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Published Articles
Jeffrey J. DiGiovanni
The effects of transient-noise reduction algorithms on speech intelligibility and user ratings among hearing-impaired users. American Journal of Audiology 20, 140-150.

Jeffery J. DiGiovanni, Erin A. Davlin, & Naveen K. Nagaraj

The Goal of the study was to asses the functional utility of transient noise reduction (TNR) algorithms available in hearing aids via speech intelligibility ans user preferences. Two different pairs of hearing aids were programmed for a group of individuals with hearing impairment. Intelligibility was then measured for each participant in a variety of different conditions. Each condition was tested with TNR activated and TNR deactivated in a counterbalanced, single-blinded format. Subjective ratings of overall speech understanding were obtained from participants after each condition. The results found a significant improvement in speech intelligibility with TNR activated when listeners were presented with multitalker babble, in the presence of chair clang transient noise, and when combining these noises. Activation of the TNR algorithm did not result in significant improvements from any other subjective ratings. TNR appears to offer an incremental step in improving listening experience for heating aid users.

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Verification of in situ thresholds and integrated real-ear measurements. (2010). Journal of the American Academy of Audiology 21, 663-670.

DiGiovanni, J.J., & Pratt, R.M.

Accurate prescriptive gain results in a more accurate fit, lower return rate in hearing aids, and increased patient satisfaction. In situ threshold measurements can be used to determine required gain. The Widex Corporation uses an in situ threshold measurement strategy, called the Sensogram. Real-ear measurements determine if prescriptive gain targets have been achieved. Starkey Laboratories introduced an integrated real-ear measurement system in their hearing aids. The purpose of this article was to determine whether the responses obtained using the Widex Sensogram were equivalent to those obtained using current clinical threshold measurement methods. To determine the accuracy of the Starkey IREMS™ (Integrated Real Ear Measurement System) in measuring RECD (real-ear to coupler difference) values compared to a dedicated real-ear measurement system. A significant difference between Sensogram thresholds and conventional audiometric thresholds was found with the Sensogram method resulting in better threshold values at 0.5, 1.0, and 2.0 kHz for both groups. In Experiment 2, a significant difference between RECD values obtained by the Starkey IREMS and the Audioscan Verifit system was found with significant differences in RECD values found at 0.25, 0.5, 0.75, 1.5, 2.0, and 6.0 kHz. The Sensogram data differ significantly from traditional audiometry at several frequencies important for speech intelligibility. Real-ear measures are still required for verification of prescribed gain, however, calling into question any claims of shortened fitting time. The Starkey IREMS does perform real-ear measurements that vary significantly from benchmark equipment. These technologies represent a positive direction in prescribing accurate gain during hearing-aid fittings, but a stand-alone system is still the preferred method for real-ear measurements in hearing-aid fittings.

The relative contribution of adaptation and temporal integration in forward masking. (2010).  Proceedings of Meetings on Acoustics,  7-1 – 7-8.

Nagaraj, N.K., DiGiovanni, J.J., & Ries, D.T.

The aim of this study was to demonstrate the relative contribution of adaptation and temporal integration on forward masking. In the first of two experiments, monaural gap-detection, thresholds were measured for broad-band noise markers having different levels on either side of the gap. In the second experiment, subjects matched the gap-duration within comparison noise markers to be perceptually equal to that of standard noise markers. Levels of standard noise markers and the first noise marker level of the comparison stimuli were set constant at 30 dB SL. Gap-duration matches were performed by decreasing the level of the second noise marker by 5 dB SL relative to the first noise marker in the comparison stimuli. The results indicated that gap-detection thresholds increased with a decrease in level of noise after the gap. Subjects required longer gaps in the gap-duration matching task with a decrease in level of the second noise marker. Both results are consistent with the notion that persistence of excitation is the dominate perceptual mechanism that results from forward masking.

Performance of phonemically-targeted processing in conjunction with compression processing with spectral enhancement.  (2010). Proceedings of Meetings on Acoustics 9(1), 5-1 – 5-6

DiGiovanni, J.J., Rutledge, J.C., Umble, C.S., & Mester, A.P.

The goal of the present study is to test, individually and in combination, two signal-processing strategies designed to improve both consonant and vowel perception. In the first strategy, specific consonants were targeted for processing to increase amplitude and duration. For consonants with a duration increase, the adjacent vowel was decreased proportionately in order to maintain overall word and sentence duration. Second, Coliga, an adaptive compression-processing strategy incorporating spectral enhancement, was used to process stimuli. Hearing-in-noise-test sentences were presented monaurally to normal-hearing and hearing-impaired adults in the presence of speech-shaped noise. For normal-hearing listeners, it appears that Coliga is improving intelligibility in certain conditions. Conservative amounts of duration and amplitude processing need to be applied in combination rather than when applied independently in order to provide benefit. For the hearing-impaired listeners, Coliga with spectral enhancement resulted in poorer performance than without, for all conditions except for the Coliga-only conditions, where the intelligibility was the same. It appears that phonemically- targeted speech processing and Coliga are working antagonistically due to either 1) the processing method negating the benefits when the two are combined, or 2) combining the duration and amplitude components leading to no benefit despite documented benefit of each when employed individually.

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Effects of recurrent tonal information on auditory working memory for pitch. (2009). Hearing Research 255, 14-21.

Ries, D.R., & DiGiovanni, J.J.

This study ascertained the influence of repeating pitch information within an intervening tonal sequence upon the extent of interference for a pitch standard held within auditory working memory as measured by the difference limen for frequency (DLF). Standard and comparison tones were presented to subjects and same/different responses were obtained using a touch screen monitor and the DLF was measured using single interval adjustment matrix (SIAM) procedure [Kaernbach, C., 1990. A single-interval adjustment-matrix (SIAM) procedure for unbiased adaptive testing. J. Acoust. Soc. Am. 88, 2645-2655]. Estimates of the DLF were obtained in a control condition with a silent inter-comparison interval and three conditions containing intervening tones within the temporal gap between the standard and comparison stimuli. The presence of intervening stimuli produced a significant increase in the DLF when the intervening tonal sequence contained tones with pitches that differed from that of the standard (Int condition) as well as when the sequence contained a tone with a pitch identical to that of the comparison (RptCmp condition). Further, the DLFs obtained for RptCmp condition were significantly higher than those measured in the Int condition. The DLFs measured in the condition where the pitch of an intervening tone was identical to the standard were significantly lower than those for the Int and RptCmp condition, but did not differ from the DLFs for the control condition. These results indicate that either a release from or an increase in interference in auditory working memory for pitch can occur dependent upon the frequency relationships between of the standard, comparison, and intervening tones.

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The role of temporal-masking patterns in the determination of subjective duration and loudness for ramped and damped sounds. (2008). Journal of the Acoustical Society of America, 124 (6).

Ries, D.R., Schlauch, R.S., & DiGiovanni, J.J.

The association between temporal-masking patterns, duration, and loudness for broadband noises with ramped and damped envelopes was examined. Duration and loudness matches between the ramped and damped sounds differed significantly. Listeners perceived the ramped stimuli to be longer and louder that the damped stimuli, but the outcome was biased by the stimulus context. Next, temporal-masking patterns were measured for ramped- and damped-broadband noises using three (0.5, 1.5, and 4.0 kHz) 10 ms probe tones presented individually at various temporal delays. Predictions of subjective duration derived form masking results underpredicted the matching results. Loudness estimates derived from models that assume persistence of neural activity after stimulus offset [Glasberg B.R., and Moore, B. C. J. (2002). “A model of loudness applicable to time-varying sounds.” J. Audio. Eng. Soc. 50, 331-341; Chalupper, J., and Fastl, H. (2002) “Dynamic loudness model (DLM) for normal and hearing-impaired listeners,” Acust. Acta Acust. 88, 378-386] were greater for ramped sounds than for damped sounds and were close to the average results obtained via the matching task. Differences in simultaneous-masked thresholds for these stimuli could not account for the loudness-matching results. Decay suppression of the later occurring portion of the damped stimulus may account for the differences in perception due to the stimulus context; however, a parsimonious implementation of this process that accounts for both subjective duration and loudness judgments remains unclear. 2008 Acoustical Scociety of America. 

The role of consonant duration and amplitude processing on speech intelligibility in noise.(2008). Proceedings of Meetings on Acoustics, 4, 050004.

DiGiovanni, J.J., & Stover, A.K.

Hearing loss can have drastic effects on communication abilities leading researchers to strive to find solutions for the associated problems. Clear speech has been studied in the past as a possible avenue for improving speech understanding in those with hearing loss. In the present study, two characteristics of clear speech, increased consonant duration and consonant amplification, were applied to Hearing-in Noise Test sentences. Stimuli were presented in sound field to hearing-impaired and normal-hearing individuals in the presence of speech-shaped noise, and percentage correct was measured. For HINT sentence presentations, both groups showed improvements for both amplitude processing and the 15-ms condition for duration processing. Results show that increasing consonant duration and amplitude is a viable approach to improving speech understanding; nevertheless, this data suggest that there is a significant cutoff point where lengthening the duration of a consonant can result in a degraded speech signal

Release from interference in auditory working memory for pitch. (2007). Hearing Research, 230, 64-72.

Ries, D.T., & DiGiovanni, J.J.

The purpose of this study was to quantify the effect of interpolated tones upon a pitch standard held within auditory working memory through measurement of the difference limen (just noticeable difference) for frequency and the usefulness of “Where” cues to ameliorate the interference produced by these intervening stimuli. To this end, we measured the degree to which tones, containing identical and disparate localization cues, presented within the retention interval altered differential sensitivity for frequency via the method of constant stimuli. The difference limen for frequency nearly tripled when tones were presented within the retention interval and sound localization cues produced a significant partial release from interference within the short-term pitch store. Interference produced by “Where” cues ranged from 4.0 to 5.2 Hz. These findings indicate that there is a possible integrative use of the “What” and “Where” pathways in forming and maintaining pitch information within the pitch array within auditory working memory. 2007 Elsevier B.V. All rights reserved.

Stapedial reflex and ears with high static acoustic admittance. (2007). American Journal of Audiology, 16, 68-74.

DiGiovanni, J.J., & Ries, D.T.

Purpose: To evaluate modified acoustic reflex diagnostic protocols for a group of individuals (n = 9) with high peak compensated static acoustic admittance (Ytm) tympanograms. Method: A modified procedure designed to improve acoustic stapedius reflex threshold (ASRT) measurements in individuals with high-admittance tympanograms was employed in both an experimental and a control group. ASRTs were measured at 0.5, 1.0, and 2.0 kHz, ipsilateral and contralateral. Measurements were obtained within each condition for 7 ear canal pressures that were set to 0, 50, 100, and 150 daPa (relative to tympanometric peak pressure [TPP]). Results: Though measuring ASRTs at –50 daPa (relative to TPP) in the high-admittance and normal groups did not result in significantly better thresholds than at TPP, the absent reflex rate was reduced when the ear canal pressure was changed by –50 daPa during ASRT measurements. Conclusions: Based on this sample, it is suggested that a patient presenting with high peak compensated static acoustic admittance (peak Ytm ≥ 2.1 mmho) undergo ASRT evaluation with the ear canal pressure set to –50 daPa (relative to TPP).

Mechanisms responsible for differences in perceived duration for rising-intensity and falling-intensity sounds. (2007). Ecological Psychology, 19, 239-264.

DiGiovanni, J.J., & Schlauch, R.S.

Sounds that are equivalent in all aspects except for their temporal envelope are perceived differently.  Sounds with rising temporal envelopes are perceived as louder, longer, and show a greater change in loudness throughout their duration than sounds with falling temporal envelopes.  Stecker & Hafter (2000) proposed that participants ignore the decay portion of sounds with falling temporal envelopes to account for observed loudness differences, but there is no empirical evidence support this hypothesis. To test this idea, two duration-matching experiments were performed. One experiment used broadband noise and the other natural stimuli.  Different groups of participants were given different instruction sets asking them to 1) simply match the duration or 2) include all aspects of the sounds. Both experiments produced the same result. The first instruction set, which represented participants’ natural biases, yielded shorter subjective durations for sounds with falling temporal envelopes than for sounds with rising temporal envelopes.  By contrast, asking participants to include all aspects of the sounds significantly reduced the size of the asymmetry in subjective duration, a result that supports Stecker and Hafter’s (2000) hypothesis. This segregation of the stimulus at the perceptual level is consistent with observed asymmetries in loudness change and overall loudness for sounds with rising and falling temporal envelopes, but it does not account for the entire effect. The remaining portion of the effect, after considering biases due to instructions, is not likely a result of adaptation but could be associated with persistence. The amount of persistence was inferred from behavioral masking data obtained for these sounds.

Response method in manual audiometry for adults. (2007). American Journal of Audiology, 16, 1-4.

DiGiovanni, J.J., & Repka, J.

Purpose: This study compared the speed, false-alarm rate, and participant preference of different response methods (rasing a hand, pushing a response button, and giving an oral respose) for measuring pure tone threshold. Method: Thirty female university students with normal hearing participated. Response method order was randomly assigned to 6 different groups. Air-conduction thresholds were measured twice for each response method for each participant in octave intervals between 250 Hz and 8000 Hz. The 2nd threshold measurements were performed on a different day but within 2 weeks of the initial measurement. Results: A significant difference was found when comparing the amount of time necessary to complete the test for each response method. On average, it took about 1 min less when using the push-button response than when using hand-raise or verbal response methods. There was also a significant participant preference for using the response button. No significant difference between response method for threshold level and number of false positives was found. Conclusion: This study supports the use of the response button when measuring auditory threshold for young adults with normal hearing.

Response growth using a low-frequency suppressor. (2007). Hearing Research, 234 15-20.

DiGiovanni, J.J., & Nair, P.

Numerous psychophysical studies on two-tone suppression have been carried out. More recently, researchers have attempted to relate the magnitude of suppression to the level of suppressee. [Wojtczak, M., Viemeister, N.F., 2005. Psychophysical response growth under suppression. In: Pressnitzer, D., de Cheveigne, A., McAdams, S., Collet, L. (Eds.), Auditory Signal Processing: Physiology, Psychoaccoustics, and Models. Springer, New York, pp. 67–74] demonstrated that the magnitude of suppression for a higher-frequency, fixed-level suppressor decreases with increasing level of the suppressee. This suggests a linearization of the basilar membrane response in presence of a high-frequency suppressor. The present study expands these results to a low-frequency suppressor of varying intensity levels. Detection of a 10-ms, 4.0-kHz probe was measured under different forward-masking conditions: one with a 200-ms, 4.0-kHz masker (suppressee) presented with no suppressor and another with the same masker paired with a 2.2-kHz, 200-ms suppressor. The 4.0-kHz masker level was varied adaptively and a range of probe levels was used to measure the growth of suppression. Results indicate that (1) the magnitude of suppression increases with increasing suppressor level and (2) generally, the probe level was not related to the magnitude of suppression. Elsevier B.V. All rights reserved.

Auditory filters and the benefit measured from spectral enhancement. (2006). Journal of the Acoustical Society of America 120, 1529-1538.

DiGiovanni, J.J., & Nair, P.

Algorithms designed to improve speech intelligibility for those with sensorineural hearing loss (SNHL) by enhancing peaks in a spectrum have had limited success. Since testing of such algorithms cannot separate the theory of the design from the implementation itself, the contribution of each of these potentially limiting factors is not clear. Therefore, psychophysical paradigms were used to test subjects with either normal hearing or SNHL in detection tasks using well controlled stimuli to predict and assess the limits in performance gain from a spectrally enhancing algorithm. A group of normal-hearing (NH) and hearing-impaired (HI) subjects listened in two experiments: auditory filter measurements and detection of incremented harmonics in a harmonic spectrum. The results show that NH and HI subjects have an improved ability to detect incremented harmonics when there are spectral decrements surrounding the increment. Various decrement widths and depths were compared against subjects' equivalent rectangular bandwidths (ERBs). NH subjects effectively used the available energy cue in their auditory filters. Some HI subjects, while showing significant improvements, underutilized the energy reduction in their auditory filters. 2006 Acoustical Society of America.

Spontaneous recovery of sudden sensorineural hearing loss: Possible association with autoimmune disorders. (2006). Journal of the American Academy of Audiology 27, 498-505.

DiGiovanni, J.J., & Nair, P.

A 46-year-old white male diagnosed with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS) was seen for audiological testing 15 minutes following a sudden onset hearing loss in the right ear. The test battery included pure-tone audiometry, word-recognition testing, speech-recognition threshold (SRT) testing, immittance testing, and distortion-product otoacoustic emissions (DPOAE) testing. Testing revealed a sensorineural hearing loss in the right ear. Shortly after testing, the patient indicated that his condition had improved. Testing was repeated, and the second round of tests revealed normal hearing in both ears. Four days later, a follow-up test again indicated normal hearing in both ears. Possible connections of this brief occurrence of idiopathic hearing loss with the patient’s medical conditions are discussed. Specifically, symptoms were consistent with a transient ischemic attack (TIA) affecting his right cochlea in the stria vascularis region, resulting in a temporary,sensorineural hearing loss. No residual effects were observed clinically.

A psychophysical evaluation of spectral enhancement. (2005). Journal of Speech-Language Hearing Research 48(5), 1121-1135

DiGiovanni, J.J., Nelson, P.B., & Schlauch, R.S.

Listeners with sensorineural hearing loss have well-documented elevated hearing thresholds; reduced auditory dynamic ranges; and reduced spectral (or frequency) resolution that may reduce speech intelligibility, especially in the presence of competing sounds. Amplification and amplitude compression partially compensate for elevated thresholds and reduced dynamic ranges but do not remediate the loss in spectral resolution. Spectral-enhancement processing algorithms have been developed that putatively compensate for decreased spectral resolution by increasing the spectral contrast, or the peak-to-trough ratio, of the speech spectrum. Several implementations have been proposed, with mixed success. It is unclear whether the lack of strong success was due to specific implementation parameters or whether the concept of spectral enhancement is fundamentally flawed. The goal of this study was to resolve this ambiguity by testing the effects of spectral enhancement on detection and discrimination of simple, well-defined signals. To that end, groups of normal-hearing (NH) and hearing-impaired (HI) participants listened in 2 psychophysical experiments, including detection and frequency discrimination of narrowband noise signals in the presence of broadband noise. The NH and HI listeners showed an improved ability to detect and discriminate narrowband increments when there were spectral decrements (notches) surrounding the narrowband signals. Spectral enhancements restored increment detection thresholds to within the normal range when both energy and spectral-profile cues were available to listeners. When only spectral-profile cues were available for frequency discrimination tasks, performance improved for HI listeners, but not all HI listeners reached normal levels of discrimination. These results suggest that listeners are able to take advantage of the local improvement in signal-to-noise ratio provided by the spectral decrements.

Duration discrimination and subjective duration of ramped and damped sounds. (2001) Journal of the Acoustical Society of America, 109, 1880-7.

The perception of stimuli with ramped envelopes (gradual attack and abrupt decay) and damped envelopes (abrupt attack and gradual decay) was studied in subjective and objective tasks. Magnitude estimation (ME) of perceived duration was measured for broadband noise, 1.0-kHz, and 8.0-kHz tones for durations between 10 and 200 ms. Damped sounds were judged to be shorter than ramped sounds. Matching experiments between sounds with ramped, damped, and rectangular envelopes also showed that damped sounds are perceived to be shorter than ramped sounds, and, additionally, the reason for the effect is a result of the damped sound being judged shorter than a rectangular-gated sound rather than the ramped sound being judged longer than a rectangular-gated sound. These matching studies also demonstrate that the size of the effect is larger for tones (factor of 2.0) than for broadband noise (factor of 1.5). There are two plausible explanations for the finding that damped sounds are judged to be shorter than ramped or rectangular-gated sounds: (1) the abrupt offset at a high level of the ramped sound (or a rectangular-gated sound) results in a persistence of perception (forward masking) that is considered in judgments of the subjective duration; and (2) listeners may ignore a portion of the decay of a damped sound because they consider it an "echo" [Stecker and Hafter, J. Acoust. Soc. Am. 107, 3358–3368 (2000)]. In another experiment, duration discrimination for broadband noise with ramped, damped, and rectangular envelopes was studied as a function of duration (10 to 100 ms) to determine if differences in perceived duration are associated with the size of measured Weber fractions. A forced-choice adaptive procedure was used. Duration discrimination was poorer for noise with ramped envelopes than for noise with damped or rectangular envelopes. This result is inconsistent with differences in perceived duration and no explanation was readily apparent. 2001 Acoustical Society of America.

Masker laterality and cueing in forward-masked intensity discrimination. (1999). Journal of the Acoustical Society of America, 105, 822-828.

Schlauch, R.S., Clement, B.R., Ries, D.T., & DiGiovanni, J.J.

Forward-masked intensity discrimination was measured as a function of level in experiments designed to reveal insights into the mechanism(s) underlying the midlevel elevation of the Weber fraction. The standard and maskers were 1.0-kHz tones that were separated by 100 ms. Performance was measured for listeners with normal hearing using an adaptive procedure. In experiment 1, intensity discrimination was measured in the presence of an ipsilateral masker (80 dB SPL), a contralateral masker (93 dB SPL), and a binaural (dichotic) masker produced by combining the ipsilateral and contralateral maskers. Listeners perceived only the contralateral masker in the binaural-masker condition. The contralateral masker produced a small midlevel elevation of the Weber fraction. The ipsilateral masker and the binaural masker produced a large, midlevel elevation of the Weber fraction. Experiment 2 found that a two-tone masker resulted in a reduction (improvement) in the Weber fraction for some conditions, but the midlevel elevation remained for all subjects in this cue-tone condition. Experiment 3 demonstrated that cross talk could not account for all of the masking observed with contralateral maskers. Taken together, the results suggest that a single complex mechanism or multiple mechanisms may be responsible for the masking seen in these experiments. On the basis of the cueing results, it is concluded that a portion of the masking is due to cognitive factors; however, a sensory mechanism cannot be ruled out for the remaining portion, based on the results of these experiments. Finally, a small but significant amount of masking due to contralateral maskers places the mechanism for this outcome central to the cochlear nucleus. 1999 Acoustical Society of America.

Basilar membrane nonlinearity and loudness. (1998). Journal of the Acoustical Society of America, 103, 2010-2020.

Schlauch, R.S., DiGiovanni, J.J., & Ries, D.T.

Loudness matching functions for tones for persons with one shifted-threshold ear (hearing loss and noise-shifted thresholds) and one ear within normal limits were used to derive the presumed basilar membrane (BM) input–output (I/O) function in a normal ear. The comparison was made by assuming that the BM I/O function for the ear with the cochlear threshold shift has a slope of one (a linearized cochlea). The function for the normal ear was derived from the loudness matching function based on this assumption. Comparisons were made for archival basilar membrane data [M. A. Ruggero, N. C. Rich, A. Recio, S. S. Narayan, and L. Robles, J. Acoust. Soc. Am. 101, 2151–2163 (1997)] for chinchilla and archival loudness matches for long-duration tones for persons with various degrees of cochlear hearing loss [F. Miskolczy-Fodor, J. Acoust. Soc. Am. 32, 486–492 (1960)]. Comparisons were made also between BM I/O functions and ones derived from loudness matches for persons with unilateral hearing loss simulated by broadband noise. The results show a close resemblance between the basilar membrane I/O function and the function derived from loudness matches for long-duration tones, even though the comparison was between human and chinchilla data. As the degree of threshold shift increases from 40 to 80 dB, the derived BM I/O functions become shallower, with slopes for losses of 60 dB or more falling in the range of values reported for physiological data. Additional measures with short-duration tones in noise show that the slope of the loudness function and the slope of the derived basilar membrane I/O function are associated with the behavioral threshold for the tone. The results for long-duration tones suggest a correspondence between BM displacement and loudness perception in cases of recruitment, but the relation between the degree of loss and the amount of BM compression and the relation between signal duration and compression suggests that other factors, such as the neural population response, may play a role. ©1998 Acoustical Society of America.

Intensity discrimination of ramped and damped tones. (1998). Proceedings of the International Congress on Acoustics and the Acoustical Society of America, 2, 885-886. 2aPP31.

Schlauch, R.S., Ries, D.T., DiGiovanni, J.J.,
Elliot, S., & Campbell, S.

Intensity discrimination of 1.0-kHz tones that had exponentially ramped or damped envelopes was measured as a function of duration and level. Standard levels ranged from near threshold to 90 dB SPL and durations ranged from 10 to 200 ms. Just-noticeable differences for some subjects were smaller for damped tones than for ramped tones for the same duration and level. For these conditions, damped tones showed more temporal integration than ramped tones. This result is somewhat surprising given that the subjective duration of ramped tones increased by a factor of 100 for stimuli ranging from 10 to 200 ms, whereas the subjective duration of damped tones increased by a much smaller factor of 28 over the same range of durations. Possible physiological and cognitive explanations for these differences will be discussed. [Work supported by NIH-NIDCD R29 DC01542.]