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Acoustic Shock Injury: Real or Imaginary?

Acoustic Shock Injury: Real or Imaginary?
Janice C. Milhinch
June 17, 2002

Isolated reports of injury following exposure to loud sounds from headsets have met with skepticism. This study involved examination of the case records of 103 call centre operators who experienced acoustic incidents, sometimes described as 'shrieks' or 'howls,' from headsets. Acoustic incidents are loud, unexpected, randomly occurring, high-pitched and startling stimuli, typically tones of 2.3 - 3.4 kHz, at intensities varying from 82 to 120 dB S.P.L. at the tympanic membrane, with rise times of 0 - 20 milliseconds and varying durations. Operators described being shocked by the incidents and experienced a range of physiological symptoms including pain (81%), tinnitus (50%), vestibular disturbance (48%) and hyperacusis (38%). Headaches and sensations of numbness, burning, tingling, blocking, pressure or fullness, echo or hollow feelings in the ear were also frequently reported.

Symptoms varied in their severity, number and recovery rate. The mean number of symptoms per incident was 2.7 but some operators reported as many as seven or eight different symptoms following acoustic incidents. While hearing loss was not a frequently reported symptom (18%), audiograms showed atypical configurations, ranging from slight dips at 4-6kHz to moderate flat sensorineural losses. A number of severe cases involving vertigo were recorded and subsequently diagnosed as perilymph fistulae. There was a range of psychological sequelae, primarily related to extreme anxiety when exposed to moderately loud sounds, potentially precluding a return to headset use.

This study documents consistent symptoms, demonstrating an atypical injury pattern which is not adequately explained by existing theories of acoustic trauma injury and which carries implications for clinical assessment and rehabilitation. It is proposed that the stressful nature of the acoustic incidents is responsible for acoustic shock injury and an explanation of the damage mechanism is offered.


In the past 20 years, there has been a rapid expansion in the number of individuals using telephones, including those who work in call center environments using headsets throughout the working day. Correspondingly, there has been an increase in the number of reports of injury associated with exposure to excessive noise emitted from both handsets and headsets. The excessive noise exposure reported in these studies comprised either continuous high levels of sound from the incoming signal, sufficient to contribute to a daily noise dose in excess of 85dB Leq8h, or comprised very loud sudden peaks (impulse sounds) reputed to be in excess of 120dB SPL .

However, there exist isolated anecdotal reports of injury which fail to conform to the usual understanding of acoustic trauma. Such reports have met with skepticism. They have been attributed to sounds which are variously described as "shrieks," "spikes," " howls," "screeches," "squawks" and "acoustic incidents." The resultant injury to the auditory system has been described as an "acoustic shock" and has been defined as "any temporary or permanent disturbance of the functioning of the ear, or of the nervous system, which may be caused to the user of a telephone earphone by a sudden sharp rise in the acoustic pressure produced by it." .

Hinke& Brask (1999) reported a Danish study in which call centre operators experienced symptoms of injury after exposure to a 'kraftig impulsstoj" or impulse sound, sudden and unexpected. They noted that the sound bursts ranged from 56 dB SPL to 108 dB SPL in the frequency range 100 - 3.8kHz. They observed that, although such exposures had been known since 1993, some of the 14 call centres in Denmark had recorded no incidents, despite the fact that all centres used the same communications system. In their study of one specific call centre they described a group of 90 call center operators, 22% of whom had experienced an "acoustic shock," and for whom tinnitus was the most frequent symptom reported. They examined psychosocial correlates of those who had experienced acoustic shock, and found that the symptoms of "shock" were correlated with stress, smoking, and neck or shoulder pain. Those who had experienced more than one "shock" appeared to develop more severe symptoms.

Kristensen (2000) Further reports of acoustic shock are noted in the United Kingdom by the Royal National Institute for the Deaf in conjunction with the Trades Union Congress in their report entitled "Indecent Exposure" . They cite anecdotal reports and evidence of litigation relating to injury following acoustic incidents, with symptoms including pain, tinnitus, and some instances of hearing impairment at 4 and 6kHz. In Australia, recent anecdotal cases of acoustic shock have been reported, such as the case of a call centre operator who suffered balance disturbances and hearing disability following exposure to a "spiking" .


This study was designed to examine evidence of significant injury as a consequence of exposure to acoustic incidents. Since the incidence of injury was presumed to be low, in order to obtain sufficient data to form some preliminary conclusions, retrospective examination of clinical records was used.


Records from a telecommunications organization were investigated for significant acoustic incidents reported during the years 1995 -1999. A 'significant' acoustic incident was defined as one which led to loss of time from work. Audiologists in four states provided comprehensive clinical histories and audiological test results of operators who had sustained significant injury. Data were recorded on the following variables: age, gender, level of injury, number of shocks, reported symptoms, audiological assessment, medical diagnosis, and clinical management recommendations.


A total of 103 cases were collated, comprising 91 females (89%) and 12 males (11%). This compares with the average gender employment figures in call centres of 74% female and 36% male (Australian Services Union, 2001). Ages ranged from 21 to 63 years with a mean age of 34.4 years. Twenty individuals (19%) experienced repeated incidents, which included a range of 1-9 incidents, with a mean of 1.64 incidents per person.

The clinical records were examined for evidence of symptoms which were reported immediately subsequent to an acoustic incident. Symptoms varied in their severity, number and recovery rate and included physiological, psychophysiological and psychological symptoms. The mean number of symptoms per shock was 2.7 but some individuals reported as many as seven or eight different symptoms. The severity of symptoms also varied between individuals and within individuals.

Of the total number of symptoms reported, pain was the most common (81%), followed by tinnitus (50%), loss of balance (48%) and hypersensitivity to sound (38%) as shown in Table 1. Hearing loss was recorded in 19 (18%) of cases but there were no statistically significant differences between the exposed and non-exposed ears except at 1.5kHz. Statistical analysis did not permit any conclusions. In those cases in which hearing impairment was recorded, hearing levels ranged from 20 dB Hearing Threshold Level (HTL) average to 55 dB HTL average. In some cases recovery was noted over time. There were 3 cases of provisionally diagnosed perilymph fistulae.

As Table 2 indicates, headaches and physical sensations of numbness, a burning feeling in the ear or face, tingling, a warm or hot ear or face, a feeling of pressure or fullness in the ear, and an echo or hollow feeling in the ear were frequently reported. In cases of moderate to severe acoustic shocks, operators typically described the acoustic incident as "like being hit over the head" or "stabbed in the ear." Some operators became tearful, began to shake, and several hours after the trauma, developed fatigue and slept for an unusually lengthy period.

In a significant number of cases, pain and unusual sensations consistently recurred when the individual experienced any moderately loud sound, similar in pitch to the acoustic incident itself. Psychological symptoms included anxiety and depression, anger, feelings of vulnerability, and persistent hypervigilance. The data also showed that individuals who had experienced more than one acoustic shock were more likely to have most severe symptoms and a longer recovery time.

Discussion and conclusions:

The consistency of reported symptoms over four different states in this country and from a wide range of call centres demonstrates that there is evidence of injury following acoustic incidents, termed 'acoustic shock injury.' The data indicate that this injury, while involving some psychological aspects, is most likely to be a neurophysiological phenomenon directly related to exposure to unexpected high-pitched sounds. Typically there is a cluster of symptoms and in severe cases, the response to the traumatic auditory stimulus caused a reaction involving both middle and inner ears. In extreme cases, severe hyperacusis or perilymph fistulae were recorded, leading to a permanent, distressing disability.

Where sensorineural hearing loss occurred, either temporarily or permanently, it suggests damage to the outer hair cells of the cochlea, but there is no characteristic pattern. Results range from very slight dips at 4 and 6 kHz to a moderate degree of hearing impairment with flat pattern, similar to that noted by Beastall (1992) and in those cases of cordless telephone handset injury reported by other researchers . However, the findings of this study did not take into account pre-acoustic shock hearing levels and did not exclude other possible factors which may have contributed to any change in hearing acuity.

These results suggest that a new understanding of the mechanism of injury following severe acoustic incidents is required. Thus the term 'acoustic shock' has been used to differentiate the injury from the more commonly understood 'acoustic trauma' which describes damage to the hair cells in the inner ear, as a consequence of exposure to excessive noise over a prolonged period of time, or to an extremely high intensity stimulus . The evidence obtained in this study shows that acoustic shock injury presents as an atypical cluster of symptoms. Its characteristics include: that it occurs as a response to an acute episode of exposure, that it involves the whole aural area, not just the auditory mechanism, that it is essentially traumatic and stressful, as in a shock, and that it may occur in response to a relatively low level of stimulus.

Patuzzi, Milhinch &Doyle (2000) attributed the primary cause of injury to an excessive startle reflex and suggested that many of the persistent symptoms reported in this study of acoustic shock are explicable in terms of a condition initially described as the "tonic tensor tympani phenomenon." . This is a condition in which the tensor tympani muscle is spontaneously active, continually and rhythmically contracting and relaxing. Klockoff and Westerberg, on the basis of clinical observations and experiments, stated that the symptoms of this chronic activity include a sense of fullness in the ear, otalgia (pain), tinnitus and/or other transient acoustic sensations, abnormalities of sound perception but normal audiometry, dizziness of a non-specific type but usually normal vestibular tests, and headache. They noted "elevated psychic tension seems to be the essential etiologic factor in almost all cases." (cited in , p.90)

In this study, the more severe and persistent of injury symptoms were experienced in those with repeated acoustic incidents. This was supported by Hinke and Brask (op.cit) in their Danish study and suggests that a persistence of symptoms may in part result from a classical conditioned response in which further exposure to a high pitched tone similar to that experienced during the injury may elicit the same stress response. Such symptoms may persist for a period ranging from months to years. It is also possible that in individuals with pre-existing high levels of stress, this psychophysical response is heightened. In view of the evidence that call centre operators report higher levels of stress than do coal miners (Call Centre Research/Deloitte and Touche Consulting Group, op. cit.) it is possible that certain individuals in particular call centers may manifest a severe reaction to a threatening stimulus, with a consequent conditioning to elicit the same painful response to ever softer levels.

In conclusion, these findings suggest that acoustic shock injury requires a new approach to clinical assessment and rehabilitation. Because it has not been recognised as a clinical entity, clinicians who work in audiology, general medicine and otology have often failed to identify the injury and have at times proposed a course of clinical management which has not assisted, but rather exacerbated, the recovery of the affected individual.

These results suggest that appropriate audiological assessment and rehabilitation may prove to be key determinants of a successful early intervention approach. This implies the need for audiological assessment (excluding loud stimuli) as soon as is practicable after the acoustic incident, acknowledgement of the potentially traumatic nature of the experience and of the individual's symptoms, preliminary counseling, monitoring of progress to document any changes in hearing levels, hyperacusis and symptoms, and early identification of complex cases, including those with psychological issues, to enable the implementation of long-term rehabilitation as may be necessary.


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I wish to thank the Health and Safety Directorate, Telstra, Australia for access to their files, Dr Janet Doyle, consultant audiologist, Melbourne Australia, Dr Kaj Hinke, occupational physician, Denmark, Dr Robert Patuzzi, senior lecturer and physiologist, University of Western Australia and Clinical audiologists Dr Ross Dineen, Melbourne, Australia Ms Pam Gabriels, Perth, Australia, Ms Vivien Sobon, Perth, Australia, Mr Iain Summerlin, Perth, Australia and Mr Paul Walsh, Adelaide, Australia

Editor's Note: Further details of the background, equipment issues, audiological assessment and rehabilitation of acoustic shock injury is available in an extended 58-page report which may be obtained by contacting Dr Janice Milhinch at

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