# Diploma in Aviation Medicine Introduction to Acoustics

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

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

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

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

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

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

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

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

Direction of travel (propagation) Sound Energy distance

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

3dB reduction per doubling of distance r2 r1

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

Noise and Man

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

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

Loudness Non-linear: Average Person Level Frequency
20 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

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

} 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

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

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

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

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

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

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

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

Communication

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

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.

Typical Fast Jet Cabin Noise Spectrum
80 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)

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)

50 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)

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

QUESTIONS?

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