rensselaer noise: quantification and perception architectural acoustics ii february 11, 2008

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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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http://www.nytimes.com/2007/06/03/arts/music/03kram.html

http://www.allposters.com/-sp/Symphony-Hall-Boston-MA-Posters_i1119076_.htm

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http://upload.wikimedia.org/wikipedia/commons/thumb/5/57/Symphony_hall_boston.jpg/800px-Symphony_hall_boston.jpg

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From Beranek, Concert and Opera Halls: How They Sound

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Outline

• Measuring noise Sound-level meters Noise metrics Speech intelligibility metrics using noise levels

• Basic noise control concepts

• Intensity measurements

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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

• Time Response Slow: Time constant = 1 sec Fast: Time constant = 125 ms Impact: Time constant = 35 ms rising,

1.5 sec falling

Image from www.bk.dk, B&K 2260 Investigator

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Sound Level Meters

• Frequency Response Linear, A-weighted,C-weighted Full bandwidth, 1/1-octave, 1/3-octave

• Classes (ANSI S1.4-1983) 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)31.5 63 125 250 500 1k 2k 4k 8k 16k

A-Weighting Adjustment

(dB)-39 -26 -16 -9 -3 0 +1 +1 -1 -7

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C-Weighting

Octave-Band Center

Frequency (Hz)31.5 63 125 250 500 1k 2k 4k 8k 16k

C-Weighting Adjustment

(dB)-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

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Noise Criterion (NC)• 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

From Paul Henderson

• 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

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Limitations of the NC Rating

• 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

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Room Criterion

• 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

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Room Criterion

MJR, Figure 8.3, p. 165

• Each line has a -5 dB per octave slope

• The RC-X line crosses X dB at 1000 Hz

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Finding the RC Limit Curve

• Draw an RC line ( ) with slope -5 dB/oct that intersects the 1000 Hz band at the SIL(3)

• 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

0

10

20

30

40

50

60

70

80

16 31 63 125 250 500 1000 2000 4000

Octave-Band Center Frequency (Hz)

Sou

nd P

ress

ure

Lev

el (dB)

RC-36

From Paul Henderson

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Determine the RC Quality Rating

0

10

20

30

40

50

60

70

80

16 31 63 125 250 500 1000 2000 4000

Octave-Band Center Frequency (Hz)

Sou

nd P

ress

ure

Lev

el (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)

From Paul Henderson

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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

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Blazier and RC Mark II• Three factors influence the subjective response

to HVAC-related background noise 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.

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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• 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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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)

A 24-hour LEQ calculated with a 10 dB penalty for levels measured between 10:00 PM and 7:00 AM

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

10

10

1010 1091015

24

1log10

ND LL

DNL

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Various Levels

• TNI (Traffic Noise Index) TNI = 4·(L10 – L90) + L90 – 30 (dBA)

• 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

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Various Levels• SEL (Sound Exposure Level)

Li = level for a given one-second period

N = number of seconds in the measurement period

• SENEL (Single Event Noise Exposure Level) 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)

N

i

Li

1

1.010 10log10SEL

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CNEL CorrectionsType 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

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Noise Source Directivity

• Q (directivity) of a source is

• For a source against a wall (for example)

AvgI

IQ

,, 24 r

WI Avg

Average intensity (I) if total power (W) is radiated uniformly over a spherical surface.

2

4

2,,

2

2

rW

rW

I

IQ

Avg

Total power (W) is radiated uniformly over a hemispherical surface.

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Noise Source Location

MJR, p. 174

?Qf

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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

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

3

3

2

2

1

1

T

C

T

C

T

CD

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OSHA and Noise Exposure

MJR Table 8.2, pg. 169

http://www.nonoise.org/hearing/hcp/25.gif

Noise dose is measured with a noise dosimeter.

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Speech Intelligibility

• Statistical Measures: Human Listeners Modified Rhyme Test: Listeners are given lists of 6 rhyming or

similar-sounding 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

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 background-noise 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…

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Articulation Index• Use this table of weighting factors to calculate

AI = Σ SNR · weighting factor

• AI ≥ 0.7 is desired, < 0.5 is unacceptable

Octave-Band Center

Frequency (Hz)

Weighting Factor

250 0.0024

500 0.0048

1000 0.0074

2000 0.0109

4000 0.0078

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Speech Interference Level

• 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

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MTF and STI

• Modulation transfer function (MTF) Start with the idea that speech is well represented

by modulated bands of noise

• Speech is interfered with by reverberation and background noise which effectively modify the modulation

Long, Fig. 4.28, p. 151

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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

SNL

m

mT

f

fm 1.0

60101

1

8.1321

1

ion timereverberat

(Hz)frequency modulation

(dB) ratio noise tosignal

60

T

f

L

m

SN

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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

• The result is a graph like this with 98 (7 x 14) values

Long, Fig. 4.28, p. 151

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MTF and STI

• Now find the apparent signal-to-noise ratio for all 98 values of m

• And the average LSNapp weighted by octave band

• Finally

Long, Fig. 4.28, p. 151

m

mL

1log10 10SNapp

i

ii LwL SNapp

7

1SNapp

30

15SNapp

LSTI

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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

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Basic Noise Control

• Address the source Enclose it Modify it to reduce noise production

• Address the path 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

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Noise Barrier Performance• Barrier attenuation: SPL reduction provided by the

barrier under free-field conditions (no ground absorption considered) From MJR

• ∆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

• 03.5 0.19- ,52tanh

2log20 10

N

N

NL

03.5 ,20 NL

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Noise Reduction• NR achieved by adding absorption in a room

A1 = total room absorption before modifications

A2 = total room absorption after modifications

• NR achieved by a partition between two spaces

TL = transmission loss of the partition

ARec = total absorption in the receiving room

S = surface area of the partition

1

210log10

A

ANR

S

ATLNR Rec

10log10

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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.

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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

• Now consider Euler’s equation:

• Solve for particle velocity

dt

du

dx

dp0

ba ppx

u0

1

pa – pb = pressure difference between two mics

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Intensity Probe

• Use particle velocity and average pressure (pa + pb)/2 to find intensity

• Orientation of the probe can be changed to find the strongest intensity, which (likely) indicates the direction toward the noise source

ba

ba ppx

ppupI

02

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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

STC 56

(a) With a 5.4-cm hole

STC 53

(b) With a 3.8-cm sealed pipe in the hole

STC 56

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Directional Arrays


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Directional Arrays

Open Spherical Array Rigid Spherical Array

More on these later in the semester…

rensselaer noise: quantification and perception architectural acoustics ii february 11, 2008

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