IB6 - Noise

Question 1

Noise

Answer 1

Noise is defined as “all sound which can result in hearing impairment or be harmful to health or otherwise dangerous”

Question 2

Sound

Answer 2

• Sound is the vibration of any substance. The substance can be air, water, wood, or any other material, and in fact, the only place in which sound cannot travel is in a vacuum.
• The vibrations occur among the individual molecules of the substance, and the vibrations move through the substance in longitudinal sound waves.
• Longitudinal pressure waves moving through air (or other media).

Question 3

Amplitude

Answer 3

Is the maximum displacement of sound wave pressure (measured in Pa).

A larger amplitude means louder

Question 4

Intensity

Answer 4

• Is the sound power transmitted per unit area (W/m2).
• Sound intensity is proportional to sound pressure squared.

Question 5

Frequency

Answer 5

• Frequency (F) is the number of times a complete wave passes a point. It is measured in hertz (Hz), or cycles per second. The slowest, lowest sound a human can hear is approximately 20 Hz. The highest sound a human can hear is approximately 20,000 Hz (or 20 kilohertz - kHz).
• subjectively described as ‘pitch’.

Question 6

Sound Pressure Level

Answer 6

• Sound pressure is the local pressure deviation from the ambient atmospheric pressure, caused by a sound wave. It is measured in Pascals.
• Sound pressure is the ‘effect’ of a disturbance (what is heard).
• Sound pressure level (SPL) is the pressure level of a sound, measured in decibels (dB).

Question 7

What is the logarithmic relation between dB and intensity?

Answer 7

• 0 dB Threshold of human hearing
• 10 dB 10 or 10¹ times more intense
• 20 dB 100 or 10² times more intense
• 30 dB 1,000 or 10³ times more intense
• 40 dB 10,000 or 10⁴ times more intense

Question 8

Decibel (dB)

Answer 8

• Used to measure sound intensity.
• The threshold of hearing is assigned a value of 0dB.
• Decibel scale is logarithmic, rather than linear.

Question 9

What is the hearing human threshold?

Answer 9

• 0 dB, it is the least intense sound a human can detect
• Above 90 dB can lead to chronic hearing damage
• 110 decibels (threshold of discomfort)
• above 130 decibels (threshold of pain)

Question 10

Weighting

Answer 10

• A perceived sound level, a filter (a factor, a weight) applied to engineering (scientific) measurements to obtain a perceived sound level by human ears.
• Humans perceive SPL at higher frequencies as louder than the same PSL at lower frequencies.
• Other weighting scales are also used, e.g. C-weighting (dB(C)) for peak sound pressure.

Question 11

A-weighting

Answer 11

• Mimics the human ear’s response across the range of frequencies
• Reduces the importance of lower frequencies at 500 Hz or less. The lower the frequency, the greater the A-weighted correction factor becomes
• Slightly increases the overall magnitude of the mid to high frequencies (2,000-4,000 Hz).
• Reduces the very high frequencies as they extend beyond normal hearing.

Question 12

C-weighting

Answer 12

• It is used for the measurement of peak noise
• Designed to approximately correspond to how humans perceive sound at higher volume levels
• The C-weighting scale is quite flat, and therefore includes much more of the low-frequency range of sounds than the A and B scales

Question 13

A, B and C Weighting

Answer 13

• A weighting (dB(A)) mimics the response of the human ear and is the one that is used most frequently to assess the effects on humans of exposure to noise;
• B weighting (dB(B)) is used to measure sounds that may have a more dominant low frequency content;
• C weighting (dB(C)) provides for some filtering at the higher frequencies and is used to measure peak levels.

Question 14

Damaging effects of noise are related to

Answer 14

The dose (total amount of energy) the ear receives.

The dose is determinated by 2 factors:

1. Level of Noise
2. Duration of Exposure

Question 15

LAeq

Answer 15

LAeq is the equivalent continuous sound pressure level that is

averaged over a period of time and takes account of fluctuations in noise that occur when the machines are running.

“when a noise varies over time, the Leq is the equivalent continuous sound which would contain the same sound energy as the time varying sound”.

Question 16

L_EP,d

Answer 16

• The equivalent continuous daily personal noise exposure level.
• The time-weighted average dose of noise calculated for a 8-hour day.

Question 17

LC_Peak

Answer 17

• Maximum C-weighted peak sound pressure level(s) to which a person is exposed.
• Not a time-weighted average exposure but a ‘spike’ in sound pressure level achieved by exposure to a single impulse noise.

Question 18

Physiology of the Ear

Answer 18

ƒ Outer ear with auditory canal - collects sound onto eardrum.

ƒ Middle ear - transmits and amplifies sound to the cochlea.

ƒ Inner ear - stimulates nerve ends at the base of hair cells and transmits sound energy patterns to auditory centre of the brain.

Question 19

Types of hearing loss (6x)

Answer 19

1. Conductive Hearing Loss : Physical breakdown of the conducting mechanism of the ear resulting from an acute acoustic trauma.
2. Sensorineural Hearing Loss: Exposure to excessive noise, resulting in varying levels of acoustic trauma.
3. Tinnitus: Persistent ‘ringing’ in the ears & an acute condition which may stop (temporary tinnitus) with time.
4. Threshold Shift

– Temporary Threshold Shift (TTS): Exposure to a high noise level; hearing sensitivity returns with time. (Fatigue of the hair cells in the cochlea.) – A dip in hearing sensitivity occurs at 4,000Hz.

– Permanent Threshold Shift (PTS): a non-reversible condition which usually follows continual TTS exposure.

5. Noise-Induced Hearing Loss (NIHL) : Permanent threshold shift as a result of exposure to excess noise. Failure of the hair cells in the cochlea to respond fully to sound intensities which have frequencies within the speech range
6. Presbycusis : Age-related hearing loss (or presbycusis) is the gradual loss of hearing in both ears. Most often, it affects the ability to hear high-pitched noises such as a phone ringing or beeping of a microwave

Question 20

A temporary threshold shift (TTS) is

Answer 20

a hearing loss which shows some recovery within 24-48 hours after the noise exposure stops. The more intense (louder/longer) the exposure, the longer the expected recovery period would be.

Question 21

permanent threshold shift (PTS)

Answer 21

Hearing loss which persists more than 30 days after the noise exposure is considered to be permanent threshold shift (PTS) and recovery is unlikely.

Question 22

Health Surveillance : Audiometry

Answer 22

Audiometry is the measurement of hearing performance in order to detect actual noise-induced hearing loss.

Question 23

Interpreting the audiogram

Answer 23

• Frequency (Hz) is plotted from low to high pitch along the x-axis (horizontal).
• Intensity (dB HL) is plotted from soft to loud along the y-axis (vertical).
• Air conducting thresholds are indicated by an ‘X’ for the left ear and an ‘O’ for the right ear.
• Noise induced hearing loss typically presents with a classic ‘notch’ at around 4kHz.

Question 24

Interpretation of Audiometric Results

Answer 24

• Mild hearing loss: 25 to 40 dB higher than normal.
• Moderate hearing loss: 40 to 55 dB higher than normal.
• Moderate-to-severe hearing loss: 55 to 70 dB higher than normal.
• Severe hearing loss: 70 to 90 dB higher than normal.
• Profound loss: 90 dB or more.

Question 25

Where the health surveillance identifies hearing damage the employer shall ensure that the employee is examined by a doctor. If the doctor considers that the damage is likely to be the result of exposure to noise, the employer shall:

Answer 25

1. ensure that a suitably qualified person informs the employee
2. review the risk assessment
3. review control measures
4. consider assigning the employee to alternative work where there is no risk from further noise exposure
5. ensure continued health surveillance and a health for any other employee who has been similarly exposed.

Question 26

Audiometry Technique

Answer 26

1. Preparation and instructions given about the test procedure.
2. Earphones fitted over the ears and the test is then carried out on each ear.
3. A threshold test is undertaken (ear subjected to sound at a frequency of 1kHz at varying levels of intensity).
4. Ears tested through a range of frequencies and hearing loss recorded for each frequency.

Question 27

The Principles of Noise Assessment

Answer 27

ƒ Surveying the workplace to gather data.

ƒ Analysing the results and comparing with action values.

ƒ Deciding if you need to do any more to control noise.

ƒ Planning what more you need to do and doing it.

ƒ Recording findings.

ƒ Reviewing assessment, as necessary.

Question 28

The risk assessment should consider:

Answer 28

• The risk of hearing impairment, the impairment of communications, and possible effects of fatigue.
• The identification of sources and tasks which are likely to generate noise.
• The expected noise emission levels from equipment.
• The expected time of exposure.
• The people likely to be affected.
• The likely levels of personal noise exposure and comparison of these levels with legal limits to make control decisions.
• Identify any workers who need to be provided with health surveillance and whether any people are at significant risk.

Question 29

Instrumentation to take measurements

Answer 29

1. Simple Sound Level Meter
2. Integrating Sound Level Meter (ISLM)
3. Personal Sound Exposure Meter (Dosimeter)

Question 30

Calculations Methods for Hearing Protection

Answer 30

SNR Method - You need a “C” weighted sound level meter and the protector’s SNR figure.

HML Method - You need the “A” and “C” weighted sound level and the protector’s HML figures.

Octave Band Method - You need a meter with Octave Band Filters and the hearing protector’s APV (assumed protected values) values.

Question 31

Calculating dB(A) from SNR (dB(C))
Real World factor is 4dB

Answer 31

L’A = LC − SNR + 4dB

Question 32

There are three methods for predicting the effects of hearing protection so that its suitability can be assessed:

Answer 32

1. The Octave Band method is the most accurate but requires a full octave band spectrum of the noise in the workplace as derived from octave band analysis.
2. The HML method is less accurate if noise is dominated by single frequencies but only requires two bits of information about the workplace noise - A-weighted and C-weighted average sound pressure levels.
3. The SNR method (Single Number Rating) is the simplest but least accurate. One piece of information is required about the workplace noise - C-weighted average sound pressure level.

Whichever method is used, a 4dB addition is always made to the predicted noise exposure at the ear to take account of ‘real world’ factors.

Question 33

Noise control strategies involve controls at the

Answer 33

source, the pathway and at the receiver

Question 34

Noise Pathways

Answer 34

ƒ Directly - the noise moves directly from the source to the individual through the air.

ƒ Reflected - some noise is reflected off surfaces before it hits the receiver.

ƒ Transmitted - some of the noise from the source will be transmitted through structures and then re-emitted into the air.

Question 35

Advantages of Audiometry Programmes

Answer 35

• May help to demonstrate legal compliance, demonstrating that hearing loss risks are well managed, it may be required by law in certain countries.
• Pre-employment testing gives a basis against which future tests can be compared.
• Can assist in defence of civil claims, since the pre-employment testing may show that hearing loss had already occurred before starting work.
• May result in lower insurance premiums since the insurer will be reassured that noise risks are being managed.
• May assist in identifying control measure deficiencies. If workers suffer hearing damage, then this should be investigated like any other accident or instance of ill-health.

Question 36

Limitations of Audiometry Programmes

Answer 36

• Increased costs to the organisation. Audiometric testing requires a soundproof booth, a room to install it in, an audiometer, personnel to carry out the testing, admin personnel to organise the tests and appointments, and releasing workers from their normal duties to be tested.
• Can be inaccurate if the subject is uncooperative since it relies on the subject signalling that they can hear the sounds.
• It is reactive in that it determines loss of hearing that has already occurred. It does not help prevent injury.
• The subject may feel claustrophobic in a sound booth.
• Finally, and significantly, an audiometric testing programme may increase civil claims. Since the workers will become aware that their hearing has been damaged, they may be inclined to then pursue the organisation for compensation.

Question 37

The following factors should be considered when selecting hearing protection:

Answer 37

• Types of protector, and suitability for the work being carried out.
• Noise reduction (attenuation) offered by the protector.
• Compatibility with other safety equipment.
• The pattern of the noise exposure.
• The need to communicate and hear warning sounds.
• Environmental factors such as heat, humidity, dust, and dirt.
• Cost of maintenance or replacement.
• Comfort and user preference.
• Medical disorders suffered by the wearer.

Question 38

The types of hearing protection include:

Answer 38

Ear muffs

o Standard ear muffs.
o Noise cancelling ear muffs.
o Helmet mounted ear muffs.

Ear plugs

o Foam.
o Rubber.
o Disposable.
o Reusable and washable.
o With neck cords.
o With neck bands.
o Custom moulded.