Components: Sound
Impact on Speech Intelligibility
- Distance
- Signal-to-Noise Ratio
- Reverberation
Notes:
Individuals who have a hearing loss have much more difficulty with intelligibility than those with no loss. Once the person with a hearing loss perceives the sound, he must still interpret it to understand it. This is why hearing aids do not provide ‘20/20 hearing’. The studies described below are from Blair (1990).
Distance: The further away from the sound one is, the softer the amount of pressure that is exerted on the eardrum, and thus the less intense (loud) the sound. Consider that the average speech is about 65 dB. At about 4 feet (for example, the first row in a classroom), the intensity drops to about 53 dB, and at 16 feet (about the 4th row) the intensity is only 41 dB. With or without a hearing aid, then, it would be beneficial to the person with a hearing loss to sit close to the sound source. In addition, it will also reduce eyestrain in speech reading.
Signal-to-Noise Ratio: This refers to how much louder speech is than the background noise. For the individual with normal hearing, speech discrimination is close to 100% as long as the speech is at least 6 dB louder than the background noise (this is an SNR of +6). Although we do not know why, SNRs must be +15 to +25 to achieve the same level of speech discrimination for individuals with a hearing loss.
Remember we said that the average speech intensity is 65 dB? Well, the average room noise level is 56 dB (SNR +9). The combined impact of distance and background noise can result in totally inaccessible listening environments for the individual with a hearing loss, even with a hearing aid. Why? Hearing aids do not improve the Signal-to-Noise Ratio because they amplify all sounds! Only ALDs will improve the SNR.
Reverberation is measured in time. It is the amount of time it takes for the intensity of a sound to drop 60 dB once it has stopped being produced. The longer the time, the more of an echo effect that is produced, and individual sounds will become slurred together. Although the definition is a little unwieldy, most of us have experienced this reverberation or echo in school gyms and sports stadiums.
For the person with normal hearing, a reverberation of up to 1 second will have little effect on their ability to discriminate speech (the reverberation in school gyms is about 1.5 seconds). Individuals who have a hearing loss, however, typically experience difficulty in speech discrimination with reverberations of only one-half a second (i.e., the individual with normal hearing will be able to repeat back words at nearly 100 percent accuracy until the reverberation time is above one second, but the individual with a hearing loss will start having trouble repeating words when the reverberation time is only about 0.5 seconds).