Posted: Oct 6, 2005  |  By: Branko Miletic, Journalist

The sound of acoustic shock injuries

Depression; mood swings; the inability to tolerate loud sounds; an inability to attend loud festivals, rock concerts or car racing venues; the cessation of noisy sports; an inability to go to hotels; continuous tinnitus, jaw and neck pain; nausea; severe pain when exposed to loud sounds; loss of enjoyment; a lower quality of life; and a general fear of going outside. In some cases, these symptoms are reported to continue for many days or weeks after an incident. These symptoms read like the after-effects of some type of torture or severe industrial accident. In reality, these symptoms are the end result of acoustic shock injury or ASI.

Recent studies show that hearing loss affects one in ten people in Australia. Of that number, one in a thousand is the result of congenital or childhood ear conditions and more than 60% result from ageing changes in the ear - from the age of 60 years on. However, a significant proportion of permanent hearing loss is avoidable, being the result of permanent damage to the inner part of the ear - know as the cochlea - after repeated exposure to excessive noise in the workplace.

The mechanism causing the adverse symptoms is not known with certainty. ASI occurs when a worker is exposed to an acoustic shriek. An acoustic shriek is an unexpected high-pitched tone (HPT) or other noise (loud or soft) that may occur in telephone networks. Examples of acoustic shrieks are incorrectly dialled fax machines, maliciously generated noises such as screaming or blowing a whistle down the line, feedback or oscillations between a cordless phone and its base station, faults within the telephone line, or even callers dropping their handset onto a desktop.

Affected workers may then feel hypersensitive to loud sounds that previously would have caused no problems. The fear that a shriek might recur lowers their threshold even further, putting them at greater risk of injury if it happens again. This is even likely to affect co-workers, who may be apprehensive about the likelihood of an incident once they are aware that it has occurred within their telephone network.

Dr Harvey Dillon, Director of Research at National Acoustic Laboratories (NAL), says: "It seems highly likely ... that the sound exposure [to an acoustic shriek] elicits an acoustic startle reflex.

The same primitive startle reflex can also be elicited by an unexpected touch or puff of air to the eyes. When startle occurs, numerous muscles in the upper limbs, shoulders, neck, eye and ear (the stapedius muscle and the tensor tympani muscle) are activated."

According to Dr Dillon, "If the noise exposure is loud, or if the person is in an aroused state (eg, anxious, fearful) prior to the startle, the magnitude of the muscular response is heightened. It seems likely that the ongoing symptoms are the after-effects on the muscles and ligaments caused by the muscles being tensed to an unusual degree."

According to audiologist and acoustic shock injury specialist, Dr Janice Milhinch, given our knowledge that noise is damaging and that hearing protection has been available for decades, the question that should be asked is: why does this problem persist? It's not for lack of government regulation. In Australia, each state has individual legislation covering acceptable limits for sound before a hearing protection program must be undertaken. Over the period since the first laws were enacted - in Victoria in 1978 - acceptable levels have been reduced to an exposure over a working day of eight hours of no more than 90 dBA. Employers are obliged to arrange noise measurements and to attempt to reduce noisy sources. On failing to achieve a reduction below 90 dBA, employers must provide education, hearing assessment and hearing protection on an ongoing basis to all exposed staff. While there is some variation between states, generally hearing tests are required with subsequent reporting to the relevant authority.

Dr Milhinch said the evidence so far is that, while claims for compensation have been reduced, this may be more a reflection of the changes to the claimable entitlements than a real reduction in the incidence of hearing loss. Hearing protection remains the most common form of noise control. If hearing protection is worn, it does not guarantee that hearing loss will not occur, since the 'prescription' of hearing protection devices done correctly (using the SLC80 figures) allows that 80% of workers should expect adequate protection but 20% of people who wear protection would still be at risk.

For example, during the large construction work while building the Crown Casino complex, Melbourne was home to more than one thousand workers. Many of them participated in research carried out to find out how they made decisions about their hearing health on site.

Predictably, the study found many sources of noise - angle grinders, riveters, pneumatic hammer drills and so on. The site required many different trades to operate in the same area at the same time - and even though one worker would not be making a noise, another next to him might be. To reduce the noise at the source is extremely difficult and costly, especially when it comes to using hand tools for interrupted periods in a confined area.

In the construction environment, hearing protection becomes the only viable solution. Yet workers rarely wore earmuffs or ear plugs.

The researchers asked workers during focus group discussions to describe how they saw the work environment, the risks and the solutions. The answer to all those questions was that hearing protection was essential and that those who did not wear it were slowly losing hearing and knew it. So why not use protection? The answer was surprising - workers had made a clear decision to not use protection. They were actually more concerned that they would not hear warning calls or signals which would prevent possible death because ear protection might restrict the level of the warning. The likelihood of hearing damage occurring was high, but the risk of death or physical injury was by far more important, despite the likelihood being very low.

The Crown Casino construction site studies showed the importance of the link between individual worksite characteristics and workers' perceptions of safety issues. And it isn't just the sound level measured which tells the story about risk.

Another example is that of the rapidly growing call centre industry. Many call centres operate at a stressful pace, with workers using headsets throughout the working day. Recent research has shown that injury may occur to the ear and associated mechanisms from sound which are not necessarily extremely loud but are loud enough to be perceived by the operator as a threat. Under certain circumstances, acoustic shock injury occurs when a sudden loud sound comes into the earpiece. Operators may react to the sound with an instinctive startle reflex response, including repercussions in the ear (involving muscles in the middle ear), head and neck including pain, undue sensitivity to sound, head noises and balance difficulties.

For businesses, the impact of ASI can be both time consuming as well as potentially expensive. In 2001 in the UK for example, 83 people sued British Telecom for exposure to acoustic shrieks resulting in ASI.

However, the real rate of acoustic shock in call centres is poorly documented and relative incidence is difficult to ascertain. In a study of 90 call centre operators in Denmark in 2000, 22% had experienced an acoustic incident (sudden loud sound) which could have led to injury. Another study in a call centre in the United Kingdom found just one case (0.6%) in 160 operators, whereas a third study found two cases (1.4%) in 140 operators, including one incident at a recorded level of 93 dBA. And the most recent UK study of 2004 cited 20% who experienced incidents.

So why did some operators in some call centres react so adversely to the loud sounds? The answer lies in the workplace and the individual. Stressful, poorly managed workplaces create stress and anxiety in workers. This in turn predisposes the person to reacting more severely to sudden sounds - hence the increased risk of injury.

As a practicing audiologist, Dr Milhinch says she once treated a worker who was injured to the extent that she was extremely sensitive to any sound which was only moderately loud, and anxiety resulted in pain whenever a sound similar to that which caused the injury occurred. This hypersensitivity affected her ability to go out into shopping centres, social groups and everyday traffic. She had chronic tinnitus - noises in the ears - and loss of balance. She could no longer use a headset and was permanently removed from her role as headset operator in a call centre. The cost to her personally and to her employer, not to mention the ensuing compensation claim, highlights the importance of firstly recognising risk factors and secondly preventing or minimising exposure to acoustic incidents.

Dr Milhinch says that given the role of anxiety and stress in contributing to injury, the more information provided to staff about ASI, the better. The likelihood of damage is low, so the fear of injury can be minimised by educating staff that low level acoustic incidents are quite normal. It also helps to inform staff that the workplace has risk prevention equipment and management protocols in place, and to conduct regular ear safety training. Minimising anxiety is part of the prevention program.

She notes that it is valuable to encourage staff to minimise background chatter. This will improve the overall noise levels within the call centre and reduce noise which can interfere with good listening. In itself, this is good work practice and acts to minimise stress. ASI as a health care issue in the workplace is legitimate, identifiable and manageable. Management can reduce the incidence and damage it causes by educating and training staff, implementing procedures to reduce ambient noise in the environment and installing protective equipment.

According to a number of companies that supply equipment to prevent ASI, such as GN Netcom and Plantronics, the answer to the problem lies in three areas: management awareness of staff stress issues; using modern appropriate headset technology to reduce the likelihood of any sudden loud sounds; and educating workers to understand the interaction between exposure to acoustic incidents and their personal reactions.

New technology now offers improved signal-to-noise ratios with digital enhancement and rapid response to limiting sudden loud sounds at the ear. The concept that 'prevention is better than cure' means that businesses are well advised to purchase equipment which reflects these new technological innovations.

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