** In July 2011 this research project migrated to become HEARing CRC commercialisation project C1.4.3 **

Personal listening devices used to listen to speech (such as headsets) may sometimes produce sounds which the listener finds uncomfortably loud or harmful. Unlike music recordings and radio programs which are carefully controlled, speech picked up in noisy environments and/or sent through less controlled technology may contain distressing or harmful sounds resulting in acoustic shock injury. 

Telephone/communications headset wearers and transcriptionists wearing headphones have reported these problems. Hearing aid and hearing protection users experience similar problems.

Simply reducing the volume unfortunately reduces the level of the desired speech signal and therefore this is not a practical solution. Limiting the maximum level is preferable however this introduces a new set of problems. Limiting to a fixed maximum level is a compromise between comfort/safety and speech clarity - if the maximum is high then noise can be too loud and if the maximum is low speech clarity will be reduced. Furthermore, to set a fixed limit one must know how sensitive the personal listening device is, otherwise the limit may be set too low or too high, unfortunately this is often unknown.

A new approach is simply to prevent sounds being louder than speech. This novel approach developed by Michael Fisher at NAL is called Speech Referenced Limiting (SRL).