Noise induced hearing loss

The outer ear and the ear canal constitute the outer ear which funnels sound to the ear drum. The middle ear begins at the ear drum (tympanic membrane). In the middle ear three small bones called the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup help to transmit and amplify the vibrations generated by the sound, such that it travels through the middle ear to the inner ear. The inner ear besides having the functions that relate to the perception of sound, is also responsible for the perception of balance/orientation, and of acceleration.

However we are only concerned with sound in this exercise - specifically that unwanted sound that we call ‘noise’. The inner ear has a snail like structure called the cochlea filled with fluid and lined by cells with very fine hairs. This sequence of sensitive cells is somewhat like the keys of a piano. In the inner ear one end of the sensitive cells in the cochlea discerns high frequency sounds and the other end discerns low frequency sounds, with a continuous spectrum in between the two.

Sound can be measured scientifically in terms of intensity, but also specifically related to particular frequency bands.

Pitch or frequency is measured in cycles per second, or Hertz (Hz). The higher the pitch of sound, the higher the frequency. It is reported that in young children even frequencies as low as 20Hz and up to 1000 times greater as in a dog whistle, can be detected.

Sound intensity is measured in decibels (dB). By definition the faintest level of hearing detected by the human ear is set at zero decibels (though some people can hear levels lower than this).

Decibel scales are usually weighted to mimic better the human ear - the 'A' weighting i.e. dBA being the preferred weighting.

Standing behind a jet aircraft on take off would cause one to experience sound levels in excess of 140 dBA. Since sound levels are expressed logarithmically, if a source of noise such as a machine generates a sound level of say 90dBA, two such machines would generate 93dBA (not 180)!

Workers at special risk of hearing damage (industrial deafness) are usually those in heavy productive industry, such as metal work, drilling and quarrying, stone cutting, or the use of noisy machinery, as in textiles, printing, wood cutting, transportation and agriculture. Noises above 90 dB, as measured with special instruments that are electronically weighted to mimic loudness functions of the human ear, are likely to cause damage to a proportion of the exposed population with continued exposure. Very high levels may cause damage after relatively short periods, even when the noise is intermittent. This may be illustrated by the frequent finding of hearing loss in people who have fired guns as an occasional hobby, as well as in people who are exposed to noise of lower levels but more constantly, such as those working on construction sites or in other industrial locations such as mines.

The harmful effects of noise are cumulative and not, of course, confined to the workplace. The use of personal stereos and frequenting of discos has resulted in young people having some early damage to hearing before they even start work.

The answer to this is not completely known. However we do know that the damage is caused to the sensitive cells in the cochlea. For reasons which are not entirely clear some of the cells part of the way along the sensory organ in the cochlea are more sensitive than others. Hence noise induced hearing loss (NIHL) will begin to affect hearing of certain frequencies. This results in a 'dip' manifest in an audiogram (see image left).

High noise levels damage the hair cells of the organ of Corti, affecting first those in the basal part of the cochlea concerned with reception of the higher frequency sounds (though not necessarily the highest) and progressing through to those receiving lower frequencies. This is reflected in audiometric changes which show loss of sound perception first in the 4-5 kilohertz (kHz) range, progressing both in severity and into lower frequency ranges.

By clicking on the link, Hearing Loss and Environmental Factors, to the right of this page, you can investigate the effects of age and length of exposure on hearing loss, provided your resources include a soundcard and speakers as well as supporting software.

The first symptom of noise-induced hearing loss is usually difficulty hearing a conversation against a noisy background. The sufferer comes to dislike parties where everyone is apparently chattering away happily, yet he or she hears just a jumble of noise. Consonants seem to be lost first. Often he or she will mention intermittent high-pitched ringing in the ears, though this is rarely sufficient to be more than an irritant. By the time these symptoms have become sufficient to prompt medical consultation, the damage as measured by audiometry will be severe and, even with cessation of noise exposure, progressive.

When hearing is reduced at 3kHz and below, conversation is significantly interfered with.

Several measures can and should be taken in a hierarchical order: Assessment of exposure, using tools and equipment which generate a lower level of noise, segregation and insulation, appropriate work practices and personal protection such as ear muffs and ear plugs. As well as steps to protect workers from noise, many companies now carry out regular audiometry.

The most reasonable way to protect the ears is to generate less noise in the first place, by better design of machinery and equipment.

Secondly steps should be taken to insulate the machinery to reduce the noise that it emits and to segregate people from it (i.e. to keep them away).

People should work in areas where they are not exposed to high levels of noise. The same goes for leisure activities (e.g. clubs).

Remember the 'two metre rule' - if you find it difficult to communicate with a workmate at this distance because of noise, then probably the intensity is high enough to damage your hearing.

At a personal level it is possible to protect the ears with ear muffs and/or ear plugs. If you must work in an excessively noisy environment, you should wear protectors. You should also wear them when you are using power tools, noisy yard equipment, or firearms.

Personal habits are also important especially in avoiding high exposures to noise resulting from the use of so called "portable media players".

Ear plugs are small, often torpedo nose shaped synthetic inserts, that are fitted inside the outer ear (canal). For them to work properly they should be of the appropriate shape, and correctly inserted. They may cause irritation especially if dirty. They are usually disposable and will not fit properly if repeatedly re-used.

Ear muffs fit over and around the outer ear, as shown in the accompanying image. They too need to be appropriately fitted with correct headband tension, and avoiding glasses or hair which will impair the seal and hence the protection.

Hearing protection may attenuate noise by between 15 and 30 dBA. The use of combined ear plugs and muffs is controversial.

As with all personal protection - this is the last line in the protection hierarchy. Appropriate low noise machinery/ processes must be in place, then followed by insulation and segregation to attenuate the workers' exposure to noise.

The main problems are:

• Undue reliance on personal protection without adequate steps to reduce noise exposure at source.
• Inappropriate personal protection - because of poor choice, incorrect fitting or inadequate maintenance.
• Hearing protection not worn as constantly and consistently as it should be. (Remember, from the physics of noise, and the logarithmic function of the decibel scale, that even if optimal hearing protection is worn, producing an average of say 30dBA of noise reduction over an eight hour shift, this will fall to less than 10dBA protection if the ears are unprotected for even one-eighth of that shift).