighted equivalent continuous noise level, LAeq
For occupational noise, the most common descriptor is
LAeq,8h , which implies a normalisation to 8 hours, even
though the contributing noises may be experienced for more
or less than 8 hours.
Thus, for sound experienced over T hours:
LAeq ,8 h
January 17, 2015
 1 T LA (t )/10 
 10 log10   10
dt 
8 0

Page 6
Occupational Noise Measures
A-weighted equivalent continuous noise level, LAeq
If the sound pressure level is measured using a sound level
meter at m different locations where an employee may spend
some time, then the previous equation becomes:
LAeq ,8 h
1
 10 log10 t110 LA1 /10  t210 LA 2 /10  ......  tm10 LAm /10 
8
where LAi are the measured equivalent A-weighted sound
pressure levels and ti are the times in hours which an
employee spends at the m locations.
Note that the sum of t1...tm does not have to equal 8 hours.
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Occupational Noise Measures
A-weighted Sound Exposure, EA,T
Industrial sound exposure may be quantified using the Aweighted Sound Exposure, EA,T, defined as the time integral of
the squared, instantaneous A-weighted sound pressure, (Pa2)
over a particular time period (hours). The units are pascalsquared-hours (Pa2.h) and the defining equation is:
t2
E A,T   p (t )dt
2
A
t1
January 17, 2015
Page 8
Occupational Noise Measures
A-weighted Sound Exposure, EA,T
The relationship between the A-weighted Sound
Exposure and the A-weighted Equivalent
Continuous Noise Level, LAeq,T , is:
E A,T  4T 10
January 17, 2015
( LAeq ,T 100)/10
Page 9
Occupational Noise Measures
A-weighted Sound Exposure Level, EAE or SEL
The A-weighted Sound Exposure Level is:
LAE
 t2 p 2 (t ) 


E
3600

A
T
,
 10 log10   A2 dt   10 log10 

2
pref


 t1 pref

where the times t1, t2 and dt are in seconds (not hours as
for A-weighted sound exposure) and T = t2 - t1.
January 17, 2015
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Occupational Noise Measures
Equivalent Continuous Noise Level, EAeq,8h
The Equivalent Continuous Noise Level for a nominal 8
hour day may be calculated from EA,8h, or LAE using:
L Aeq,8h
 E A,8 h 
 1
LEA , 8 h 10 
  10 log10 
 10 log10 
10

9 
 28,800

 3.2 10 
January 17, 2015
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Occupational Noise Measures
Based on OSHA standards
Continuous exposure to noise levels greater than 115dBA
are not permitted for any duration.
Exposure to any noise above 85dBA must be acted upon
with some kind of exposure mitigation strategy.
Permissible noise levels for various times of exposure, T,
are determined from the following equation.
5 log10 16 / T 
LA  85 
log10 2
January 17, 2015


 for T  16 hours 


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Occupational Noise Measures
Based on OSHA standards
Permissible times of exposure to a continuous noise level,
LA, are determined from the following equation.
T
January 17, 2015
16
2 LA 85 / 5


 for 85dBA  LA  115dBA 


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Occupational Noise Measures
Based on OSHA standards
When the daily noise exposure is composed of two or
more periods of noise exposure at different levels, the
combined effect must be considered
The Noise Exposure Dosage (NED) for periods where the
noise exposure level varies during the working period can
be determined from the following equation.
Cn
C1 C2 C3
NED  


T1 T2 T3
Tn
January 17, 2015
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Occupational Noise Measures
Based on OSHA standards
Cn
C1 C2 C3
NED  


T1 T2 T3
Tn
The quantities C1, C2, etc., are the total times of exposure
to the noise levels LA1, LA2, etc., in hours per day.
The quantities T1, T2, etc,. Are the total permitted exposure
times at the noise levels LA1, LA2, etc.
For noise levels LA < 85dBA, T = infinity.
For noise levels LA > 115dBA, T = 0.
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Occupational Noise Measures
Different standards in different jurisdictions
All Canadian provinces except Quebec
Noise Exposure Limits when Criterion Level = 85 dB(A)
3 dB(A) Exchange Rate
5 dB(A) Exchange Rate
Allowable Level dB(A)
Maximum Permitted
Daily Duration (hours)
85
8
85
88
4
90
91
2
95
94
1
100
97
0.5
105
100
0.25
110
January 17, 2015
Allowable Level dB(A)
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Occupational Noise Measures
Different standards in different jurisdictions
Quebec
Noise Exposure Limits when Criterion Level = 90 dB(A)
3 dB(A) Exchange Rate
5 dB(A) Exchange Rate
Allowable Level dB(A)
Maximum Permitted
Daily Duration (hours)
90
8
90
93
4
95
96
2
100
99
1
105
102
0.5
110
105
0.25
115
January 17, 2015
Allowable Level dB(A)
Page 17
Occupational Noise Measures
Different standards in different jurisdictions
Instruments capable of measuring noise doses automatically
are called “noise dose meters” (more on these later).
Employees who are exposed to noise levels over 85dB(A)
must have options available to limit their exposure.
These options may include…
• rotation between “quiet” and “noisy” areas
• personal hearing protestors
• engineered noise control
January 17, 2015
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Environmental Noise Measures
A-weighted Day-Night Average Sound Level, Ldn
The Day–Night Average Sound Level is used sometimes to
quantify traffic noise in some standards regarding the
intrusion of traffic noise into the community and is written in
terms of this quantity, Ldn , which is defined as:
07:00
22:00


1
LA ( t )/10
LA ( t )/10
Ldn  10 log10
dt   10
dt 
  10  10
24  22:00
07:00

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Environmental Noise Measures
A-weighted Day-Night Average Sound Level, Ldn
For traffic noise, the Day–Night Average Sound Level for a
particular vehicle class is:
Ldn  LAE  10 log10  N day  N eve  10  N night   49.4
N: number of vehicles
where, LAE is the A-weighted Sound Exposure Level for a single vehicle
pass-by, Nday , Neve and Nnight are the numbers of vehicles in the particular
class that pass by in the daytime (0700 to 1900 hours), evening (1900 to
2200 hours) and nighttime (2200 to 0700 hours), respectively and the
normalization constant, 49.4, is 10 log10 of the number of seconds in a day.
To calculate the Ldn for all vehicles, the above equation is used for each class
and the results added together logarithmically.
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Environmental Noise Measures
Community Noise Equivalent Level, Lden
The Community Noise Equivalent Level is used sometimes to
quantify industrial noise and traffic noise in the community and
some regulations are written in terms of this quantity, Lden, which
is defined as: …
07:00
19:00
22:00

1
LA /10
LA /10
LA /10
Lden  10log10   10 10 dt   10 dt   310 dt 
24 22:00
07:00
19:00

January 17, 2015
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Environmental Noise Measures
Community Noise Equivalent Level, Lden
For traffic noise, the Community Noise Equivalent Level for a
particular vehicle class is related to the Sound Exposure Level
by: …
L den  L AE  10 log 10 N day  3  N eve  10  N night   49 . 4
N: number of vehicles
LAE is the A-weighted Sound Exposure Level for a single vehicle pass-by,
Nday , Neve and Nnight are the numbers of vehicles in the particular class that
pass by in the daytime (0700 to 1900 hours), evening (1900 to 2200 hours)
and nighttime (2200 to 0700 hours), respectively and the normalization
constant, 49.4, is 10 log10 of the number of seconds in a day.
January 17, 2015
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Noise Criteria Curves
Specification of an A-weighted level is easy and
convenient, but it gives no indication of which
frequency components may be the source of noncompliance.
For most acoustic design purposes it is more
useful to make use of a weighting curve, which
defines a spectrum of band levels in terms of a
single number.
January 17, 2015
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Noise Criteria Curves
•
•
•
•
•
Noise rating (NR)
Noise criteria (NC)
Balanced noise criteria (NCB)
Room criteria (RC)
Room noise criteria (RNC)
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Noise Criteria Curves
• Noise rating (NR) curves have been adopted
by ISO and are intended for general use,
particularly for rating environmental and
industrial noise levels.
• They are also used in many cases by
machinery manufacturers to specify
machinery noise levels.
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Noise Rating
Curves
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Noise Criteria Curves
To determine the NR rating of a noise, measured octave band
sound pressure levels are plotted on the preceding figure and
the rating is determined by the highest weighting curve which
just envelopes the data.
If the highest level falls between two curves, linear
interpolation to the nearest integer value is used.
It is also possible to use 1/3 octave band data on 1/3 octave
band NR curves, which are obtained by moving the octave
band curves down by 10log10(3) = 4.77 dB.
January 17, 2015
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Noise Criteria Curves
Specification of an NR number means that in no
frequency band shall the octave band sound
pressure in the specified space exceed the
specified curve.
In practice, a tolerance of ±2 dB is implied.
In specifications, an allowance of 2 dB above the
curve is usually acceptable in any one octave
band, provided that the levels in the two adjacent
bands are only 1 dB below the criterion curve.
January 17, 2015
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Noise Criteria Curves
• Noise criteria (NC) curves were developed in
response to the need for specification of just
acceptable noise in occupied spaces with all
systems running.
• They are still used extensively in the building
services industry even though they have
been largely superseded by NCB criteria.
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Noise Criteria Curves
• Noise criteria curves are not defined in the 31.5
Hz octave band and thus do not account for very
low frequency rumble noises.
• They are also regarded as too permissive in the
2000 Hz and higher octave bands and do not
correlate well with subjective response to airconditioning noise.
• This has resulted in them now being considered
generally unsuitable for rating interior noise.
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Noise Criteria
Curves
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Noise Criteria Curves
• Balanced noise criteria (NCB) curves are
used to specify acceptable background noise
levels in occupied spaces and include airconditioning noise and any other ambient
noise.
• They apply to occupied spaces with all
systems running and are intended to replace
the older NC curves.
January 17, 2015
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Noise Criteria Curves
• The designation number of an NCB curve is
equal to the Speech Interference Level (SIL) of a
noise with the same octave band levels as the
NCB curve.
• The SIL of a noise is the arithmetic average of the
500 Hz, 1 kHz, 2 kHz and 4 kHz octave band
decibel levels, calculated to the nearest integer.
• To determine whether the background noise is
“rumbly”, the octave band sound levels of the
measured noise are plotted on a chart containing
a set of NCB curves.
January 17, 2015
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Noise Criteria Curves
• If any values in the 500 Hz octave band or lower
exceed by more than 3 dB the curve
corresponding to the NCB rating of the noise,
then the noise is labeled “rumbly”.
• If any of the octave band sound levels between
1000 Hz and 8000 Hz inclusive exceed this NCB
curve, then the noise is rated as “hissy”.
January 17, 2015
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Noise Criteria Curves
• Interest in the low frequency (31.5 Hz and 16 Hz)
bands stems from the fact that a level on the order
of 70 dB or greater may result in noise-induced
vibrations that are just “feelable”, especially in
lightweight structures. Such vibration can also give
rise to objectionable rattle and buzz in windows,
doors and cabinets, etc.
January 17, 2015
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Balanced Noise
Criteria Curves
A – high probability of
noise induced
vibrations in light
weight structures
B – moderate
probability
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Balanced Noise Criteria Curves
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Balanced Noise Criteria Curves
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Balanced Noise Criteria Curves
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Noise Criteria Curves
• Room criteria (RC) curves have been developed to
replace Noise Criteria curves for rating only air
conditioning noise in unoccupied spaces. Like the
NCB curves, the RC curves include 16 Hz and
31.5 Hz octave band levels, although few sound
level meters with external octave band filters
include the 16 Hz octave band.
January 17, 2015
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Room Criteria
Curves
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Noise Criteria Curves
• Room noise criteria (RNC) curves are intended to
address the limitations associated with the RC
and NCB curves.
• The intention is for noise criteria to be above the
threshold of hearing for well-behaved systems,
while at the same time preventing a turbulenceproducing, fan-surging HVAC systems (that
generate high levels of low frequency noise for
which the level can vary by up to 10 dB) from
being labeled acceptable.
January 17, 2015
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Noise Criteria Curves
Also, the RC curves could unnecessarily penalize a well
designed HVAC system such as may be used in a concert hall,
requiring 10 dB or more of unnecessary noise attenuation at
low frequencies.
On the other hand, NCB curves are intended for well designed
HVAC systems and they do not sufficiently penalize poorly
designed systems that are characterized by high levels of
turbulence induced low frequency noise.
It is unlikely that the RNC curves will receive general
acceptance because of the complexity of the rating process.
January 17, 2015
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Room Noise
Criteria Curves
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Noise Criteria Curves
All of the various noise rating schemes are widely used. The
following table shows comparisons between them.
This table is intended as a guide, to be used with caution.
Judgment is often necessary in specifying a noise rating for a
particular application. Consideration must be given to any
unusual aspects, such as people’s attitudes to noise, local
customs and need for economy.
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Noise Criteria Curves
January 17, 2015
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Speech privacy
• When designing an office building, it is important to ensure
that offices have speech privacy so that conversations
taking place in an office cannot be heard in adjacent
offices or corridors.
• When speech privacy is essential, there are two
alternative approaches that may be used: sound
insulation and acoustic “perfume”.
• Increased sound insulation in the walls can be achieved in
several ways, but perhaps the most straight forward is to
use double stud instead of single stud walls so that the
same stud does not contact both sides).
January 17, 2015
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Speech privacy
• The second approach is to add acoustic “perfume”
to the corridors and offices adjacent to those where
privacy is important. This “perfume” could be
introduced using a random-noise generator,
appropriate filter, amplifier and speakers, with the
speakers mounted above the suspended ceiling.
The filter would need to be adjusted to produce an
overall noise spectrum (existing plus introduced
noise), which followed the shape of one of the RC
curves.
January 17, 2015
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Speech privacy
• Generally, the higher the background noise
levels from air conditioning and other
mechanical equipment, the less one has to
worry about speech privacy and the more
“flimsy” can be the office partitions.
• One can deduce that speech privacy is likely
to be a problem in a building with no air
conditioning or forced ventilation systems.
January 17, 2015
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Environmental noise level criteria
• Noise level criteria at the property line or
plant boundary for intrusive noise.
• Noise measured at the nearest noise
sensitive area must not exceed existing
background levels by more than 5 dB in any
octave band.
• Permissible plant-boundary noise levels
generally are dependent upon the type of
area and the time of day.
January 17, 2015
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Environmental noise surveys
• To document existing environmental noise,
one or more ambient sound surveys must be
undertaken.
• If a new facility is being planned, the
calculated emissions of the facility must be
compared with existing noise levels to assess
the potential noise impact.
• When undertaking a noise survey to establish
ambient sound levels, it is important to
exclude transient events and noise sources.
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Environmental noise surveys
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January 17, 2015
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Next Time
Human Hearing and Hearing Loss
January 17, 2015
Page 54