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Noise: Quantification and Perception
Architectural Acoustics II
February 11, 2008
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Symphony Hall, Boston
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Symphony Hall, Boston
http://www.nytimes.com/2007/06/03/arts/music/03kram.html
http://www.allposters.com/-sp/SymphonyHall-Boston-MA-Posters_i1119076_.htm
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Symphony Hall, Boston
http://upload.wikimedia.org/wikipedia/commons/thumb/5/57/Symphony_hall_boston.jpg/800px-Symphony_hall_boston.jpg
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Symphony Hall, Boston
From Beranek, Concert and Opera Halls: How They Sound
Outline
• Measuring noise

Sound-level meters
 Noise metrics
 Speech intelligibility metrics using noise levels
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• Basic noise control concepts
• Intensity measurements
Sound Level Meters
• Time constants



Given a sound raised instantaneously to an
SPL of L, the meter should display (L – 2)
dB within one time constant.
Why 2 dB? If SPL L has energy E, the meter
registers (1 – 1/e)·E in one time constant.
e = 2.718, 10log10(1-1/e) = -2
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• Time Response



Slow: Time constant = 1 sec
Fast: Time constant = 125 ms
Impact: Time constant = 35 ms rising,
sec falling
Image from www.bk.dk, B&K 2260 Investigator
1.5
Sound Level Meters
• Frequency Response


Linear, A-weighted,C-weighted
Full bandwidth, 1/1-octave, 1/3-octave
• Classes (ANSI S1.4-1983)


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
0 (Laboratory): ±0.2 dB, 22.4 – 11200 Hz
1 (Precision): ±0.5 dB, 22.4 – 11200 Hz
2 (General Purp.): ±0.5 dB, 63.0 – 2000 Hz
±1.0 dB, 22.4 – 11200 Hz
• Orientation


For free-field measurements, point the meter at the
noise source (normal incidence)
For diffuse-field measurements, the meter orientation
is not too important (random incidence)
Image from www.bk.dk, B&K 2260 Investigator
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A-Weighting Review
Octave-Band
Center
Frequency (Hz)
A-Weighting
Adjustment
(dB)
31.5
63
125
250
500
1k
2k
4k
8k
16k
-39
-26
-16
-9
-3
0
+1
+1
-1
-7
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C-Weighting
Octave-Band
Center
Frequency (Hz)
C-Weighting
Adjustment
(dB)
31.5
63
125
250
500
1k
2k
4k
8k
16k
-3
-1
0
0
0
0
0
-1
-3
-8
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From dB(A) to NC/RC
• dB(A) is typically insufficient to describe interior
noise conditions (no spectral information)
• NC (Noise Criterion) and RC (Room Criterion)
metrics were developed to better describe interior
noise, specifically that generated by mechanical
systems
• These metrics better approximate the human
response to various noise spectra and provide us
with more detailed analysis information
From Paul Henderson
Noise Criterion (NC)
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•
•
•
•
Single number rating based on octave band levels
63 Hz to 8,000 Hz frequency range
Compare measured spectra with NC curves (tangent basis)
5 point resolution (NC-15 to NC-65)
From Paul Henderson and MJR Fig. 8.2
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Calculating the NC Rating
• The NC Rating is the
lowest NC curve that
lies entirely above all
measured data points
• In this example, the
noise is NC-40, and it
is limited by the 500
Hz octave band
From Paul Henderson
Limitations of the NC Rating
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• Provides no limits to low frequency noise below
the 63 Hz octave band
• Permits excessive high frequency noise above
2,000 Hz
• Provides no information on spectrum balance or
sound quality
From Paul Henderson
Room Criterion
•
•
•
•
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Introduced in 1981, approved by ASHRAE in 1995
Two-parameter rating based on octave band levels
16 Hz to 4,000 Hz octave band range
First parameter is the SIL(3) (arithmetic average of
noise levels in the 500, 1k, and 2k Hz octave bands)
• Second parameter is a sound quality rating (Hissy,
Neutral , Rumbly, Tonal, Vibration)
From Paul Henderson
Room Criterion
• Each line has a -5 dB
per octave slope
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• The RC-X line crosses
X dB at 1000 Hz
MJR, Figure 8.3, p. 165
Finding the RC Limit Curve
• Draw an RC line ( ) with
slope -5 dB/oct that intersects
the 1000 Hz band at the SIL(3)
80
Sound Pressure Level (dB)
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• The limit curve (- - -) is 5 dB
above the RC line at and below
500 Hz and 3 dB above the RC
line at and above 1000 Hz
SIL(3) = (43+36+29)/3 = 36 dB
70
60
50
40
30
RC-36
20
10
0
16
31
63
125
250
500 1000 2000 4000
Octave-Band Center Frequency (Hz)
From Paul Henderson
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Determine the RC Quality Rating
From Paul Henderson
80
70
Sound Pressure Level (dB)
• (R) for rumbly if data exceeds
limit curve at or below 500 Hz
• (H) for hissy if data exceeds
limit curve at or above 1000 Hz
• (N) for neutral if spectrum is
below limit curve
• (T) for tone if audible (any one
band is at least 5 dB above both
of its neighboring bands)
• (V) for noise induced vibrations
in light-weight structures (above
75 dB at 16 or 31 Hz, 80 dB at
63 Hz)
60
50
40
30
20
10
0
16
31
63
125
250
500 1000 2000 4000
Octave-Band Center Frequency (Hz)
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Other Noise Metrics
• Balanced Noise Criterion (NCB)
• Proposed by Beranek in 1989
• Extend lower in frequency than
original NC curves
• More stringent at high
frequencies than original NC
curves
• Similar quality ratings (e.g.
rumbly and hissy) to RC rating
system
http://ceae.colorado.edu/~muehleis/classes/aren4020/handouts/lecture24/nc_rc.pdf
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Other Noise Metrics
• Room Criterion Mark II
• Proposed by Blazier in 1997
• More stringent at low
frequencies than the original RC
curves
• Uses a Quality Assessment
Index (deviations from RC curve
in low, mid, and high
frequencies) to qualify the
numeric rating
http://ceae.colorado.edu/~muehleis/classes/aren4020/handouts/lecture24/nc_rc.pdf
Blazier and RC Mark II
• Three factors influence the subjective response
to HVAC-related background noise
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
The loudness of the noise relative to the noise
created by “normal” activities in the space
 The potential for “task interference” e.g. the
reduction of speech intelligibility
 The “quality” of the noise, e.g. a neutral-sounding
noise spectrum will be judged mainly by its
loudness but a hissy or rumbly noise spectrum is
inherently more irritating regardless of loudness
Blazier, W., "RC Mark II: A refined procedure for rating the noise of heating, ventilating, and air-conditioning
(HVAC) systems in buildings," Noise Control Eng. J. Vol. 45, no. 6, pp. 243-150. Nov-Dec 1997.
Blazier and RC Mark II
• RC Mark II Rating takes the form RC xx(yy)

xx is the value of the RC reference curve
corresponding to the arithmetic average of the
levels in the 500, 1k, and 2k Hz octave bands
 yy is a qualitative descriptor
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• N = neutral
• LF = low-frequency dominant (rumble)
▪ LFA = substantial sound-induced vibration
▪ LFB = moderate sound-induced vibration
• MF = mid-frequency dominant (roar)
• HF = high-frequency dominant (hiss)
Blazier, W., "RC Mark II: A refined procedure for rating the noise of heating, ventilating, and air-conditioning
(HVAC) systems in buildings," Noise Control Eng. J. Vol. 45, no. 6, pp. 243-150. Nov-Dec 1997.
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Recommended Background Noise
Levels
MJR Table 8.1, pg. 168
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Recommended Background Noise
Levels
MJR Table 8.1, pg. 168
Various Levels
• LEQ (Equivalent (Continuous) Sound Level)

Given a time-variant sound-pressure level measured over time T, the LEQ is the
constant SPL which contains an equal amount of energy over time T
• LDN (Day Night Equivalent Sound Level)
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
A 24-hour LEQ calculated with a 10 dB penalty for levels measured between
10:00 PM and 7:00 AM
 LN 10  
LD
 1 

10
10
 10 log 10  15 10  9 10

 24 
 

LDN

LD = daytime LEQ, LN = nighttime LEQ
• Ln (Exceedance Level)

SPL equaled or exceeded n% of the time during a measurement period. L10 is
often used to represent the maximum level and L90 is often used to represent
the ambient level
Various Levels
• TNI (Traffic Noise Index)

TNI = 4·(L10 – L90) + L90 – 30 (dBA)
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• NPL or LNP (Noise Pollution Level)

NPL = LEQ + σk

σ = standard deviation of the time varying level

k = 2.56 (found from studies of subjective
response to time-varying noise levels)

Uses A-weighted LEQ
Various Levels
• SEL (Sound Exposure Level)
N

0.1 Li 
 SEL  10 log 

10 10
 i 1


Li = level for a given one-second period

N = number of seconds in the measurement period
• SENEL (Single Event Noise Exposure Level)
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
SEL of a single sound event calculated over a period in which the level
is within 10 dB of the maximum level. Often used to quantify noise for
individual aircraft fly-overs
• CNEL (Community Noise Equivalent Level)

CNEL = SENEL +10log10(ND + 3NE + 10NN) – 49.4 (dB)
• ND = number of daytime flights (7 AM to 7 PM)
• NE = number of evening flights (7 PM to 10 PM)
• NN = number of nighttime flights (10 PM to 7 AM)
CNEL Corrections
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Type of Correction
Description
Correction
(dB)
Seasonal
Summer (windows open)
Winter (windows closed)
0
+5
Outdoor Noise Level
Quiet suburban or rural community
“Normal” suburban community
Urban residential community
Noisy urban residential community
Very noisy urban res. community
+10
+5
0
-5
-10
Previous Exposure
No prior exposure to intruding noise
Some previous exposure
Considerable previous exposure
+5
0
-5
Pure Tone or Impulse
Pure tone or impulsive character
+5
http://www.sfu.ca/sonic-studio/handbook/Community_Noise_Equivalent.html
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A Few More…
Long Figure 4.22, p. 143
Noise Source Directivity
• Q (directivity) of a source is
I  ,  
Q ,   
I Avg
I Avg
W

4r 2
Average intensity (I) if total power (W) is
radiated uniformly over a spherical surface.
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• For a source against a wall (for example)
W
I  ,   2r 2
Q ,   

2
W
I Avg
4r 2
Total power (W) is radiated uniformly
over a hemispherical surface.
Noise Source Location
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f Q  ?
MJR, p. 174
OSHA and Noise Exposure
• OSHA is the Occupational Safety and Health
Administration
• They provide guidelines (legal limits) for workplace
noise exposure or noise dose
• Noise Dose
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C1 C2 C3
D



T1 T2 T3

where Ci is the total daily exposure time to a specific noise
level (e.g. 90 dBA) and Ti is the maximum permissible
exposure time for that level
• D > 1is illegal
OSHA and Noise Exposure
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Noise dose is measured with a noise dosimeter.
http://www.nonoise.org/hearing/hcp/25.gif
MJR Table 8.2, pg. 169
Speech Intelligibility
• Statistical Measures: Human Listeners


Modified Rhyme Test: Listeners are given lists of 6 rhyming or similarsounding words (e.g. went sent bent dent tent rent OR cane case cape
cake came cave) and are asked to choose which has been spoken
Diagnostic Rhyme Test: Listeners are given pairs of rhyming words and
are asked to choose which has been spoken
• Machine Measures

Percentage Articulation Loss of Consonants (%ALCons)
• Calculated using RT, speaker-to-listener distance, room volume, and
speaker directivity
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
Speech Transmission Index (STI)
• Changes in the modulation of speech intensity are measured for listener
positions


Articulation Index
Speech Interference Level
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Articulation Index
• Combines the effects of source level,
background noise, and hearing sensitivity
• Given the source level and the backgroundnoise level (in octave bands), calculate the
signal-to-noise ratio in each band:
SNR = LSource – LNoise (dB)
• If SNR > 30, SNR = 30
• If SNR < 0, SNR = 0
• Then…
Articulation Index
• Use this table of weighting factors to calculate
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AI = Σ SNR · weighting factor
Octave-Band
Center
Frequency (Hz)
Weighting
Factor
250
0.0024
500
0.0048
1000
0.0074
2000
0.0109
4000
0.0078
• AI ≥ 0.7 is desired, < 0.5 is unacceptable
Speech Interference Level
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• SIL (or PSIL) evaluates the impact of
background noise on speech communication
• SIL(3) is the arithmetic average of the SPL in the
500, 1,000, and 2,000 Hz octave bands
• SIL(4) is the arithmetic average of the SPL in the
500, 1,000, 2,000 and 4,000 Hz octave bands
From Paul Henderson
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SIL and Distance
MJR Figure 8.1, pg. 162
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Speech Interference Level
Foreman, Sound Analysis and Noise Control, Fig. 7.4
MTF and STI
• Modulation transfer function (MTF)

Start with the idea that speech is well represented
by modulated bands of noise
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• Speech is interfered with by reverberation and
background noise which effectively modify the
modulation
Long, Fig. 4.28, p. 151
MTF and STI
• The effect of background noise is independent of
the modulation frequency, while the effect of
reverberation is not
• Skipping a few details, the modulation reduction
factor is
1
m f m  
0.1LSN 
1

10
T


1  2f m 60 
13.8 

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1
LSN  signal to noise ratio (dB)
f m  modulation frequency (Hz)
T60  reverberat ion time
MTF and STI
• m(fm) is calculated for


fm from 0.63 to 12.5 Hz in 14 1/3-octave steps
7 octave bands of noise, from 125 Hz to 8 kHz
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• The result is a graph like this with 98 (7 x 14)
values
Long, Fig. 4.28, p. 151
MTF and STI
• Now find the apparent signal-to-noise ratio for all
98 values of m
m
LSNapp  10 log 10
1 m
• And the average LSNapp weighted by octave band
LSNapp   wi LSNapp i
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7
i 1
• Finally

L
STI 
SNapp
Long, Fig. 4.28, p. 151

 15
30
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STI Comparisons
Long, Fig. 4.29
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STI Comparisons
Long, Figs. 4.30
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STI Comparisons
Long, Figs. 4.29
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RASTI = RApid STI
Long, Figs. 4.33
Basic Noise Control
• Address the source

Enclose it
 Modify it to reduce noise production
• Address the path

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Add a barrier between the source and receiver
 Add absorption
• Address the receiver

Distribute ear plugs or other hearing protection and
enforce their use
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Noise Barrier Performance
http://www.ashraeregion7.org/tc26/pastprograms/Outdoor_Noise/barriers.pdf
Noise Barrier Performance
• Barrier attenuation: SPL reduction provided by the
barrier under free-field conditions (no ground
absorption considered)

From MJR
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• ∆L = 10·log10(20N + 3) where
▪ N = 2δ/λ (called the Fresnel Number)
▪ δ = length of shortest path from S to R over the barrier minus
the length of the direct path from S to R
▪ λ = wavelength

From every other noise control reference

2N

• L  20  log 10 
 tanh 2N
L  20,

  5,


- 0.19  N  5.03
N  5.03
Noise Reduction
• NR achieved by adding absorption in a room
 A2 
NR  10 log 10  
 A1 


A1 = total room absorption before modifications
A2 = total room absorption after modifications
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• NR achieved by a partition between two spaces
 ARec 
NR  TL  10 log 10 

 S 

TL = transmission loss of the partition

ARec = total absorption in the receiving room

S = surface area of the partition
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Other Measurement Options
• Thus far, we’ve only considered noise
measurements based on sound pressure.
Is that all we can measure?
• Pressure is a scalar value (as opposed to
a vector) so it provides no directional
information.
• Intensity probes are becoming popular as
tools to locate noise sources/leaks.
• Arrays can be used for this too.
Intensity Probe
• Two omni mics are mounted face to face
at a known separation distance (∆x)
• Recall (for a plane wave) I = p·u

p = pressure, u = particle velocity
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• Now consider Euler’s equation:
dp
du
  0
dx
dt
• Solve for particle velocity
1
 pa  pb 
u

 0 x
pa – pb = pressure difference
between two mics
Intensity Probe
• Use particle velocity and average
pressure (pa + pb)/2 to find intensity
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pa  pb
I  p u  
2  0 x
p
a
 pb 
• Orientation of the probe can be
changed to find the strongest intensity,
which (likely) indicates the direction
toward the noise source
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Directional Arrays
B. Gover and J. Bradley, “Identification of Weak Spots in the Sound Insulation of Walls Using
a Spherical Microphone Array,” in Proc. NOISE-CON 2005, Minneapolis, October 2005.
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Directional Arrays
Original Wall
(a) With a 5.4-cm hole
STC 56
STC 53
(b) With a 3.8-cm sealed
pipe in the hole
STC 56
B. Gover and J. Bradley, “Identification of Weak Spots in the Sound Insulation of Walls Using
a Spherical Microphone Array,” in Proc. NOISE-CON 2005, Minneapolis, October 2005.
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Directional Arrays
B. Gover and J. Bradley, “Identification of Weak Spots in the Sound Insulation of Walls Using
a Spherical Microphone Array,” in Proc. NOISE-CON 2005, Minneapolis, October 2005.
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Directional Arrays
B. Gover and J. Bradley, “Identification of Weak Spots in the Sound Insulation of Walls Using
a Spherical Microphone Array,” in Proc. NOISE-CON 2005, Minneapolis, October 2005.
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Directional Arrays
B. Gover and J. Bradley, “Identification of Weak Spots in the Sound Insulation of Walls Using
a Spherical Microphone Array,” in Proc. NOISE-CON 2005, Minneapolis, October 2005.
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Directional Arrays
B. Gover and J. Bradley, “Identification of Weak Spots in the Sound Insulation of Walls Using
a Spherical Microphone Array,” in Proc. NOISE-CON 2005, Minneapolis, October 2005.
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Directional Arrays
Open Spherical Array
Rigid Spherical Array
More on these later in the semester…