||Effects of compression in hearing aids on the envelope of the speech signal, Signal based measures of the side-effects of the compression and their relation to speech intelligibility
||Dau, Torsten (Acoustic Technology, Department of Electrical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Johannesson, René Burmand
||Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark
||A research project is presented, where the influence of the compression in hearing aids on the
envelope of the speech signal was investigated. Signal-based measures were implemented to
quantify three types of the side-effects of the compression: an across source modulation
correlation (ASMC), an envelope alternation by the envelope difference index (EDI) and a
change of the target-to-background ratio (TBR) at the output of the compression system. The
measures were implemented for the setup, where a hearing-aid device was used. The
dependence on the compression ratio, the release time and the compression strategy was studied,
whereby compression was applied either before or after mixing foreground and background
stimuli. The target speech was presented in a background speech or a speech-like noise.
ASMC occurs when two fluctuating signals are subjected to compression and obtain a common
modulation due to the compressor action. The ASMC measure was calculated for the
unprocessed foreground and background as well as when the signals were compressed
independently before mixing (INDEP) or compressed together after mixing (COM). As
expected from data reported by Stone and Moore (2007, 2008), a negative correlation was
observed only in the COM condition for the compression system with a short release time.
The difference in temporal contrast of the speech envelope before and after the compression was
quantified with the EDI. It was observed that the EDI increased with increasing compression
ratio and decreased with increasing release time. Additionally, it was observed that the values
are generally higher for the background of the one-talker speech than for the background of 2-
talker modulated speech-shaped noise.
The TBR was measured at the output of the compression system. It occurred that, despite of
identical input TBR, the output TBR was of different value for various compression settings.
The speech intelligibility test for situations, where mainly the envelope cues were preserved,
was performed for various parameters of the compression and two types of fluctuating
background. In the perceptual test, the intelligibility of speech was measured in the presence of
background speech and speech-like noise, similarly as for the measurements of the signal-based
measures. The signals were processed by an 8-channel noise vocoder, such as mainly the
envelope cues were remaining. The experiment was performed with normal hearing subjects.
For both types of background there was a significantly worse performance for the fast-acting
compression system. For the speech-like noise background, there was also a significant interaction between the speed and the compression strategy (compression before versus after
mixing foreground and background) and the pair-wise comparison of Tukey HSD test revealed
that when the mixture of the foreground and background were processed with fast-acting
compression system, the recognition was significantly worse.
The speech intelligibility scores were compared with the pattern of signal-based measures. The
observed effects could be attributed to the side-effects of the compression. These observations
suggest that the signal-based measures could be relevant for predicting the intelligibility pattern
for target speech presented in the presence of speech or speech-like noise that was processed
with a compression hearing aid.
Creation date: 2009-06-09
Update date: 2009-06-09