Schroeder-phase Maskers Research Articles

Optimizing masker phase effects for use in a portable hearing screening tool

The current study is a continuation of work toward development of a preclinical indicator for noise induced hearing loss. Masked threshold differences produced by Schroeder-phase maskers have been demonstrated with long duration tones at different frequencies (1, 2, and 4 kHz) as well as with IEEE sentences (Summers and Leek, 1998) for normal hearing listeners, but these differences are not found for hearing impaired listeners. Similar results are also found for repeated short duration tone bursts and greater differences occur with systematically changed tone burst frequencies (Hoglund et al., 2012). Using the enhanced channel model (Oh, 2013), threshold differences are predicted, and the Schroeder-phase masker characteristics are adjusted to maximize the difference. This leads to a greater range of masker phase effect differences between the positive and negative Schroeder-phase maskers, and should allow greater sensitivity to preclinical hearing threshold changes. These optimized maskers were also applied to a digit triplets test designed for telephone hearing screening (Watson et al., 2012). Testing with both the digit triplets and the short duration tone bursts are compared for ease of use and portability, as well as sensitivity to small changes in post-exposure thresholds. [Work supported by the Office of Naval Research.]

Phase effects using chirp maskers

Schroeder-phase maskers appear to interact with the phase curvature of the basilar membrane to produce significantly different amounts of masking depending on the direction of the instantaneous frequency sweep (Oxenham and Dau, 2001). Schroeder-phase maskers may not optimally compensate for the phase-curvature of the basilar membrane, which is not assumed to be linear over its length. This study presents a comparison of masking produced by harmonic complexes that differ only in the phase relationships of their components. Four “chirp” complexes were constructed according to Elberling et al. (2007), with both positive and negative signs, and were compared to Schroeder-phase signals (Smith et al., 1986). Masking observed with pure-tones at three signal frequencies (1, 2, and 4 kHz) and with spectro-temporal burst signals (Hoglund et al., 2013) is reported at two presentation levels (50 and 60 dB SPL). The results are discussed in the context of Schroeder-phase masking and current understanding of the phase-curvature of the human basilar membrane.

Testing a nonlinear computational channel model for masker phase effects

The masked threshold differences produced by Schroeder-phase maskers are most often attributed to the non-linear response of the normal cochlea (Summers et al., 2003). The nonlinear properties of the basilar membrane (BM) cause different response to the positive and negative Schroeder-phase maskers (i.e., + SCHR and -SCHR maskers, respectively) based on signal level, temporal synchrony, and on- and off-frequency harmonic components (Kohlrausch and Sander, 1995, Carlyon and Datta, 1997a,b, and Summers, 2000). In this study, manipulation of harmonic components of the maskers was used to explore nonlinear aspects of BM motion produced by the two different maskers. Specifically, masking period patterns (MPPs) for the + SCHR and -SCHR maskers were measured to show the influence of the phase relationships, and thus, the spectrotemporal characteristics of harmonic complexes on masking effectiveness. An enhanced channel model (Oh, 2013) provides a quantitative explanation for masking differences between + SCHR and -SCHR maskers by introducing nonlinear channel correlation across both frequency and time and a nonlinear decision criterion. [Research supported by a grant from the Office of Naval Research #N000140911017.]

Relationship of distortion product otoacoustic emission components to psychoacoustic measures of noise induced hearing loss

Distortion product otoacoustic emissions (DPOAEs) have promise as a tool to detect and monitor cochlear hair cell damage in noise-induced hearing loss (NIHL). However, variability in OAE amplitudes and thresholds across individuals limits their potential for detecting the early impact of NIHL. The DPOAE has two components, one generated at the point of interaction between the primaries (generator), the second reflected from the characteristic place of the DPOAE frequency (reflected). This study investigated the components of the DPOAE and how they correlate with injury. A total of 109 men, including 54 with varying degrees of occupational noise exposure, were tested. We recorded DPOAEs using logarithmically swept pure-tones and extracted the components of the DPOAE using a least-squares fit approach. Audiometry was conducted at 1.5 and 4kHz with 1dB resolution. Compressive nonlinearity was evaluated psychoacoustically with 1.5 and 4kHz pure-tones using Schroeder-phase maskers. Overall there were associations between the generator component and the psychoacoustic measures: stronger DPOAE amplitudes were associated with better thresholds and greater difference in Schroeder-phase masking threshold. Associations were stronger at 4kHz than 1.5kHz. Similar, but weaker effects were observed for the reflected component. Findings suggest that separation of DPOAE components enhances the assessment of cochlear noise-induced injury.

Extending Schroeder-phase masking: Influence of direction and shape of masker instantaneous frequency

Schroeder (1970) devised an algorithm to produce low peak factor signals. Schroeder signals with equal amplitude spectra, but reversed phase spectra, reveal large differences in masker effectiveness for listeners with normal hearing [Smith et al. (1986)]. Results reported here extend previous work to include detection of multiple bursts of the same frequency, and multiple bursts that increase or decrease in frequency. Signal frequencies were selected to correspond with harmonics in the maskers. Results indicate that changing the frequency of the signal amplifies the difference between the Schroeder-phase maskers, but the direction of the change does not. When a frequency modulated tone is substituted for the Schroeder maskers, masked threshold depends on the shape of the instantaneous frequency (IF) function, as well as the direction of change. For linear FM (with IF similar to the Schroeder maskers), masked thresholds are comparable to the Schroeder-phase maskers. However, for logarithmic FM, IF changes at a constant ERB/s and directional differences are much smaller. A modified channel model [Oh (2012)] shows substantial differences in the basilar membrane response to these different maskers. [Research supported by a grant from the Office of Naval Research #N000140911017.]

Hearing preservation surgery: Psychophysical estimates of cochlear damage in recipients of a short electrode array

In the newest implementation of cochlear implant surgery, electrode arrays of 10 or 20 mm are inserted into the cochlea with the aim of preserving hearing in the region apical to the tip of the electrode array. In the current study two measures were used to assess hearing preservation: changes in audiometric threshold and changes in psychophysical estimates of nonlinear cochlear processing. Nonlinear cochlear processing was evaluated at signal frequencies of 250 and 500 Hz using Schroeder phase maskers with various indices of masker phase curvature. A total of 15 normal-hearing listeners and 13 cochlear implant patients (7 with a 10 mm insertion and 6 with a 20 mm insertion) were tested. Following surgery the mean low-frequency threshold elevation was 12.7 dB (125-750 Hz). Nine patients had postimplant thresholds within 5-10 dB of preimplant thresholds. Only one patient, however, demonstrated a completely normal nonlinear cochlear function following surgery--although most retained some degree of residual nonlinear processing. This result indicates (i) that Schroeder phase masking functions are a more sensitive index of surgical trauma than audiometric threshold and (ii) that preservation of a normal cochlear function in the apex of the cochlea is relatively uncommon but possible.

Representation of Auditory-Filter Phase Characteristics in the Cortex of Human Listeners

Harmonic tone complexes with component phases, adjusted using a variant of a method proposed by Schroeder, can produce pure-tone masked thresholds differing by >20 dB. This phenomenon has been qualitatively explained by the phase characteristics of the auditory filters on the basilar membrane, which differently affect the flat envelopes of the Schroeder-phase maskers. We examined the influence of auditory-filter phase characteristics on the neural representation in the auditory cortex by investigating cortical auditory evoked fields (AEFs). We found that the P1m component exhibited larger amplitudes when a long-duration tone was presented in a repeating linearly downward sweeping (Schroeder positive, or m(+)) masker than in a repeating linearly upward sweeping (Schroeder negative, or m(-)) masker. We also examined the neural representation of short-duration tone pulses presented at different temporal positions within a single period of three maskers differing in their component phases (m(+), m(-), and sine phase m(0)). The P1m amplitude varied with the position of the tone pulse in the masker and depended strongly on the masker waveform. The neuromagnetic results in all cases were consistent with the perceptual data obtained with the same stimuli and with results from simulations of neural activity at the output of cochlear preprocessing. These findings demonstrate that phase effects in peripheral auditory processing are accurately reflected up to the level of the auditory cortex.

Upward spread of Schroeder-phase maskers

Harmonic complexes with phases selected according to the Schroeder algorithm can produce large differences in masking, depending on whether phases increase (positive Schroeder) or decrease (negative Schroeder) with frequency. This finding has been attributed to an interaction between the phase characteristics of auditory filters and the stimulus. The current study investigates effects of filter asymmetry on masking by harmonic complexes using an upward spread of masking paradigm. Maskers were Schroeder phase maskers with frequencies from 200 to 2000 Hz, and signal frequencies ranged from 1000 to 4000 Hz. When the signal fell within the masker bandwidth, it was added in-phase with the identical masker component. At signals below 2000 Hz, negative Schroeder maskers produced more masking than the positive maskers. When the signal frequency was above the masker, there was a rapid decrease in masking for both maskers, but for most subjects, the positive masker became more effective than the negative. This shift in the Schroeder masking effect may be related to phase changes occurring in the lower skirts of the auditory filters. [Work supported by NIH.]

Effects of hearing impairment and presentation level on masking period patterns for Schroeder-phase harmonic complexes.

Masking period patterns (MPPs) for Schroeder-phase harmonic complexes containing equal-amplitude harmonics of a 100-Hz fundamental were determined for 5-ms tonal probes at 4,000 and 1,000 Hz. Maskers consisted of harmonics 2-50 (200-5,000 Hz bandwidth) for 4,000-Hz probes and harmonics 2-20 (200-2,000 Hz) for 1,000-Hz signals. Masked thresholds were determined for probe onsets 153, 155.5, 158, 160.5, and 163 ms following masker onset (masker duration=460 ms). Overall, results were similar for both probe frequencies. For listeners with normal hearing, MPPs for positive Schroeder-phase complexes masking 60 dB SPL probes were highly modulated and became flatter when probe level was increased to 80 dB SPL. MPPs were less modulated for listeners with sensorineural hearing loss than for normally hearing listeners at both 60 and 80 dB SPL probe levels. Thresholds in negative Schroeder-phase maskers were more similar across the two groups of listeners and across differences in probe position and probe level. The findings support an interpretation involving differences in the shape of the basilar-membrane waveform generated by each masker and possible influences of nonlinear cochlear processing on these internal responses. For normally hearing listeners, 60 dB SPL probes were most difficult to detect when temporally positioned so that probe frequency and masker instantaneous frequency were closely matched. For 80 dB SPL probes and for hearing-impaired listeners, probes presented at these same positions were often more easily detected than probes at other positions. The latter result appears to involve benefit associated with in-phase addition of the probe to a portion of the masker similar to the probe in both frequency and phase. This benefit was reduced or entirely eliminated when probe phase was altered so that this in-phase addition did not occur.

Masking of tones and speech by Schroeder-phase harmonic complexes in normally hearing and hearing-impaired listeners

Tone detection and sentence recognition were measured for normally hearing and hearing-impaired listeners using maskers consisting of harmonic series with components summed in positive or negative Schroeder phase. Each task was carried out with the signal set at 60, 70, or 80 dB SPL. For listeners with normal hearing, positive Schroeder-phase complexes masked tones and sentences less than negative Schroeder-phase maskers. In the two experimental tasks, to achieve the same amount of masking, positive Schroeder-phase complexes had to be set as much as 12–15 dB higher than negative Schroeder-phase complexes. Large phase effects were observed on both tasks at all three test levels. The two maskers were more nearly equal in effectiveness in the presence of cochlear damage. The findings support an interpretation that involves differences in the shape of the basilar-membrane waveform generated by each masker and active cochlear processing which enhances the internal signal-to-masker ratio for signals presented in the positive Schroeder masker. This spectral enhancement appears to require nonlinear active gain that is characteristic of normal auditory processing at moderate presentation levels. The results of the sentence recognition task suggest that group differences observed in tone detection transfer fairly directly to speech perception under masking.