Meaningful improvements in speech intelligibility
Increases in speech intelligibility are conventionally reported as either (a) the change in relative levels of the target speech and noise(s) – the signal-to-noise ratio (SNR) – for a given percentage of utterances (e.g., 79%) or (b) the change in the percentage of utterances heard for a given SNR. What is considered an important change has been considered solely on account of the statistical distribution of scores, not what is a noticeable or convincing improvement in (a) SNR or (b) intelligibility. Through a series of experiments, we have determined what is the just noticeable difference (JND) in SNR and intelligibility, as well as inferred for both measures what is the just meaningful difference (JMD): the scale of improvement necessary to prompt an individual to seek intervention. For JNDs, participants of varying hearing ability and age listened to paired examples of sentences, words or digit triplets presented in various noises, and judged which of the two examples were clearer. The SNR JND ranged from 2.4 dB for digit triplets in noise to 4.4 dB for sentences in two-talker babble. The corresponding intelligibility JNDs, estimated from psychometric functions, ranged from 14% for words in same-spectrum noise to 33% for sentences in babble. JNDs were not correlated with hearing ability. For JMDs, participants also listened to paired examples of speech (sentences only) in same-spectrum noise: one at a reference SNR and the other at a variably higher SNR. In different experiments, different hearing-impaired adults performed various tasks: (a) better/worse rating, (b) better/worse rating of a corresponding level change, (c) ease-of-listening rating, (d) conversation tolerance, (e) device-swap, or (f) clinical importance. The SNR JMD was determined to be approximately 6 dB to reliably motivate participants to seek intervention. An intelligibility JMD was also estimated, albeit from mean psychometric functions. The SNR JMD did not correlate with hearing loss, age nor SNR JND, and only correlated weakly with hearing-aid usage and general quality-of-life. The SNR and Intelligibility JND provide perceptual benchmarks for performance beyond statistical relevance; the SNR JMD further adds clinical relevance to speech-intelligibility improvements.
[Supported by intramural funding from the Medical Research Council (grant number U135097131) and the Chief Scientist Office of the Scottish Government]