noise desibels fundamentals
TRANSCRIPT
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Noise 101
What is a dB?What are Hz?
Noise 101
What is a dB?What are Hz?
Quiet Asphalt 2005
Tire/Pavement Noise Symposium
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Session ObjectivesSession Objectives
To introduce:
How sound is created
The terminology of noise
How sound is measured and described
When we are done youll be able to talk
dBese and you might become a psycho-
acoustician
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Basic Terminology - Sound
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Basic Terminology - Noise
Source: Brel & Kjr
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Basic TerminologyBasic Terminology Anything that causes small, fast pressure
changes will create sound
Vibration
Aerodynamic
Sound pressure level is a measure of these
pressure changes at the receiver
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Basic TerminologyBasic Terminology
The loudness of sound is
physiological It is related to how the
pressure fluctuations excite
the ear, and the transient
and spectral nature ofsound
Annoyance is
psychological
It is related topersonal preference
and expectation
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Sound AmplitudeSound Amplitude
Sound waves are small air pressure changes
(at least, as far as were concerned)
Units of pressure are psi or Pascal (N/m)
Can be measured with a microphone, a
device sensitive to these small pressure
changes
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Sound AmplitudeSound Amplitude
Air Pressure
TimeSource: Brel & Kjr
AtmospheriPressure
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time
airpressure
Sound AmplitudeSound Amplitude
Sound Waves
Amplitude (p)
Source: Steve Karamihas
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dB
0
80
140
40
20
60
100
120
Space shuttle
Rock concert
Jet plane (@1000)
Truck (@30)
Normal conversation
Whispering
Quiet bed room
Sound AmplitudeSound Amplitude
Threshold of Pain
Hearing Hazard
Hearing threshold
FHWA threshold
Pa
0.00002
0.2
200
0.002
0.0002
0.02
2
20
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Sound Amplitude Loudness
Change in Sound Level( dB)
Change in Loudness
1 to 3 dB
5 dB
10 dB
15 dB
20 dB
Just perceptible
Noticeable change
Twice (or ) as loud
Large change
Four times (or ) as loud
Source: Brel & KjrTrueonlyforthesamesound!
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Sound Amplitude MetricsSound Amplitude Metrics
Lmax
Time
dB
LeqL10
Sound pressure levels vary with time Lmax
Maximum (peak) noise level during a defined time interval Commonly used to pick out the noise of a single vehicle in pass-by testing
Lxx Noise level that is only exceeded xx% of the time during a defined time interval
L10 (1 hr) is commonly used in traffic noise reporting
Leq
Equivalent (constant) SPL that would net the same total acoustical energy Often used to report for continuous traffic stream of different vehicles
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FrequencyFrequency
Source: Purdue
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time
soundpressure
FrequencyFrequencyPeriod = 1 / Frequency (f)
Source: Steve Karamihas
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1 10 100 1000 10 000 [Hz]
Frequency
Hearing RangeHearing Range
Source: Brel & Kjr
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(Hz)1 10 100 1000 10,000
Frequency
Common Sources and their
Frequencies
Common Sources and their
Frequencies
Source: Brel & Kjr
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FrequencyFrequency
Source: Brel & Kjr
p Lp
time
Frequency
time
time
p
p
Lp
FrequencyLp
Frequency
+
=
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p Lp
time
Frequency
time
time
p
p
Lp
FrequencyLp
Frequency
FrequencyFrequency
Source: Brel & Kjr
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FrequencyFrequency
Aco
usticLevel
63 125 250 500 1k 2k 4k 8k
Frequency, Hz
Narrow Band
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FrequencyFrequency
63 125 250 500 1k 2k 4k 8k
Frequency, Hz
Aco
usticLevel
1/3 Octave Band
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FrequencyFrequency
63 125 250 500 1k 2k 4k 8k
Frequency, Hz
Aco
usticLevel
Octave Band
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FrequencyFrequency
1/3 Octave Bands
Useful in manyenvironmental,
building acoustics
and noise control
applications
Common way that
traffic and tire-
pavement noise isreported
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70
75
80
85
90
95
100
105
110
500 630 800 1 1.25 1 .6 2 2 .5 3 .15 4 5
1 /3 O c ta v e B a n d C e n te r F req u e n c y , k H z
Sound
IntensityLevel,dBA
FrequencyFrequency
Overall: 104 dBA
OBSI Measurement
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CPX Measurements of
Tire/Pavement Noise
CPX Measurements of
Tire/Pavement Noise
Tire on DGA asphalt 97.5 dB
Tire on open graded asphalt 92.5 dB
Tire on uniformly tined PCC 102.1 dB
250 500 1000 2500 500050
60
70
80
90
100
110
1/3 octave center frequency (Hz)
A-weightedsoundpressurelevel(dBA)
Pavement Spectra: Treaded Tire, CPX Lead Mic., 60 mph
Trans.Tined PCC
DGAPFC
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Sound Generation and
Propagation
Sound Generation and
Propagation
r: Lp
2r: Lp 6 dB
Point source
r: Lp
2r: Lp 3 dB
Line source
Source: Brel & Kjr
Vehicles are point sources,
respectively
However, a series of vehicles
act as a line source
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Sound Generation and
Propagation
Sound Generation and
Propagation
Doubling the traffic volume =
Source: Arizona DOT
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Basic Sound MeasurementBasic Sound Measurement
Sound is measured by sensing
impulses of acoustical energy (pressurefluctuations)
In the air, we usually use a microphone The microphone is connected to
instruments that analyze, record, and/or
display data
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Basic Sound MeasurementBasic Sound Measurement
Source: mediacollege.com
SoundWaves
Diaphragm
Audio Signal
SoundWaves
Front Plate
(Diaphragm)
Back Plate Battery
OutputAudio Signal
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Basic Sound MeasurementBasic Sound Measurement
Source: G.R.A.S.
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Basic Sound MeasurementBasic Sound Measurement
Source: G.R.A.S., Larson-Davis, Sony, M-Audio
MicrophonePreamp
Power
Module
RT
ADAT Recorder PC Sound
Card
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0
-20
-40
10 100 1 k 10 kSoun
dPressure
LevelAdjustment
(d
B)
A
B
CD
AB + C
D
Lin.
Frequency
(Hz)
-60
20 k2 k 5 k200 50020 50
Basic Sound Analysis
Frequency Filtering
Basic Sound Analysis
Frequency Filtering
A-weighted moderate sounds
(most often used)
B-weighted intense sounds
C-weighted very loud sounds
D-weighted noisiness measure
(sometimes used for aircraft noise)
Source: Brel & Kjr
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Session SummarySession Summary Brief discussion of sound to use later when
we discuss how tire-pavement interaction
creates noise, and how it can be controlled
Brief introduction to dB, Hz, A-weighting,
octave bands, etc. so you can talk the talk
A few rules of thumb about the mathematics
of decibels
Basic sound measurement