The Effects of Signal to Noise Ratio, T60 , Wide-Dynamic Range Compression Speed, and Digital Noise Reduction in a Virtual Restaurant Setting

Abstract

Objectives: Hearing aid processing in realistic listening environments is difficult to study effectively. Often the environment is unpredictable or unknown, such as in wearable aid trials with subjective report by the wearer. Some laboratory experiments create listening environments to exert tight experimental control, but those environments are often limited by physical space, a small number of sound sources, or room absorptive properties. Simulation techniques bridge this gap by providing greater experimental control over listening environments, effectively bringing aspects of the real-world into the laboratory. This project used simulation to study the effects of wide-dynamic range compression (WDRC) and digital noise reduction (DNR) on speech intelligibility in a reverberant environment with six spatialized competing talkers. The primary objective of this study was to determine the efficacy of WDRC and DNR in a complex listening environment using virtual auditory space techniques.

Design: Participants of greatest interest were listeners with hearing impairment. A group of listeners with clinically normal hearing was included to assess the effects of the simulation absent the complex effects of hearing loss. Virtual auditory space techniques were used to simulate a small restaurant listening environment with two different reverberation times (0.8 and 1.8 sec) in a range of signal to noise ratios (SNRs) (-8.5 to 11.5 dB SNR). Six spatialized competing talkers were included to further enhance realism. A hearing aid simulation was used to examine the degree to which speech intelligibility was affected by slow and fast WDRC in conjunction with the presence or absence of DNR. The WDRC and DNR settings were chosen to be reasonable estimates of hearing aids currently available to consumers.

Results: A WDRC × DNR × Hearing Status interaction was observed, such that DNR was beneficial for speech intelligibility when combined with fast WDRC speeds, but DNR was detrimental to speech intelligibility when WDRC speeds were slow. The pattern of the WDRC × DNR interaction was observed for both listener groups. Significant main effects of reverberation time and SNR were observed, indicating better performance with lower reverberation times and more positive SNR.

Conclusions: DNR reduced low-amplitude noise before WDRC-amplified the low-intensity portions of the signal, negating one potential downside of fast WDRC and leading to an improvement in speech intelligibility in this simulation. These data suggest that, in some real-world environments that include both reverberation and noise, older listeners with hearing impairment may find speech to be more intelligible if DNR is activated when the hearing aid has fast compression time constants. Additional research is needed to determine the appropriate DNR strength and to confirm results in wearable hearing aids and a wider range of listening environments.

Figure 1.

Air-conduction thresholds for listeners with impaired hearing. Red and open circles: right ear thresholds; Blue and “x’s”: left ear thresholds. Bold lines are average thresholds. Individual audiograms are plotted with faded, dotted lines. The N3 standard audiogram is plotted in black for reference.

Figure 2.

Diagram of virtual test environment. Yellow: listener position. Red: target position. Green: masker positions. The listener is located at the point (1.98, 1.98). The target is located at the point (1.98, 3.05). In clockwise order, the maskers are located at the points (3.05, 4.88), (4.27, 4.88), (4.88, 3.97), (4.88, 2.74), (4.27, 0.91), and (3.05, 0.91).

Figure 3.

Diagram of DNR processing. The blue line depicts the peak track. The grey line depicts the valley track. The green line depicts the gain applied. The minimum gain is plotted as −10 dB because that was the maximum allowed attenuation. Only the first two seconds of the sentence are pictured here. Tracks plotted in dB re: maximum peak value. Spatialized sentence is plotted in grey in the background of the figure.

Figure 4.

Scatterplots with boxplots overlaid to show the distribution of SNRs within each processing condition and for each of the listening groups. The center of the boxplot represents the median score. The ends of the boxplot mark the 25^th and 75^th percentiles. The whiskers each span 1.5 times the interquartile range (distance between the 25^th and 75^th percentiles). Points were offset and jittered along the y-axis only for ease of visualization. Lighter/green points and boxplots represent data from listeners with normal hearing, Darker/purple points and boxplots represent data from listeners with hearing impairment. The columns separate data by DNR condition (OFF on the left, ON on the right) and the rows separate data by WDRC condition (Fast on the top, Slow on the bottom).

Figure 5.

The same legend as Figure 6, but the boxplots show the distribution of proportion corrects for the listening groups. Again, data were jittered along the y-axis for ease of visualization.

Figure 6.

Conditional predictions from the generalized linear model. In both panels, DNR condition is plotted along the x-axis and predicted percent correct is plotted along the y-axis. The left panel contains predictions for the listeners with hearing impairment. The right panel contains predictions for listeners with normal hearing. Red and blue filled circles depict mean prediction, with error bars representing 95% confidence intervals for Fast and Slow WDRC, respectively. Lighter points show the raw data, jittered along the y-axis for ease of visualization.

Figure 7.

Model predictions for the interaction between listener group and SNR condition. SNR is plotted along the x-axis and predicted percent correct is plotted along the y-axis. The red (listeners with normal hearing) and blue (listeners with hearing impairment) line and shaded area represent model predictions and 95% prediction intervals.