| dc.description.abstract | This thesis presents the research, development and evaluation associated with a
novel method of improving listening comfort and acoustic safety for people listening
primarily to speech produced by electronic devices such as headsets, telephones,
headphones, hearing aids, cochlear implants, level-dependent hearing protectors and
public address systems.
The method involves a novel technique of amplitude limiting audio signals that
convey speech. Time-varying, frequency-specific levels of speech generate a set of
time-varying speech reference levels. The method limits the level of the audio to these
speech reference levels and hence is called speech referenced limiting (SRL). In
principle, SRL provides the greatest limiting of noise for the least limiting of speech,
making it arguably the optimal method for limiting noise in speech systems.
Two schemes based on the method were developed, the SRL MKI and the SRL MKII
schemes. The latter scheme was far superior, with the ability to estimate the speech
loudness and power from frequency regions where speech was dominant, while
ignoring frequency regions where it was not. It contained a novel method of
determining the amount of additional control needed to correct for the loudness
summation of noises with a bandwidth exceeding that of speech, as well as providing
fast speech referenced control over the power of abrupt sounds while introducing only
a very short delay.
Subjective evaluation of the SRL MKI and SRL MKII schemes conducted in the
laboratory confirmed large reductions in noise loudness and preservation of speech
quality. It was hypothesised that the SRL MKII scheme would provide the greatest
reduction in the excess loudness of an audio signal compared with the loudness of
the preceding speech conveyed by the audio signal for the least reduction in the
speech loudness and quality. Using stimuli typical of those experienced in the three
main intended applications (hearing aids, level-dependent hearing protectors and
telephone headsets), this hypothesis held true and noise control was shown to be far
superior to a conventional fixed-reference limiter while speech loudness and quality
were maintained. | en_US |
