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Slide 2 Diploma in Aviation Medicine Introduction to Acoustics Noise & Vibration Division, RAF Centre of Aviation Medicine Slide 3 Program Lt Col Mark Adams Aircrew Hearing Protection: The Future 1400-1500 NVD Personnel Demonstrations1115-1215 Sqn Ldr Andy Thomason Hearing and Hearing Conservation 1015-1115 Matt Peacock Basic Acoustics 0900-1000PresenterSubjectTime Slide 4 Introduction to Acoustics u Basic Acoustics decibel (dB) u Noise & man: Frequency & loudness response u Noise and Communications Slide 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. Slide 6 Describing Amplitude u The ear detects pressure changes rather than absolute pressure u Range from 0.00002 Pa to ~200 Pa u Using pascals gives a large, unmanagable scale (over 1 million) u Use Decibel Scale Slide 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 Slide 8 140 120 110 100 90 80 60 70 40 50 30 20 10 0 Chainsaw Library Jet Take-off at 50metres Wood Busy Office Lorry Helicopter Threshold of Pain Threshold of Hearing Decibel Scale / dBA Slide 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 Slide 10 Direction of travel (propagation) distance Sound Energy Slide 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 Slide 12 Line Source u Sound radiates as a cylinder u surface area radius u Line Source 3dB reduction per doubling of distance r1r1 r2r2 Slide 13 Met: Wind Effects Height Ground Level Velocity Profile Sound waves bent by wind Shadow Focusing Slide 14 Noise and Man Slide 15 Audible Range Frequency Hz SPL dB -50 -40 -30 -20 -10 0 10 -60 -70 12.5 2050 100 200 400800 1600 3150 6300 1250 20000 The average young adult with healthy ears can detect frequencies over the range 20 Hz to 20,000 Hz Slide 16 u Equal Loudness curves: Same Loudness as a 1kHz tone Stevens & Davis 1938 u Unit - Phon Describing the Human Reaction Frequency Hz 202002k20k SPL dB 30 50 70 90 110 130 140 0 130 10 40 70 90 110 30 50 1k Slide 17 Loudness u Non-linear : Level Frequency u Average Person Threshold of hearing SPL dB Frequency Hz 202002k20k 20 40 60 80 100 120 140 0 Audible range Music Speech Threshold of feeling Slide 18 SPL dB Frequency Hz 202002k20k 30 50 70 90 110 130 140 0 130 10 40 70 90 110 30 50 1k Slide 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 60 80 100 40 L max LeqLeqLeqLeq L 65 L Ae (SEL) Slide 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 Slide 21 Units (Average level when noise level is varying) u Equivalent Continuous Sound Level L Aeq u Daily Personal Noise Exposure L EP,d Slide 22 Peak Pressure u Impulse noise: Very short duration Very high noise level u Human ear reacts differently Slide 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 Slide 24 Management u Risk assessment u Preventative measures Noise Control PPE u Health and safety arrangements u Training u Health surveillance Slide 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 Slide 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 Slide 27 Communication Slide 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 Slide 29 Aircraft Noise Sources u Aerodynamic noise u Propulsion noise u Cabin conditioning u Avionics u Weapons systems Slide 30 638k1252505001k2k4k 80 90 100 110 Typical Fast Jet Cabin Noise Spectrum SPL (dB) Frequency (Hz) 420 kt, 250 ft Slide 31 638k1252505001k2k4k 80 90 100 110 Effect of Altitude on Cabin Noise SPL (dB) Frequency (Hz) 420 kt, 250 ft 420 kt, 33,000 ft Slide 32 50 70 90 110 SPL (dB) Frequency (Hz) Added Noise due to Communications Noise level at ear Noise level at ear Speech level required Speech level required 631252505001k2k4k Slide 33 Example - Tristar Flight Deck Noise u Take-off from Calgary to Edmonton u Sennheiser headset u Background 74dBA u Speech +12-14dBA u L EP,d 84-86dBA Slide 34 QUESTIONS?