noise desibels fundamentals

7/27/2019 Noise Desibels Fundamentals

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Institute for Safe Quiet and Durable Highways

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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Air Pressure

TimeSource: Brel & Kjr

AtmospheriPressure


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time

airpressure


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


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


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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Propagation


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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Source: mediacollege.com

SoundWaves

Diaphragm

Audio Signal

SoundWaves

Front Plate

(Diaphragm)

Back Plate Battery

OutputAudio Signal


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Source: G.R.A.S.


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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

noise desibels fundamentals

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