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

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Presentation on theme: "Diploma in Aviation Medicine Introduction to Acoustics Noise & Vibration Division, RAF Centre of Aviation Medicine."— Presentation transcript:


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

3 Program Lt Col Mark Adams Aircrew Hearing Protection: The Future NVD Personnel Demonstrations Sqn Ldr Andy Thomason Hearing and Hearing Conservation Matt Peacock Basic Acoustics PresenterSubjectTime

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

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

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

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

8 Chainsaw Library Jet Take-off at 50metres Wood Busy Office Lorry Helicopter Threshold of Pain Threshold of Hearing Decibel Scale / dBA

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

10 Direction of travel (propagation) distance Sound Energy

11 Inverse Square law u Sound Energy per Unit area decreases u surface area  radius 2 u Point Source –6dB reduction per doubling of distance source r1r1 r2r2 I r1 Ir2Ir2

12 Line Source u Sound radiates as a cylinder u surface area  radius u Line Source –3dB reduction per doubling of distance r1r1 r2r2

13 Met: Wind Effects Height Ground Level Velocity Profile Sound waves ‘bent’ by wind Shadow Focusing

14 Noise and Man

15 Audible Range Frequency Hz SPL dB The average young adult with healthy ears can detect frequencies over the range 20 Hz to 20,000 Hz

16 u Equal Loudness curves: –Same Loudness as a 1kHz tone –Stevens & Davis 1938 u Unit - Phon Describing the Human Reaction Frequency Hz k20k SPL dB k

17 Loudness u Non-linear : –Level –Frequency u Average Person Threshold of hearing SPL dB Frequency Hz k20k Audible range Music Speech Threshold of feeling

18 SPL dB Frequency Hz k20k k

19 Noise Indices u SPL u L max u L min u L Eq,t u L n u L pk } rms } Peak pressure Time sec SPL dBA L max LeqLeqLeqLeq L 65 L Ae (SEL)

20 Units ( Instantaneous) u Sound Pressure Level (SPL or L p ) - Unit - dB(Lin) Instantaneous sound pressure at a given position u 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

21 Units (Average level when noise level is varying) u Equivalent Continuous Sound Level L Aeq u Daily Personal Noise Exposure L EP,d

22 Peak Pressure u Impulse noise: –Very short duration –Very high noise level u Human ear reacts differently

23 European and UK Occupational Noise Legislation u Control Noise at Work Regulations 2005 (6 April 2006) –Lower Exposure Action Value 80dBA average for 8 hours (L EP,d ) –Upper Exposure Action Value L EP,d 85dBA –Exposure Limit Value L EP,d 87dBA –Peak Exposure Limit Value (Lc Pk ) of 200Pa (140dBC), Peak Action Values 135dBC and 137dBC

24 Management u Risk assessment u Preventative measures –Noise Control –PPE u Health and safety arrangements u Training u Health surveillance

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

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

27 Communication

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

29 Aircraft Noise Sources u Aerodynamic noise u Propulsion noise u Cabin conditioning u Avionics u Weapons systems

30 638k k2k4k Typical Fast Jet Cabin Noise Spectrum SPL (dB) Frequency (Hz) 420 kt, 250 ft

31 638k k2k4k Effect of Altitude on Cabin Noise SPL (dB) Frequency (Hz) 420 kt, 250 ft 420 kt, 33,000 ft

32 SPL (dB) Frequency (Hz) Added Noise due to Communications Noise level at ear Noise level at ear Speech level required Speech level required k2k4k

33 Example - Tristar Flight Deck Noise u Take-off from Calgary to Edmonton u Sennheiser headset u Background 74dBA u Speech dBA u L EP,d 84-86dBA


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