speech sounds understandable to individuals with high-frequency hearing loss when the loss becomes severe (Ching, Dillon, and Byrne, 1998; Hogan and Turner, 1998). In individuals who have profound sensorineural hearing loss across the frequency range, hearing aids may not be as effective in improving hearing as a cochlear implant. Infections (viral or bacterial), disorders such as Meniere’s disease and autoimmune inner ear disease, hereditary disorders, trauma, and ototoxic drugs are other causes of sensorineural hearing loss.
Acoustic trauma can be a significant problem in the workplace. If the level of sound is intense, especially if the sound lasts for a long time, listeners exposed to such intense sounds may experience either a temporary or a permanent threshold shift—that is, their threshold for detecting sound is either temporarily or permanently elevated above that measured in quiet and before the exposure. A temporary threshold shift (TTS) can recover to normal detection threshold after a few minutes to a few days, depending on the parameters of the exposing sound and their relationship to those of the sound to be detected. Permanent threshold shifts (PTS) never recover and therefore indicate a permanent hearing loss that can range from mild to severe.
There is a trade-off between sound level and duration in terms of producing TTS and PTS. The greater the level or the longer the duration of the exposing sound, the greater the threshold shift and the longer it takes to recover from TTS. Most TTS occurs at frequencies the same or slightly higher than the frequency of the exposing sound. The Occupational Safety and Health Administration (OSHA) and the National Institute of Occupational Safety and Health provide regulations and guidance (e.g., Occupational Safety and Health Administration, 2002) for occupational noise exposure to mitigate its effects in the workplace.
It is estimated that 1 person in 1,000 has a severe to profound hearing loss. The number with bilateral (both ears) profound deafness is lower. Loss of hearing has a significant impact on the development of speech and spoken language skills, which are dependent on the age of onset of deafness. Children born with bilateral profound hearing loss or who acquire profound loss before the acquisition of speech and spoken language (approximately age 2 years) are identified as prelingually deafened. Children and adults deafened at any age after developing speech and spoken language are referred to as postlingually deafened.