Noise and hearing

The looms have made McDermott deaf. Well not deaf exactly, but they have changed sound, damaged sound, so that sometimes spoken words seem to come from the bottom of a well, and others have halos around them, gauzy halos that slur sound.

This passage, from Anita Shreve’s novel “Sea Glass” , which I finished recently, is one of the best descriptions I’ve read of the impact of noise induced hearing loss on an individual.

File:Spinning room in indian orchard cotton mill.jpg

A ring spinning mill (picture source Wikimedia commons)

In the novel, which is set in and around the cotton mills in New England in the USA in the 1930s, McDermott is a loom fixer in a mill. He has been exposed to high levels of noise for all his relatively short working life. And it is this exposure that has caused him to become deaf. But, as come across in the passage, noise induced hearing loss doesn’t just make it more difficult to hear, it distorts the sound making it difficult to understand speech and spoiling the enjoyment of music and other activities that involve listening

The development of noise induced hearing loss follows a characteristic pattern, although the severity will be dependent upon a number of factors including the intensity of the noise experienced, the duration of exposure, the pattern of exposure, individual susceptibility and many other complex considerations.  Noise-induced hearing impairment occurs predominantly in the high-frequency range of 3 to 6 kHz (3,000 to 4,000 Hz), the effect being greatest at 4 kHz. Frequencies above this range are less affected resulting in a characteristic dip in audiogram charts created during hearing tests usually known as the “4 Khz dip”.

An audiogram for a worker who is starting to suffer from noise induced hearing loss. The “4 kHz dip” is clearly identifiable (Source: American Academy of Family Physicians website)

When a worker first starts to suffer from noise induced hearing loss difficulties are experienced during conversation, and speech on the TV or radio begins to become indistinct.  Some higher frequency domestic sounds, for example a clock ticking, may also become difficult to hear.  As hearing deterioration progresses, further difficulties are experienced in conversation – even in face-to-face situations, speech and music on the television and radio begins to sound even more muffled, and it may not be possible to hear many ordinary domestic sounds.  The ability to determine the direction from which a sound comes is also affected.  Even small values of hearing impairment  may have an effect on the understanding of speech.

It’s difficult for someone with normal hearing to appreciate how the world sounds to someone who has been made deaf by exposure to noise. The UK Health and Safety Executive have an audio demonstration that tries to get this across here.

Once noise induced hearing loss has occurred the damage is permanent, and cannot be reversed.  All that can be done is to take measures to prevent hearing deteriorating further once the  hearing loss has been detected.  So the important thing is to control exposure to prevent hearing damage occurring.

Selecting ear defenders

The use of personal protection should always be the last resort when looking for ways to control health and safety risks. Although this principle applies to ear defenders as much as any other type of PPE, it’s difficult to avoid their use in many industries where exposure to noise can be difficult to control, at least in the short term, by engineering measures.

If ear defenders have to be used, it’s important that they are suitable and will provide adequate protection. Their performance is frequency dependant – like most noise control measures they are more effective against higher frequencies. So when selecting ear defenders for a particular application it’s important that this is taken into consideration. There are three methods that can be used to achieve this, which are described in Appendix 3 of the Health and Safety Executive’s publication  L108 Controlling noise at work.

I’ve recently uploaded a presentation to Slideshare which describes these methods and which illustrates them with some worked examples. It also briefly covers some of the other factors that need to be considered when selecting ear defenders.