Diploma inAviation Medicine

Introductionto Acoustics

Noise & Vibration Division,

RAF Centre of Aviation Medicine

ProgramProgram

Lt Col Mark AdamsLt Col Mark AdamsAircrew Hearing Aircrew Hearing Protection: The Protection: The FutureFuture

1400-15001400-1500

NVD PersonnelNVD PersonnelDemonstrationsDemonstrations1115-12151115-1215

Sqn Ldr Andy Sqn Ldr Andy ThomasonThomason

Hearing and Hearing and Hearing Hearing ConservationConservation

1015-11151015-1115

Matt PeacockMatt PeacockBasic AcousticsBasic Acoustics0900-10000900-1000

PresenterPresenterSubjectSubjectTimeTime

Introduction to AcousticsIntroduction to Acoustics

Basic AcousticsBasic Acoustics

decibel (dB)decibel (dB)

Noise & man:Noise & man:

Frequency & loudness responseFrequency & loudness response

Noise and CommunicationsNoise and Communications

What is Sound?What is Sound?

Sound:Sound:

– Changes in pressure which Changes in pressure which

can be detected by the earcan be detected by the ear

– CompressionCompression and and

rarefraction rarefraction of the air of the air

molecules.molecules.

– LongitudinalLongitudinal wave. wave.

Describing AmplitudeDescribing Amplitude

The ear detects pressure changes The ear detects pressure changes rather than absolute pressurerather than absolute pressure

Range from 0.00002 Pa to Range from 0.00002 Pa to ~200 Pa~200 Pa

Using pascals gives a large, Using pascals gives a large, unmanagable scale unmanagable scale (over 1 million)(over 1 million)

Use Decibel ScaleUse Decibel Scale

The DecibelThe Decibel

Based on a logarithmic scaleBased on a logarithmic scale

– compresses huge rangecompresses huge range

log1=0 log10=1 log100=2… log100,000=5log1=0 log10=1 log100=2… log100,000=5

– human ear works logarithmicallyhuman ear works logarithmically

Bel - ratio of 2 numbers using logarithmsBel - ratio of 2 numbers using logarithms

Decibel - Bel divided by 10.Decibel - Bel divided by 10.

Unit dBUnit dB

140

120

110

100

90

80

60

70

40

50

30

20

10

0

Chainsaw

Library

Jet Take-offat 50metres

Wood

Busy Office

Lorry

Helicopter

Threshold of Pain

Threshold of Hearing

Decibel Scale / dBADecibel Scale / dBA

What Makes up a Sound?What Makes up a Sound?

Very few sounds have a single frequency Very few sounds have a single frequency

ie are pure tonesie are pure tones

Sounds with a dominant frequency are Sounds with a dominant frequency are

called tonalcalled tonal

Most sounds contain various frequencies Most sounds contain various frequencies

at different intensities and are called at different intensities and are called

broadbandbroadband

Direction of travel(propagation)

distance

Sound Energy

Inverse Square lawInverse Square law

Sound Energy per Unit Sound Energy per Unit

area decreasesarea decreases

surface area surface area radius radius22

Point SourcePoint Source

– 6dB reduction per 6dB reduction per

doubling of distancedoubling of distance2

2

2

1

21

1

r

r

I

I

dI

r

r

1

2

21log20

r

rLL

rprp

source

r1

r2

Ir1 Ir2

Line SourceLine Source

Sound radiates as a Sound radiates as a cylindercylinder

surface areasurface arearadiusradius

Line SourceLine Source

– 3dB reduction per 3dB reduction per

doubling of distancedoubling of distance

1

2

2

11

r

r

I

I

dI

r

r

1

221 log10

r

rLL rr

r1

r2

Met: Wind EffectsMet: Wind Effects

HeightHeight

Ground LevelGround Level

Velocity ProfileVelocity Profile Sound waves ‘bent’ by windSound waves ‘bent’ by wind

ShadowShadow FocusingFocusing

Noise and ManNoise and Man

Audible RangeAudible Range

Frequency Hz

SPL dB

-50

-40

-30

-20

-10

0

10

-60

-70

12.5

20 50 100

200

400

800

160

0

315

0

630

0

125

0

200

00

The average young adult with healthy ears can detect frequencies over the range 20 Hz to 20,000 Hz

Equal Loudness curves:Equal Loudness curves:

– Same Loudness as a Same Loudness as a

1kHz tone1kHz tone

– Stevens & Davis 1938Stevens & Davis 1938

Unit - Unit - PhonPhon

Describing the Human ReactionDescribing the Human Reaction

Frequency Hz20 200 2k 20k

SPL dB

30

50

70

90

110

130

140

0

130

10

40

70

90

110

30

50

1k

LoudnessLoudness

Non-linearNon-linear::

– LevelLevel– FrequencyFrequency

Average PersonAverage Person

Threshold of hearing

SPL dB

Frequency Hz

20 200 2k 20k

20

40

60

80

100

120

140

0

Audible range

Music

Speech

Threshold of feeling

SPL dB

Frequency Hz20 200 2k 20k

30

50

70

90

110

130

140

0

130

10

40

70

90

110

30

50

1k

Frequency Hz

SP

L d

B

-70

-60

-50

-40

-30

-20

-10

0

10

20

16 31.5 63 12

525

050

010

0020

0040

0080

00

2000

00

dB(A)

dB(B)

dB(C)

dB(D)

Noise IndicesNoise Indices

SPLSPL

LLmaxmax

LLminmin

LLEq,tEq,t

LLnn

LLpkpk

}rms

} Peakpressure Time sec

SPL

dBA

60

80

100

40

LLmaxmax

LLeeqq

LL6565

LLAe Ae (SEL)(SEL)

UnitsUnits( Instantaneous)( Instantaneous)

Sound Pressure Level (SPL or Sound Pressure Level (SPL or LLpp) - Unit - dB(Lin) ) - Unit - dB(Lin)

Instantaneous sound Instantaneous sound

pressure at a given positionpressure at a given position

Sound Level (SL) - A-weighted Sound Level (SL) - A-weighted - Unit - dB(A) - Unit - dB(A) Instantaneous sound at Instantaneous sound at given position, measured with a meter that given position, measured with a meter that takes account of the sensitivity of the takes account of the sensitivity of the

human earhuman ear

Units Units (Average level when noise level is varying)(Average level when noise level is varying)

Equivalent Continuous Equivalent Continuous Sound Level LSound Level LAeq Aeq

Daily Personal Noise Daily Personal Noise Exposure LExposure LEP,d EP,d

Peak PressurePeak Pressure

Impulse noise:Impulse noise:

– Very short durationVery short duration

– Very high noise levelVery high noise level

Human ear reacts Human ear reacts

differentlydifferently

European and UK Occupational NoiseEuropean and UK Occupational NoiseLegislationLegislation

Control Noise at Work Regulations 2005 (6 April 2006)Control Noise at Work Regulations 2005 (6 April 2006)

– Lower Exposure Action Value 80dBA average for 8 hours Lower Exposure Action Value 80dBA average for 8 hours

(L(LEP,dEP,d))

– Upper Exposure Action Value LUpper Exposure Action Value LEP,dEP,d 85dBA 85dBA

– Exposure Limit Value LExposure Limit Value LEP,dEP,d 87dBA 87dBA

– Peak Exposure Limit Value (LcPeak Exposure Limit Value (LcPkPk) of 200Pa (140dBC), Peak ) of 200Pa (140dBC), Peak

Action Values 135dBC and 137dBCAction Values 135dBC and 137dBC

Management Management

Risk assessmentRisk assessment Preventative Preventative

measuresmeasures– Noise ControlNoise Control– PPEPPE

Health and safety Health and safety arrangementsarrangements

TrainingTraining Health surveillanceHealth surveillance

Prevention: PrinciplesPrevention: Principles

Avoid Risk- remove Avoid Risk- remove sourcesource

Reduce the Noise Reduce the Noise At At SourceSource

Move source to Move source to minimise effect on minimise effect on wider workforcewider workforce

PPEPPE

Personal Hearing ProtectionPersonal Hearing Protection

Last resort Last resort Must be compatible with other protective Must be compatible with other protective

clothing and equipmentclothing and equipment Protection provided limited Protection provided limited Active noise reduction (ANR) only effective Active noise reduction (ANR) only effective

at certain frequenciesat certain frequencies Helmets/Headsets do not necessarily Helmets/Headsets do not necessarily

provide hearing protectionprovide hearing protection Any covering over the ears may affect the Any covering over the ears may affect the

ability to localise soundsability to localise sounds

CommunicationCommunication

CommunicationsCommunications

Speech Intelligibility:Speech Intelligibility:

– Consonants 80% Intelligibilty, 20% Consonants 80% Intelligibilty, 20%

EnergyEnergy

s, f, t, p, ks, f, t, p, k

– E.g. Zoo/Sue, Pack/Pat, E.g. Zoo/Sue, Pack/Pat,

Fazed/Phased, Sap/SatFazed/Phased, Sap/Sat

Intelligibility Indexes weighted to Intelligibility Indexes weighted to

Higher FrequenciesHigher Frequencies

Aircraft Noise SourcesAircraft Noise Sources

Aerodynamic noiseAerodynamic noise

Propulsion noisePropulsion noise

Cabin conditioningCabin conditioning

AvionicsAvionics

Weapons systemsWeapons systems

63 8k125 250 500 1k 2k 4k

80

90

100

110

Typical Fast JetTypical Fast JetCabin Noise SpectrumCabin Noise Spectrum

SPL(dB)

Frequency (Hz)

420 kt, 250 ft420 kt, 250 ft

63 8k125 250 500 1k 2k 4k

80

90

100

110

Effect of Altitude on Cabin NoiseEffect of Altitude on Cabin Noise

SPL(dB)

Frequency (Hz)

420 kt, 250 ft420 kt, 250 ft

420 kt, 33,000 ft420 kt, 33,000 ft

50

70

90

110

SPL(dB)

Frequency (Hz)

Added Noise due to CommunicationsAdded Noise due to Communications

Noise levelat ear

Noise levelat ear

Speech levelrequired

Speech levelrequired

63 125 250 500 1k 2k 4k

Example - Tristar Flight Deck NoiseExample - Tristar Flight Deck Noise

Take-off from Calgary Take-off from Calgary

to Edmontonto Edmonton

Sennheiser headsetSennheiser headset

Background 74dBABackground 74dBA

Speech +12-14dBASpeech +12-14dBA

LLEP,dEP,d 84-86dBA 84-86dBA

QUESTIONS?QUESTIONS?