Noise And Vibration Flashcards
The nature of sound
Sound is a mechanical radiant ENERGY that is transmitted by longitudinal pressure waves in a material medium (such as air) and is the objective cause of hearing
*Sound is ENERGY
Sound
Produced when an object or surface vibrates rapidly
Transmitted through any elastic substance such as air, water, or bone
Density of the substance determines the speed at which the sound and pressure waves will travel
Perception of sound
Sound energy wave enters ear flap (gives direction, intensifies), into ear canal, to the eardrum and middle ear, finally to inner ear and converted to nerve impulses to brain.
Cochlea
Inside inner ear
Receptors of sound
Detect fluid movement in cochlea
Transmit electrical impulses to the brain where sound is interpreted
Noise
Sound that is - loud (perception of listener); unpleasant, unwanted
Effects of noise in aviation enviornment
Annoyance and fatigue
Speech interference
Hearing loss
Measurable characteristics of noise
Frequency
Intensity
Duration
Frequency
Gives sound pitch
Number of times per second air pressure oscillates
CPS = Hertz (HZ)
Intensity
Measure that correlates sound pressure to loudness
Measured in decibels (dB)
Decibel levels
0dB - hearing threshold 65dB - average human conversation **85dB - damage-risk noise limit** 120dB - threshold for discomfort 140dB - threshold of pain 160dB - ear drum rupture *190dB - death
Duration
How long you are exposed to noise - steady noise vs. impulse
Steady noise
Continuous noise at high intensity
Wide range of frequencies
Most encountered in Army AV
Engines, drive shafts, transmissions, rotors and propellers
Impulse noise
Explosive noise
High intensity with low duration
Measured in milliseconds with less than 1 second in duration
Army noise criteria
The Surgeon General has established 85dB as max level of continuous unprotected exposure to steady-state noise for 8 hours
Army noise exposure criteria (chart)
8hrs - 85dB 4hrs - 90dB 2hrs - 95dB 1hr - 100dB 1/2hr - 105dB
Every 5 decibel increase, time exposure allowed cut in half
Types of hearing loss
*Conductive hearing loss - CAN be treated/fixed
Sensorineural hearing loss
Mixed
Conductive hearing loss
Defect or impediment in the external or middle ear
Impede with the mechanical transmission of sound to the inner ear
Sensorineural loss
When cochlea is damaged
Most frequently produced by noise
Occurs in the higher frequencies first
Could be associated with aging
Acoustic trauma
Sudden and could cause hearing loss
In excess of 140dB
From impulse noise (blast, gunfire)
Usually predictable and preventable
Mixed hearing loss
Combination of conductive and/or sensorineural loss
Example) crew-member with middle ear infection (conductive) and a high frequency hearing loss (sensorineural). One is treatable (conductive), one is not.
Noise induced hearing loss
Temporary Threshold Shift (TTS)
Permanent Threshold Shift (PTS)
TTS
Temporary Threshold Shift:
Single exposure to high level noise
May last for few minutes/hours
Depends upon frequency, intensity, and duration of the noise
Recovery when noise is removed, usually complete
PTS
Permanent Threshold Shift:
If continued for 15 hours, eventually permanent loss induced
No recovery when exposure is terminated
TTS could become permanent (cannot be predicted)
Damaged hair cells in ear
Looks like blown over grass
After exposure to 120dB for 5 hours
Long term hair cell damage
Prolonged, unprotected exposure to noise could cause irreversible damage
Characteristics of noise induced hearing loss
Insidious and undetectable
Noise intensity below 140dB but above 85dB
Physical pain not evident
Initially higher frequencies affected (3000 to 6000 Hz)
Audiograms
Used to determine hearing loss
First one is a “reference audiogram”
Considered normal if hearing thresholds are 20dB or less for all frequencies tested
Noise in Army aircraft
Overall noise levels are equal to or exceed 100dB’s
Most intense noise below 300Hz
Low frequency noise will produce high frequency hearing loss
MUST USE HEARING PROTECTION
Noise in Army fixed-wing aircraft
Engines and propellers in close proximity to the cockpit
Other fixed wing aircraft are beter insulated to attenuate noise levels
Rotary-wing aircraft noise
Noise levels equal or exceed 100dB
Originates from engines, rotor systems and transmissions
Observation helicopters (100-103dB)
Attack helicopters (104dB); increased during missions with weapon systems firing
Utility and cargo helicopters (108-112dB)
Noise protection
Aircraft design, silent operation or noise suppression
Isolate, distance or enclose source (isolate)
**Personal Protective Equipment ([PPE or PPD] most economical/practical protection). Max attenuation is about 50dB from dB level of device/event.
Ear plugs
Foam, single flange, and triple flange (foam is best)
Inexpensive, easy to carry
Attenuates 18-45dB across frequency band
Effective when worn with HGU-56, and IHADSS flight helmets
Ear muffs
10-41dB attenuation across the frequency band
Comfortable to wear
Ground personnel can lose their hearing too
Good control measure to ensure crew wear it - visible
Headsets
Hearing protection as well as radio comm
Attenuation could decrease due to damaged ear seals (put in bag!)
Commonly worn on VIP aircraft
Lack crash-worthiness of flight helmet
Protective helmets
Provide crash and noise protection
Great protection against higher frequencies
Low frequency is the concern in the AV environment
Must fit properly
Ear cups must be soft, unwrinkled, tear free
Noise attenuation will bring noise exposure within confines of damage risk criteria for every Army air raft EXCEPT THE UH-60 and CH-47 (wear extra protection for those… COMBINED HEARING PROTECTION)
Vibration
The motion of an object relative to a reference position (object at rest) involves series of oscillations resulting in the displacement and acceleration of the object
Sources of vibration
Increased air speed
Internal and external loading
Environmental factors (turbulence)
Most intense during transition from hover to cruise and cruise to hover
Physical effects of vibration on performance
Manual coordination - coordination and control “touch” degraded, aircrew member may over-control during turbulence/transition from hover to level flight (pilot induced oscillation)
Vision - instrument panel may be difficult to read, helmet mounts start to vibrate at 4-12Hz
Speech - can be distorted during oscillations of 4-12Hz, above 12Hz speech becomes increasingly difficult to interpret
Physical effects of vibration (short term)
Fatigue - vibration causes muscles to contract, pressure receptors must constantly measure angular position - causes muscles to contract for balance
Respiratory effects - hyperventilation is caused when diaphragm vibrates at 4-8Hz
Circulatory effects - body interprets vibrations, therefore muscular tension of bracing causes pulse rate and blood pressure to increase
Spatial disorientation - vibration affects the semi-circular canals and otolith organs
Pain - pre-existing injuries aggravated by vibration (stress fractures, back pain, degenerative disc disease)
Long-term effects of vibration
Raynaud’s disease - (white finger) prolonged exposure to vibration. Trauma to nerve endings in extremities
Back ache/pain - may result at earlier age than normal, spine subject to higher pressure while sitting, vibration can cause premature degradation
Kidney/lung damage - is under study, blood in urine or lung damage is a sign of over-exposure
Reduction of vibration effects
Good posture
Restraint systems used correctly (protect against high vibration at turbulence)
Aircraft maintenance
Isolate crew-members, passengers, and patients from aircraft structures
Limit exposure time (short flights/frequent breaks)
Let the aircraft do the work
Maintain good physical condition
Fat multiplies vibration, muscles dampen them
Decreases the effects of fatigue
Maintain sufficient hydration! Dehydration with vibration can cause fatigue twice as fast and increase recovery time.
Frequency
Frequency - number of oscillations of any object in a given time. 1 cycle per second (CPS) = 1 Hertz (Hz)
Amplitude
Amplitude - the max displacement about a position of rest
Duration
Duration - time of exposure
Natural Body Resonance
Natural body resonance - mechanical amplification of vibration by the body occurring at specific frequencies (whole body: 4-8Hz; shoulder girdle: same; head: 25Hz; Eyes: 30-90Hz)
Damping
Damping - Loss of mechanical energy in vibrating system; slows vibration (like body tissue)
