1. Diploma in Aviation Medicine Introduction to Acoustics
Noise & Vibration Division,
RAF Centre of Aviation Medicine

2. Program Presenter Subject Time Lt Col Mark Adams
Aircrew Hearing Protection: The Future
NVD Personnel
Demonstrations
Sqn Ldr Andy Thomason
Hearing and Hearing Conservation
Matt Peacock
Basic Acoustics
Presenter
Subject
Time

3. Introduction to Acoustics
Basic Acoustics
decibel (dB)
Noise & man:
Frequency & loudness response
Noise and Communications

4. What is Sound?
Sound:
Changes in pressure which can be detected by the ear
Compression and rarefraction of the air molecules.
Longitudinal wave.

5. Describing Amplitude
The ear detects pressure changes rather than absolute pressure
Range from Pa to ~200 Pa
Using pascals gives a large, unmanagable scale (over 1 million)
Use Decibel Scale

6. The Decibel Based on a logarithmic scale
compresses huge range
log1=0 log10=1 log100=2… log100,000=5
human ear works logarithmically
Bel - ratio of 2 numbers using logarithms
Decibel - Bel divided by 10.
Unit dB

7. Jet Take-off
at 50metres
Decibel Scale / dBA
140
Threshold of Pain
120
Chainsaw
110
Helicopter
100 90
Lorry 80 70
Busy Office 60 50 40
Library
Wood 30 20 10
Threshold of Hearing

8. What Makes up a Sound?
Very few sounds have a single frequency ie are pure tones
Sounds with a dominant frequency are called tonal
Most sounds contain various frequencies at different intensities and are called broadband

9. Direction of travel
(propagation)
Sound Energy
distance

10. Inverse Square law Sound Energy per Unit area decreases
surface area  radius2
Point Source
6dB reduction per doubling of distance r2
source
Ir1
Ir2 r1

11. Line Source Sound radiates as a cylinder surface arearadius
3dB reduction per doubling of distance r2 r1

12. Met: Wind Effects Height Velocity Profile Sound waves ‘bent’ by wind
Wind speed increases with increased height above ground level. The wind carries the sound and refracts (bends) it.
If sound is carried away from the ground, then a shadow zone is created where noise levels are lower than expected.
If sound is carried towards the ground, then the noise levels are concentrated in certain areas. This phenomenon is called a focus.
When measuring sound you must take account of wind speed. If the wind speed exceeds 15 knots or 5 metres per second, then the sound created by the wind hitting the microphone diaphragm is greater than the noise being measured, thus making any measurements invalid.
Focuses and Shadow zones:
Those of you who live a mile or two from railway lines, have you ever wondered why on one night you can hear the train quite clearly and the next night when the wind speed and direction are different you cannot hear the train at all?
Focusing
Shadow
Ground Level

13. Noise and Man

14. The average young adult with healthy ears can detect frequencies over the range 20 Hz to 20,000 Hz
Audible Range
20000
12.5
1600
3150
6300
1250 20 50
100
200
400
800 10
-10
SPL dB
-20
-30
-40
-50
-60
-70
Frequency Hz

15. Describing the Human Reaction
Frequency Hz 20
200 2k
20k
SPL dB 30 50 70 90
110
130
140
Equal Loudness curves:
Same Loudness as a 1kHz tone
Stevens & Davis 1938
Unit - Phon
130
110 90 70 50 40 30 10 1k

16. Loudness Non-linear: Average Person Level Frequency20 40 60 80
100
120
140
Threshold of feeling
Non-linear:
Level
Frequency
Average Person
Audible range
Music
SPL dB
Speech
Threshold of hearing 20
200 2k
20k
Frequency Hz

17. SPL dB Frequency Hz Frequency Hz SPL dB dB(A) dB(B) dB(C) dB(D) 30 50
-70
-60
-50
-40
-30
-20
-10 10 20 16
31.5 63
125
250
500
1000
2000
4000
8000
200000
dB(A)
dB(B)
dB(C)
dB(D) 30 50 70 90
110
130
140
130
110
SPL dB 90 70 50 40 30 10 20
200 1k 2k
20k
Frequency Hz

18. } Noise Indices } SPL Lmax Lmin LEq,t Ln Lpk rms Peak pressure
LAe (SEL) 60 80
100 40
SPL
Lmax
Lmin
LEq,t Ln
Lpk
Lmax
Leq
SPL
dBA
L65
rms }
Peak
pressure
Time sec

19. Units ( Instantaneous)
Sound Pressure Level (SPL or Lp) - Unit - dB(Lin) Instantaneous sound pressure at a given position
Sound Level (SL) - A-weighted Unit - dB(A) Instantaneous sound at given position, measured with a meter that takes account of the sensitivity of the human ear

20. Units (Average level when noise level is varying)
Equivalent Continuous Sound Level LAeq
Daily Personal Noise Exposure LEP,d

21. Peak Pressure Impulse noise: Human ear reacts differently
Very short duration
Very high noise level
Human ear reacts differently

22. European and UK Occupational Noise Legislation
Control Noise at Work Regulations 2005 (6 April 2006)
Lower Exposure Action Value 80dBA average for 8 hours (LEP,d)
Upper Exposure Action Value LEP,d 85dBA
Exposure Limit Value LEP,d 87dBA
Peak Exposure Limit Value (LcPk) of 200Pa (140dBC), Peak Action Values 135dBC and 137dBC

23. Management Risk assessment Preventative measures
Noise Control
PPE
Health and safety arrangements
Training
Health surveillance

24. Prevention: Principles
Avoid Risk- remove source
Reduce the Noise At Source
Move source to minimise effect on wider workforce
PPE

25. Personal Hearing Protection
Last resort
Must be compatible with other protective clothing and equipment
Protection provided limited
Active noise reduction (ANR) only effective at certain frequencies
Helmets/Headsets do not necessarily provide hearing protection
Any covering over the ears may affect the ability to localise sounds

26. Communication

27. Communications Speech Intelligibility:
Consonants 80% Intelligibilty, 20% Energy
s, f, t, p, k
E.g. Zoo/Sue, Pack/Pat, Fazed/Phased, Sap/Sat
Intelligibility Indexes weighted to Higher Frequencies

28. Aircraft Noise Sources
Aerodynamic noise
Propulsion noise
Cabin conditioning
Avionics
Weapons systems
Elicit sources of ac noise from class
Aerodynamic - greater at low level.
Propulsion - different noise spectra for jets, props & helicopter rotors
Cabin conditioning - varies from ac to ac (Jaguar noisy : Tornado quiet)
Avionics - Includes amplification of noise by coms system.

29. Typical Fast Jet Cabin Noise Spectrum80 90
100
110
420 kt, 250 ft
SPL
(dB)
Broad Band noise spectrum for fast jets 63 8k
125
250
500 1k 2k 4k
Frequency (Hz)

30. Effect of Altitude on Cabin Noise
110
420 kt, 250 ft
100
SPL
(dB) 90
Marked reduction in noise with altitude due to decreased aerodynamic noise
420 kt, 33,000 ft 80 63
125
250
500 1k 2k 4k 8k
Frequency (Hz)

31. Added Noise due to Communications50 70 90
110
Speech level
required
SPL
(dB)
Noise level
at ear
To be understood, the coms signal needs to be 15 dB above the noise level at all frequencies - the coms signal itself may be damaging!
Even so unvoiced sounds (primarily consonants) may be difficult to hear.
Standard RT procedures overcome this problem to some extent. 63
125
250
500 1k 2k 4k
Frequency (Hz)

32. Example - Tristar Flight Deck Noise
Take-off from Calgary to Edmonton
Sennheiser headset
Background 74dBA
Speech dBA
LEP,d 84-86dBA

33. QUESTIONS?