Todd W. Fortune

A new technique for quantifying temporal envelope contrasts.

A new technique has been developed for precisely quantifying the temporal contrasts that exist between two sound samples. This technique is based on envelope subtraction, and generates an Envelope Difference Index that may be used to help clarify whether alteration of the natural speech envelope via amplification improves or degrades speech intelligibility. The Envelope Difference Index method may also be used to assess hearing aid saturation, and may have other applications as well. The technique is applicable whenever a precise quantitation of the difference between two temporal envelopes is required, regardless of stimulus duration.

Duration, compression, and the aided loudness discomfort level.

Objective: The purpose of this investigation is to determine how the unaided and aided loudness discomfort level (LDL) varies with the duration of the input signal and whether the electroacoustic characteristics of compression circuits affect this relationship in a manner that may alter the listeners dynamic range for short duration sounds. Design: Ten hearing‐impaired and 20 normal‐hearing listeners participated. LDLs were determined for noise bursts of durations ranging in six steps from 32 to 1024 msec, using a two‐alternative, forced‐choice adaptive tracking procedure in which input level varied until LDL was achieved. LDLs were also obtained for continuous discourse, using a clinical procedure. Subjects were also given the opportunity to self adjust maximum output SPL to their LDL using either output limiting or volume controls in response to fixed 90 dB SPL noise bursts. Testing was conducted unaided and with hearing aids representing two analog (output compression limiting, wide dynamic range compression) and four digital compression circuits. Primary circuit contrasts included compression threshold, compression ratio, attack time and the presence or absence of unity gain at high levels. Results: For the unaided condition, both normal‐hearing and hearing‐impaired subjects showed increasing LDLs with decreasing signal duration. Under aided conditions, circuits with compression thresholds of 45 to 50 dB SPL and compression ratios of 2:1 produced LDL functions that were similar in slope to the impaired listeners unaided functions. Slopes were steeper when the attack time was slow (128 msec) than when it was fast (2 msec). Circuits with compression ratios of 8:1 produced flat LDL duration functions (i.e., a loss of duration‐dependent effects). Similar duration‐dependent LDL effects were also observed when subjects adjusted their own hearing aid output characteristics in response to 90 dB noise bursts. Conclusion: For the unaided condition, results suggest that normal‐hearing and hearing‐impaired listeners can tolerate short duration sounds at higher levels than long duration sounds, a finding that has implications for hearing aid design. Circuits that preserve the relationship between duration and LDL should allow brief phonemes to be presented at higher levels without discomfort than circuits that do not, possibly resulting in greater audibility or speech recognition. Current results suggest that circuits with low compression thresholds, low compression ratios, and slow attack times might accomplish this objective better than circuits with high compression thresholds, high compression ratios and fast attack times.

Aided growth of masking for speech and nonspeech signals.

OBJECTIVE Growth of masking (GOM) functions for speech and nonspeech signals were obtained in normal and impaired ears, with an emphasis on aided listening conditions. The purpose of the investigation was to determine whether electroacoustic contrasts between hearing aid circuits would manifest themselves in aided GOM functions and to determine whether amplification alters the slopes of GOM functions in impaired ears. DESIGN Five normal-hearing and 10 hearing-impaired listeners with sloping high-frequency losses participated. GOM functions were obtained in the sound field using a two-interval forced choice forward masking paradigm. Four sets of test signals consisted of narrowband noise maskers and sinusoidal probes. Two additional stimulus sets consisted of phonemes used as masker and probe. Both on-frequency and off-frequency masking conditions were examined. Test circuits included wide dynamic range compression (low compression threshold and low compression ratio) and output compression limiting (high compression threshold and high compression ratio), each adjusted to the DSL[i/o] prescriptive target. GOM slopes were calculated for normal-hearing and hearing-impaired subjects unaided, and for impaired subjects aided, in an effort to determined whether amplification compensates for abnormal growth of masking and whether electroacoustic contrasts between circuits differentially affect level-dependent masker effectiveness. RESULTS Results indicated large differences between normal and impaired functions unaided. Under aided conditions, narrowband noise masked thresholds were reduced re: unaided at lower masker levels but not at higher masker levels. Masked thresholds for phoneme maskers were reduced at all masker levels. In general, aided GOM slopes changed in the direction of normal but did not match normal slopes. Aided results were highly dependent on stimulus test conditions and were consistent with the compression thresholds and release times of the aids under test. Circuit contrasts were more apparent when phonemes were used as masker and probe than when conventional signals were used. CONCLUSION Results suggest that GOM functions can be altered by amplification in ways that are consistent with the electroacoustic characteristics of the circuits being used. Although these data address only a small number of conditions, they suggest a possible application for aided growth of masking in the evaluation of hearing aid performance.

Channel interactions measured by forward‐masked “place” tuning curves with multichannel electrical stimulation

Simultaneous stimulation of multichannel intracochlear electrodes can give rise to peripheral and central channel interactions. The ability to eliminate or predict and control the interactions produced by a given electrode geometry is a processing goal for optimizing performance with a multichannel cochlear implant. Previous studies have used loudness summation and forward‐masking pattern techniques to estimate interactions between channels of electrical stimulation. In this study, interactions between bipolar channels of analog electrical stimulation were estimated using a forward‐masking paradigm with a fixed‐level, fixed‐location probe. By varying the electrode location of a 200‐Hz, 300‐ms sinusoidal masker and determining the level of the masker at each location necessary to just mask a 200‐Hz, 10‐ms probe, a “place” tuning curve was derived. The level of masker required at a given location to mask the probe depends on the amount of excitation produced by the probe and reflects, in part, the degree to...

Masker envelopes and simultaneous‐masking psychophysical tuning curves

Recent reports demonstrate that the temporal envelope of a masking stimulus can influence its effectiveness. To examine the probable effects of masker envelope on psychophysical measures of tuning, simultaneous‐masking psychophysical tuning curves (PTCs) were obtained from two normal‐hearing listeners. The probe was a 250‐ms 1‐kHz tone burst presented at 60 dB SPL, which was temporally centered within the 500‐ms maskers. Maskers included SAM and QFM tones, narrow‐band noise, and pure tones. PTCs were obtained in quiet and in the presence of a continuous broadband noise to mask combination tones. Results indicate that fluctuating envelope maskers are less effective than flat envelope maskers only on the low‐frequency side of the PTC. These data suggest that the listener is able to listen for the presence of a probe during periods of low masker energy. Four of the six maskers interacted with the probe producing audible combination tones that could be masked by the broadband noise. Combination tones were mos...