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Harvey Dillon

Occasionally, intense, unwanted signals accidentally occur within the telephone network. These signals are variously called acoustic shocks, audio shocks, acoustic shrieks, or high-pitched tones. The exact source of an individual acoustic shock is usually unknown, but various sources are possible, such as alarm signals, signalling tones, or feedback oscillation.

The last may be the most common and can easily occur, such as when a cordless telephone is brought too close to its base station while the base station has its hands-free loudspeaker operating. A high-pitched tone then results in just the same way that a public address system squeals when the amplification is increased too much.

Although these high-pitched tones can affect anyone, people using a regular hand-held telephone can quickly move the phone away from their ear, thus limiting their sound exposure to a fraction of a second.

Call-centre operators, however, usually use a head-set, which takes considerably longer to remove from the ear were an intense sound to occur.  They thus receive a greater noise exposure than for people using hand-held phones. The problem may be exacerbated if call centres are so noisy that the operators need to have the volume controls on their telephones turned up higher than would be necessary in a quieter place.

Unexpected high-level sounds have been reported to cause a variety of symptoms. Symptoms that have been reported during the exposure include discomfort and pain. Symptoms that have been reported in the few minutes after the exposure include shock and nausea. Symptoms that have been reported to continue for some time after the exposure include headaches, nausea, tenseness, and hypersensitivity (discomfort) to loud sounds that would previously have caused no problems. In some cases, these symptoms are reported to continue for many days or weeks after the incident, although more commonly the symptoms are short-lived. Some operators who experience an acoustic shock may feel apprehensive about using the phone or about loud sounds in general.

The mechanism causing the adverse symptoms is not known with certainty. It seems highly likely, however, that the sound exposure elicits an acoustic startle reflex. (The same 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. If the noise exposure is loud, or if the person is in an aroused state (e.g. 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 (Patuzzi, 2001).

It is well established that the emotional state of a person affects the startle response (Butler et al., 1990; Cook et al., 1991, Grillon et al., 1993). A fearful state, for instance, lowers the threshold of sound at which the startle reflex occurs, and increases the magnitude of the response when it does occur (Cook et al., 1992). It thus seems possible that call-centre operators who fear that they will be injured by an acoustic shock may truly be at greater risk of injury than those who are not apprehensive about the likelihood of an incident. If this is true, then incidents are more likely to occur in call centres in which incidents have previously occurred than in call centres in which there have been no previous incidents.  Operators in a call centre in which there had been a high incidence of acoustic shock were found to report an abnormally rapid growth of loudness as the physical levels of test tones were increased (unpublished NAL study).

The link between startle response and emotional state opens the possibility that the after-effects of an incident have a self-perpetuating element even without further headset use: Loud sounds normally elicit the stapedius muscle, either with or without a startle response. If such muscle action causes further pain or discomfort soon after an incident, the person affected may become more apprehensive about loud sounds in general, thus increasing the likelihood of further startle reactions.

Note that while NAL has extensively researched means to minimize the incidence of acoustic shock (see below), it has not directly investigated the underlying physiological and/or psychological damage mechanisms. The statements regarding damage mechanisms in this report are inferences based on reported symptoms and the known properties of the startle response.

It may be of interest to note that the writer (a research scientist at NAL) once experienced an acoustic shock while wearing headphones connected to some (faulty) laboratory equipment. In this case the symptoms during the exposure (of approximately one-second duration) were a high level of pain and felt similar to being hit about the head. Symptoms in the 30 or so minutes after the exposure included nausea and disorientation. The physical sensations during and after exposure were similar to that caused by an electric shock (which the writer has also experienced).

Solutions to the problem

1. Limiting. Simple headset amplifiers that limit the amount of sound produced by the headsets have not solved the problem. This is understandable; output levels cannot be limited to too low a level, or the clarity and quality of speech is adversely affected, particularly in noisy call centres. As a startle response can occur at levels as low as 60 dB SPL (Blumenthal et al, 1991), it is not possible to prevent startle by limiting alone while still preserving speech clarity. Limiting is, however, an important part of the solution so that all sounds, including high-pitched tones, are no louder than they need be. Limiting should be carried out in such a way that it introduces the minimum possible distortion of speech. Ideally, limiting should allow for the frequency response of the headset on the average listener. Such frequency-dependent limiting is necessary if the optimal amount of limiting is to be provided at each frequency.

2. Shriek rejection. A more sophisticated headset amplifier, known commercially as the Sound Shield has recently been developed. This device connects to the telephone line and has a socket into which a headset is plugged. As well as providing frequency-dependent limiting, the Sound Shield continuously monitors the incoming signal with the aim of differentiating between speech and high-pitched tones. When a high-pitched tone occurs, the device measures its frequency, and blocks the transmission of any sound at this frequency. Most of the speech signal passes through unaffected. As the tone is typically detected and blocked within a few hundredths of a second, the duration and loudness of the acoustic shriek is greatly diminished without speech being much affected. This development has been carried out at NAL in conjunction with Telstra and Polaris Pty Ltd. 

3. Call centre design. The design of the call centre will greatly affect the level of ambient noise experienced by the operators. Achieving low noise levels enables the average level and limiting level of the headset amplifier to be reduced, which minimises the level at which any unwanted sound occurs. 

4. Confidence building. To the extent that the problem has a psychological component, the solution also requires a psychological aspect. If apprehensive operators are more likely to be adversely affected by high-pitched tones, then demonstrating the protective qualities of a headset amplifier to operators may increase their confidence in their equipment and thus decrease the likelihood of incidents. (This assumes that the headset amplifiers are sufficiently sophisticated to provide a high level of protection.)

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