The term acoustic shock is used to describe the symptoms a person may experience after hearing an unexpected, loud sound via a telephone. The loud sound, called an Acoustic Incident, can be caused by feedback oscillation, fax tones, or signalling tones. Telemarketers and call centre employees are thought to be most at risk.
During the exposure, most people will experience discomfort and pain. After the exposure, some people might report shock, nausea and anxiety or depression. Headache, fatigue, hypersensitivity to loud noise and tinnitus may continue for days, weeks or indefinitely. It has not been established how such unrelated symptoms might be caused by an acoustic exposure, or whether such symptoms are even a direct result of exposure.
There are many methods of attempting to reduce the risk of AS. Several devices attempt to remove potentially harmful sound signals by digital signal processing. None has yet been shown to be fully effective. Devices which solely limit noise levels to about 85dB have been shown in field trials to be ineffective (data from these trials has not been released into the public domain). Limiting background noise and office stress may also reduce the chance of an Acoustic Shock. Proper use of the headset and preventing mobile phones from being used in call centers reduces the chance of feedback. 
84 BT employees suffering from depression, headaches and other health problems, are demanding compensation for injury sustained from acoustic shock at work. BT has already paid £90,000 to one worker that suffered from tinnitus.
- ITU-T Recommendation p.10 (12/98): Vocabulary of terms on telephone transmission quality and telephone sets, Geneva, 1998.
- Acoustic Shock
- Appendix 7 – Acoustic shock
- "HSE - Noise: Acoustic shock". Retrieved 2012-07-10.
- BBC News | UK | Legal action over 'acoustic shock'