Diploma in Aviation Medicine Introduction to Acoustics

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

2. Time Subject Presenter 0900-1000 Basic Acoustics Matt Peacock 1015-1115 Hearing and Hearing Conservation Sqn Ldr Andy Thomason 1115-1215 Demonstrations NVD Personnel 1400-1500 Aircrew Hearing Protection: The Future Lt Col Mark Adams Program

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 0.00002 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 Threshold of Pain 140 120 Chainsaw 110 Helicopter 100 90 Lorry 80 70 Busy Office 60 50 40 Wood Library 30 20 10 Threshold of Hearing 0

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 Ir2 Ir1 r1

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

12. Met: Wind Effects Height Sound waves ‘bent’ by wind Velocity Profile 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 3150 6300 12.5 1600 1250 200 400 800 100 20 50 10 0 -10 SPL dB -20 -30 -40 -50 -60 -70 Frequency Hz

15. 140 130 110 90 SPL dB 70 50 30 0 20 200 2k 20k Frequency Hz Describing the Human Reaction • Equal Loudness curves: • Same Loudness as a 1kHz tone • Stevens & Davis 1938 • Unit - Phon 130 110 90 70 50 40 30 10 1k

16. 140 120 100 80 60 40 20 0 20 200 2k 20k Loudness Threshold of feeling • Non-linear: • Level • Frequency • Average Person Audible range Music SPL dB Speech Threshold of hearing Frequency Hz

17. 140 130 20 10 110 0 16 63 125 250 500 31.5 1000 2000 4000 8000 200000 90 -10 -20 70 SPL dB -30 50 -40 dB(A) -50 30 dB(B) dB(C) -60 dB(D) 0 -70 Frequency Hz 130 110 SPL dB 90 70 50 40 30 10 1k 20 200 2k 20k Frequency Hz

18. 100 80 60 40 Noise Indices } LAe (SEL) • 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: • Very short duration • Very high noise level • Human ear reacts differently

22. European and UK Occupational NoiseLegislation • 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

29. 110 100 90 80 63 125 250 500 1k 2k 4k 8k Typical Fast JetCabin Noise Spectrum 420 kt, 250 ft SPL (dB) Frequency (Hz)

30. Effect of Altitude on Cabin Noise 110 420 kt, 250 ft 100 SPL (dB) 90 420 kt, 33,000 ft 80 63 125 250 500 1k 2k 4k 8k Frequency (Hz)

31. 110 90 70 50 Added Noise due to Communications Speech level required SPL (dB) Noise level at ear 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 +12-14dBA • LEP,d 84-86dBA

33. QUESTIONS?