B6 Noise And Vibration Flashcards
Definition of noise
Any audible sound.
In short any noise that is audible noise that is within the human ear frequency range 20Hz to 20KHz
What is dB(A)
dB(A) is the weighted scale used to best mimic that the human ear would assimilate noise.
As the human ear doesn’t efficient across all frequencies, the human ear is better at assimilating mid range frequencies.
This is a good indicator of physical harm caused to hearing.
dB(C)
dB(C) is a measure of peak sound pressure
Amplitude
Amplitude is the maximum displacement of sound wave pressure.
This equates to loudness the higher the wave peak the louder the noise.
Frequency
Frequency in the number of cycles per second the pass a given location
Pitch
Pitch is the brains interpretation of the frequency of sound. Shrill or piecing sounds are High pitch and are associated with high frequencies. Rumbling or drones are low pitched and associated with low frequencies.
Make up of the ear
Outer ear and auditory canal Ear drum Hammer anvil and stirrup Semicircular canal Cochlea Nerve fibres
Two headings of hearing loss
- Conductive hearing loss (also known as instantaneous), this type of hearing loss is usually as a result of exposure to acute acoustic trauma eg explosion or gun fire. The ear drum, ossicles (hammer, anvil, stirrup) becomes damaged.
- Sensorineural hearing loss, as a result of the hair cells in the cochlea becoming damaged. This is the most common occupational hearing loss.
Threshold shift
Threshold shift is a reduction in a persons ability to hear ie they need more sound intensity to stimulate their hearing. This can temporary (TTS) or permanent (PTS).
Temporary threshold shift (TTS)
TTS occurs after exposure to load noise where the hearing acuity returns to normal in time.
It can be described as as a fatigue of the hair cells in the cochlea.
The shift can be described as the raising sound level required to hear the sound for eg an increase in 20dB(A) would mean that is the value of TTS.
The higher and longer the exposure to noise the longer it will take to recover
Permanent Threshold Shift (PTS)
PTS is due to long repeated exposure to noise where the TTS has been repeated so often the cell hairs do not return regardless of removal noise. This is seen as a dip in the 4k Hz range
This is known as noise induced hearing loss (NIHL)
Presbycusis
Presbycusis is the term used to describe natural reduction in hearing loss with age. The young are able to detect higher pitch noises that middle aged to old.
Hearing loss naturally occurs from the age of 30 and becomes more significant from the age of 60 onwards.
Audiometry
Audiometry should be carried out on all workers exposed to significant noise levels.
Base line on employment/ pre employment, Repeat testing annually for first two years, The every three years after that (unless problems arise or noise exposure changes significantly)
Audiometry should only be carried out by a suitably qualified person using a standard method and calibrated equipment. In a sound proof booth.
Ideally test should be done before work has began to prevent any TTS
Before carrying out audiometry
The following should be done/ known before carrying out audiometry
- Past medical history (not only concerning ears but any medical condition that may affect hearing)
- Visual examination of ears to ensure no excessive ear wax, damage that may have an impact on the test.
Factors that can affect audiometry
- TTS
- Poor fitting of head phones
- Lack of sound proof booth/ external noises
- Poorly calibrated or maintained equipment
- Learning effect - person being tested becomes more proficient at detecting the threshold
- lack of cooperation of person being tested
HSE categorisation scheme
The Control of Noise at Work Regulations (CNAW) appendix 5 of the guidance gives four categories for the results
Cat 1 = indicates acceptable hearing ability with no follow up
Cat 2 = indicates mild hearing impairment triggering the issue of formal verbal and written notification of impairment and action required to prevent further deterioration.
Cat 3 = indicates poor hearing triggering a referral to a GP or other medical practitioners for further investigation
Cat 4 = indicates rapid hearing loss triggering a referral to a GP or other medical practitioners for further investigation
Advantages of audiometry
- early detection of loss can prevent further deterioration
- early detection of poorly controlled noise exposure can prevent others from being put at risk
- negative results ie no hearing loss can confirm noise control measures are effective
- establishment of pre employment baseline will protect employers in event of claim for compensation
- compliance with legal requirements
- Staff feel their health is being taken as important
Disadvantages of audiometry
- As a form of surveillance it is inherently reactive by only confirming loss has occurred
- can be inaccurate if test conducted wrongly
- can be difficult and expensive to practice and manage with large numbers.
- persons falling into CAT 2 to 4 may be a trigger to pursue claims
Legal requirement (action values)
80-85dB(A) and 135dB(C)
- Provide information and training
- Make hearing protection available (mandatory for)
- individuals may be susceptible to hearing loss
- Where a worker is known to have a pre existing noise induced hearing loss
- Where there is a known family history of hearing loss
- Provide Health surveillance if RA indicates it
85dB(A) and 137dB(C)
- Reduce exposure
- Provide hearing protection
- Designate hearing protection zones
- Ensure hearing protection is worn
- Provide health surveillance
87dB(A) and 140dB(C) - limit must not be exceeded - If exceeded: *Investigate reason Identify and implement actions to prevent a *reoccurrence
Principles of a noise assessment
CNAW Regulations requires Where noise levels exceed 80dB(A) employers must carry out a suitable and sufficient risk assessment.
Principles are:
- Identify people at risk of hearing damage
- Determine the daily personal noise exposure (Lep,d) of those who are likely to be exposed at or above the lower exposure action level
- Identify additional information to comply with the regulations such as noise control and hearing protection maybe required.
Planning a noise survey
- Who should be assessed? Who will be exposed
- What equipment is required?
- What should be measured?
- Where?
- For how long? (Needs to capture full variety of working day)
- Group sampling?
- Mobile workers eg maintenance engineers
- Very short duration eg gunfire
- is a second more detailed survey required
Basic noise assessment instruments
- Simple sound level meter - not adequate for establishing compliance but may be useful for spot checks.
- Integrated sound level meter (ISLM)
for measuring noise in compliance with CNAWR is must be
-Class 2 instrument or a Type 2 instrument
and - Capable of measuring A weighted sound pressure and maximum C weighted peak sound pressure level.
An octave band analyser is a type of ISLM that allows frequency analysis of the noise.
- Personal sound exposure meter (Dosimeter)
To measure the total noise dose over a whole working period.
Hierarchy of noise control
- Noise Reduction at source eg relocation, redesign and maintenance.
- Attenuation in transmission (reduce the transmission of noise before it reaches the worker) eg isolation, barriers and enclosures.
- Control at receiver eg acoustic havens and hearing protection.
Hierarchy of noise controls - Reduction at source.
Reduce noise at source
- Eliminate hazardous noise at source
If not possible the: - Change the source to one that generates less noise
- Relocate the source of noise
- Redesign the source eg damping, modifying guarding so it doesn’t resonate.
- Carry out Maintenance
