Function Of Masker Bandwidth Research Articles

Reconciling frequency selectivity and phase effects in masking.

The effects of auditory frequency selectivity and phase response on masking were studied using harmonic tone complex maskers with a 100-Hz fundamental frequency. Positive and negative Schroeder-phase complexes (m+ and m-), were used as maskers and the signal was a long-duration sinusoid. In the first experiment, thresholds for signal frequencies of 1 and 4 kHz were measured as a function of masker bandwidth and number of components. A large difference in thresholds between the m+ and m- complexes was found only when masker components were presented ipsilateral to the signal over a frequency range wider than the traditional critical band, regardless of the absolute number of components. In the second experiment, frequency selectivity was measured in harmonic tone complexes with fixed or random phases as well as in noise, using a variant of the notched-noise method with a fixed masker level. The data showed that frequency selectivity is not affected by masker type, indicating that the wide listening bandwidth suggested by the first experiment cannot be ascribed to broader effective filters in complex-tone maskers than in noise maskers. The third experiment employed a novel method of measuring frequency selectivity, which has the advantage that the overall level at the input and the output of the auditory filter remains roughly constant across all conditions. The auditory filter bandwidth measured using this method was wider than that measured in the second experiment, but may still be an underestimate, due to the effects of off-frequency listening. The data were modeled using a single-channel model with various initial filters. The main findings from the simulations were: (1) the magnitude response of the Gammatone filter is too narrow to account for the phase effects observed in the data; (2) none of the other filters currently used in auditory models can account for both frequency selectivity and phase effects in masking; (3) the Gammachirp filter can be made to provide a good account of the data by altering its phase response. The final conclusion suggests that masker phase effects can be accounted for with a single-channel model, while still remaining consistent with measures of frequency selectivity: effects that appear to involve broadband processing do not necessarily require across-channel mechanisms.

Forward- and simultaneous-masked thresholds in bandlimited maskers in subjects with normal hearing and cochlear hearing loss.

Forward- and simultaneous-masked thresholds were measured at 0.5 and 2.0 kHz in bandpass maskers as a function of masker bandwidth and in a broadband masker with the goal of estimating psychophysical suppression. Suppression was operationally defined in two ways: (1) as a change in forward-masked threshold as a function of masker bandwidth, and (2) as a change in effective masker level with increased masker bandwidth, taking into account the nonlinear growth of forward masking. Subjects were younger adults with normal hearing and older adults with cochlear hearing loss. Thresholds decreased as a function of masker bandwidth in forward masking, which was attributed to effects of suppression; thresholds remained constant or increased slightly with increasing masker bandwidth in simultaneous masking. For subjects with normal hearing, slightly larger estimates of suppression were obtained at 2.0 kHz rather than at 0.5 kHz. For hearing-impaired subjects, suppression was reduced in regions of hearing loss. The magnitude of suppression was strongly correlated with the absolute threshold at the signal frequency, but did not vary with thresholds at frequencies remote from the signal. The results suggest that measuring forward-masked thresholds in bandlimited and broadband maskers may be an efficient psychophysical method for estimating suppression.

Psychophysical suppression measured with bandlimited noise extended below and/or above the signal: effects of age and hearing loss.

The objectives of this study were to measure suppression with bandlimited noise extended below and above the signal, at lower and higher signal frequencies, between younger and older subjects, and between subjects with normal hearing and cochlear hearing loss. Psychophysical suppression was assessed by measuring forward-masked thresholds at 0.8 and 2.0 kHz in bandlimited maskers as a function of masker bandwidth. Bandpass-masker bandwidth was increased by introducing noise components below and above the signal frequency while keeping the noise centered on the signal frequency, and also by adding noise below the signal only, and above the signal only. Subjects were younger and older adults with normal hearing and older adults with cochlear hearing loss. For all subjects, suppression was larger when noise was added below the signal than when noise was added above the signal, consistent with some physiological evidence of stronger suppression below a fiber's characteristic frequency than above. For subjects with normal hearing, suppression was greater at higher than at lower frequencies. For older subjects with hearing loss, suppression was reduced to a greater extent above the signal than below and where thresholds were elevated. Suppression for older subjects with normal hearing was poorer than would be predicted from their absolute thresholds, suggesting that age may have contributed to reduced suppression or that suppression was sensitive to changes in cochlear function that did not result in significant threshold elevation.

Leaky integrators, Fletcher curves, and period histograms: A new link

Fletcher curves, thresholds from tone-in-noise detection tasks plotted as a function of masker bandwidth, are thought to reflect characteristics of peripheral filters. This notion, however, is compromised by the finding that a roving level procedure affects thresholds only for very narrow bandwidths [Kidd et al., J. Acoust. Soc. Am. 86, 1310–1317 (1989)]. Alternatively, a three-stage, leaky integrator (i.e., initial, relatively broad bandpass filter, followed by half-wave rectification and low-pass filtering) yields frequency-dependent, level-invariant Fletcher curves, thus entertaining the possibility that sensitivity to envelope cues underlies detection at all masker bandwidths. The decision statistic is a measure of similarity (Mahalanobis distance) between the observed amplitude spectrum of the leaky integrator output and expected spectra with and without a signal. Regarding physiological plausibility, the model also describes the temporal response of auditory nerve fibers to complex stimuli. Horst et al. [J. Acoust. Soc. Am. 85, 1898–1901 (1985)] used multicomponent, periodic stimuli to obtain period histograms from auditory nerve recordings. Fourier transforms of the histograms were used to assess temporal responses of fibers. For various stimulus configurations, the spectra of the period histogram and leaky integrator output are nearly identical. Theoretical implications are discussed. [Work supported by ONR.]

Temporal integration at 6 kHz as a function of masker bandwidth.

Thresholds were measured for a 6-kHz sinusoidal signal presented within a 500-ms masker. The masker was either a bandpass Gaussian noise of varying bandwidth, or a sinusoid of the same frequency as the signal. The spectrum level of the noise masker was kept constant at 20 dB SPL, and the level of the sinusoidal masker was 40 dB SPL. Thresholds for signal durations between 2 and 300 ms were measured for masker bandwidths ranging from 60 to 12,000 Hz. The masker was spectrally centered around 6 kHz. For masker bandwidths less than 600 Hz, the slope of the temporal integration function decreased with decreasing masker bandwidth. The results are not consistent with current models of temporal integration or temporal resolution. It is suggested that the results at narrow bandwidths can be understood in terms of changes in the power spectrum of the stimulus envelope or modulation spectrum. According to this view, the onset and offset ramps of the signal introduce detectable high-frequency components into the modulation spectrum, which provide a salient cue in narrowband maskers. For broadband maskers, these high-frequency components are masked by the inherent rapid fluctuations in the masker envelope. Additionally, for signal durations between 7 and 80 ms, signal thresholds decreased by up to 5 dB as the masker bandwidth increased from 1200 to 12,000 Hz. The mechanisms underlying this effect are not yet fully understood.

The masking level difference in children: Effects of signal and masker bandwidths.

The age at which the magnitude of the masking level difference (MLD) reaches adult levels depends partly upon masker bandwidth (BW). For wider BWs, children perform like adults by 5–6 years of age. For narrow BWs, adult-like performance is not reached until an older age. To examine this age dependence further, this study measured the MLD for a 500-Hz tone as a function of masker BW in a group of 20 children aged 5–10 years, and in a group of 10 adults. Masker BWs were 20, 40, 80, 160, 320, and 1000 Hz, centered at 500 Hz. The MLDs of the children were generally smaller than those of the adults for BW = 20 Hz. Children over 7 years of age had adult-like MLDs for all BWs ≥ 40 Hz. The youngest children did not reach adult levels until BWs ≥ 320 Hz. To determine whether the smaller MLDs at narrow BWs were due to the perceptual similarity between the masker and the signal, MLDs were also measured in conditions where both masker and signal had BWs = 20 or 320 Hz. Children under 8 years of age showed the same MLDs for both BWs. Older children and adults had larger MLDs for the narrow BW. [This work supported by NIH NIDCD.]

Temporal integration as a function of masker bandwidth.

Thresholds were measured for a 6‐kHz sinusoid, temporally centered in a 500‐ms masker which was either a bandpass Gaussian noise (20 dB SPL spectrum level) or a 6‐kHz sinusoid (40 dB SPL). A notched noise centered on 6 kHz prevented the use of off‐frequency cues. The signal, gated with 2‐ms ramps, ranged in half‐amplitude duration from 2 to 300 ms. The noise bandwidth was arithmetically centered on 6 kHz and was varied from 60 Hz to 12 kHz. For masker bandwidths below 300 Hz, the slope of integration for signal durations between 2 and 20 ms decreased with decreasing masker bandwidth. For the tonal masker, increasing signal duration from 2 to 20 ms had no effect on threshold. These results cannot be accounted for by lowpass‐filter or temporal‐window models of temporal integration or resolution. Instead, it is proposed that the auditory system performs a spectral analysis of the stimulus envelope, so that the rapid fluctuations (high modulation frequencies) introduced by gating the signal can be used as a cue for brief signals in narrowband noise and tones. For broadband maskers, this cue is not available due to the masker’s inherent rapid envelope fluctuations. [Work supported by the Wellcome Trust.]

Growth of low‐pass masking of pure tones and speech for hearing‐impaired and normal‐hearing listeners

Tonal thresholds and consonant recognition were measured in low-pass maskers as a function of masker bandwidth and spectrum level. Thresholds and consonant-recognition scores were obtained for normal-hearing subjects, and for pairs of normal-hearing subjects (who listened in threshold-equating background noise) and hearing-impaired subjects. Consonant-recognition scores were compared to scores predicted by a modified articulation index. Mean thresholds measured in low-pass maskers were higher for hearing-impaired than for normal-hearing subjects for signal frequencies above the masker. Slopes of functions relating thresholds for signals above the masker to masker spectrum level were not significantly different between hearing-impaired and normal-hearing subjects listening in spectrally shaped broadband noise (SSBB), but were shallower than slopes of masking functions for normal-hearing subjects listening without SSBB. Slopes of masking functions for signals within the masker were equivalent for all subjects. Slopes of functions relating consonant recognition to masker spectrum level were similar within subject pairs, whereas, in some cases, slopes of functions relating consonant recognition to speech level were shallower for hearing-impaired subjects than for their normal-hearing counterparts. Although greater improvement in consonant recognition with speech level was predicted for hearing-impaired than for normal-hearing subjects, on average, less improvement with speech level was observed. Shallower slopes of functions relating score to speech level observed for some hearing-impaired listeners may result from more shallow growth of speech sensation levels in spectral regions above the low-pass masker.

Level discrimination in roving-level, notched-noise maskers as a function of masker bandwidth and rove range

This experiment examines how level discrimination (also called intensity discrimination) of a three-tone complex depends on the bandwidth and rove range of a notched-noise masker. Previous experiments used fixed-level maskers, which may provide profile-discrimination cues. In the present experiment, a roving masker obviated profile cues. Level discrimination thresholds, ΔL’s (=20 log[(p+Δp)/p], where p is pressure), were measured in four listeners. The equality intense signal components at 0.84, 1, and 1.17 kHz were set to 25, 55, or 85 dB SPL. The two masker bands extended 1 or 7 critical bands below 0.77 kHz and above 1.27 kHz. Results show that ΔL’s increase somewhat as the rove range increases from 0 to 20 dB, but for rove ranges less than 10 dB, the increase is small. Contrary to predictions based on profile discrimination, ΔL’s at 60 and, especially, 90 dB SPL tend to be larger with 7-than with 1-Bark-wide masking bands. At 30 dB SPL there is almost no effect of masker bandwidth. These results indicate that profile cues probably did not affect appreciably previous measurements of level discrimination in notched-noise maskers. [Work supported by NIDCD.]

Signal detection in temporally modulated and spectrally shaped maskers

The first part of this paper presents several experiments on signal detection in temporally modulated noise, yielding a general approach toward the concept of comodulation masking release (CMR). Measurements were made on masked thresholds of both long- and short-duration, narrow-band signals presented in a 100% sinusoidally amplitude-modulated (SAM) noise masker (modulation frequency 32 Hz), as a function of masker bandwidth from 1/3 oct up to 13/3 octs, while the masker band was geometrically centered at signal frequency. With the short-duration signals placed in the valley of the masker, a substantial CMR (i.e., a decrease of masked threshold with increasing masker bandwidth) was found, whereas for the long-duration signals CMR was smaller. Furthermore, investigations were carried out to determine whether CMR changes when the bandwidth of the signals, consisting of bandpass impulse responses, is increased. The data indicate that substantial CMR remains even when all masker bands contain a signal component, thus minimizing across-channel differences. This finding is not in line with current models accounting for the CMR phenomenon. The second part of this paper concerns signal detection in spectrally shaped noise. Also investigated was whether release from masking occurs for the detection of a pure-tone signal at a valley or a peak of a simultaneously presented masking noise with a sinusoidally rippled power spectrum, when this masker was preceded and followed by a second noise (temporal flanking burst) with an identical spectral shape as the on-signal noise. Similar to CMR effects for temporal modulations, the data indicate that coshaping masking release (CSMR) occurs when the signal is placed in a valley of the spectral envelope of the masker, whereas no release from masking is found when the signal is placed at a peak of the spectral envelope of the masker. The implications of these experiments for measures of spectral and temporal resolution are discussed.

Frequency resolution for diotic and dichotic listening conditions compared using the bandlimiting measure and a modified bandlimiting measure

Two experiments were performed that examined the relation between frequency selectivity for diotic and dichotic stimuli. Subjects were eight normal-hearing listeners. In each experiment, a 500-Hz pure tone of 400-ms duration was presented in continuous noise. In the diotic listening conditions, a signal and noise were presented binaurally with no interaural differences (So and No, respectively). In the dichotic listening conditions, the signal or noise at one ear was 180 degrees out-of-phase relative to the respective stimulus at the other ear (S pi and N pi, respectively). The first experiment examined frequency selectivity using the bandlimiting measure. Here, signal thresholds were determined as a function of masker bandwidth (50, 100, 250, 500, and 1000 Hz) for SoNo, S pi No, and SoN pi listening conditions. The second experiment used a modified bandlimiting measure. Here, signal thresholds (So and S pi) were determined with a relatively narrow No band of masker energy (50 Hz wide) centered about the signal. Then, a second No narrow-band masker (30 Hz wide) was added at another frequency region, and signal thresholds were reestablished. The results of the two experiments indicated that listeners process a wider band of frequencies when resolving dichotic stimuli than when resolving diotic or monotic stimuli. The results also indicated that the bandlimiting measure may underestimate the spectral band processed upon dichotic stimulation. Results are interpreted in terms of an across-ear and across-frequency processing of waveform amplitude envelope.

Comodulation masking release as a function of bandwidth and time delay between on-frequency and flanking-band maskers

The threshold for a signal masked by a narrow band of noise centered at the signal frequency (the on-frequency band) may be reduced by adding to the masker a second band of noise (the flanking band) whose envelope is correlated with that of the first band, an effect called comodulation masking release (CMR). This paper examines CMR as a function of masker bandwidth and time delay between the envelopes of the on-frequency and flanking bands. The 1.0-kHz sinusoidal signal had a duration of 400 ms. The on-frequency band was presented alone (reference condition) or with the flanking band. The flanking-band envelope was either correlated or uncorrelated with that of the on-frequency band. Flanking-band center frequencies ranged from 0.25-2.0 kHz. The flanking band was presented either in the same ear as the on-frequency band (monaural condition) or in the opposite ear (dichotic condition). The noise bands had bandwidths of 6.25, 25, or 100 Hz. In the correlated conditions, the flanking-band envelope was delayed with respect to that of the on-frequency band by 0, 5, 10, or 20 ms. For the 100-Hz bandwidth, CMRs were small (typically less than 1 dB) in both monaural and dichotic conditions at all delay times. For the 25-Hz bandwidth, CMRs were about 3.5 dB for the 0-ms delay, and decreased to about 1.5 dB for the 20-ms delay. For the 6.25-Hz bandwidth, CMRs averaged about 5 dB and were almost independent of delay time. The results suggest that the absolute delay time is not the critical variable determining CMR. The magnitude of CMR appears to depend on the correlation between the envelopes of the on-frequency and flanking bands. However, the results do not support a model of CMR that assumes that signal threshold corresponds to a constant change in across-band envelope correlation when the correlation is transformed to Fisher's z.

NoSo and NoSπ detection as a function of masker bandwidth in normal-hearing and cochlear-impaired listeners

NoSo and NoS pi detection thresholds for a 500-Hz pure-tone signal were measured as a function of masking noise bandwidth in normal-hearing and cochlear hearing-impaired subjects. NoSo and NoS pi critical bands were derived from the bandlimited noise functions. A notched noise measure of the monaural critical band was also obtained for each ear. One hypothesis tested was that an asymmetrical monaural critical band would result in a relatively steep improvement of the NoS pi detection threshold as a function of decreasing masker bandwidth and would, therefore, be associated with a wider binaural critical band. This was hypothesized because the outputs of the left and right auditory filters would be more decorrelated the greater the interaural difference in the monaural critical band. However, as the noise bandwidth was narrowed, the decorrelation would lessen, resulting in a relatively steep improvement in NoS pi detection. Results indicated that the masking level difference (MLD) was smaller and that the monaural critical bands were generally wider in cochlear-impaired listeners. NoSo and NoS pi critical bands were somewhat larger in the cochlear hearing-impaired listeners having relatively wide monaural critical bands. There was a significant correlation between monaural critical band asymmetry and the NoS pi critical band; however, this correlation was insignificant when a control was employed for the critical band in the worse ear. Therefore, the present results did not support a strong association between monaural critical band asymmetry and the width of the NoS pi critical band.

Binaural signal detectability in noise amplitude‐modulated (NAM) noise as a function of masker bandwidth

Thresholds were determined for a 500‐Hz tonal signal in the presence of a noise amplitude‐modulated (NAM) noise, where the carrier was a wideband Gaussian noise and the modulator was a 20‐Hz‐wide noise band centered at 20 Hz. The NAM noise was filtered after modulation to produce a band centered on the 500‐Hz signal. Both diotic (So) and interaurally phase‐reversed (Sπ) signal thresholds were determined as a function of masker bandwidth (10–1000 Hz). When the masker was presented diotically, So thresholds (and to a lesser extent, Sπ thresholds) first increased, then decreased as a function of masker bandwidth, thus showing the effects of comodulation masking release at the wider bandwidths. The masking‐level difference (MLD, i.e., the difference between So and Sπ thresholds) increased as masker bandwidth decreased, as it did with a random (unmodulated) noise masker. When the masker was presented such that the noise carrier was diotic but the noise modulators were independent at the two ears, a sizable MLD still occurred (5–10 dB), but the MLD decreased with decreasing bandwidth. The decreased MLD at small bandwidths resulted from the higher Sπ thresholds with this stimulus than with a true diotic masker, which can be explained by the slowly fluctuating interaural intensity differences that this type of masker produces at narrow bandwidths. [Work supported by NIH.]

Comodulation masking release (CMR) as a function of masker bandwidth, modulator bandwidth, and signal duration.

These experiments examine how comodulation masking release (CMR) varies with masker bandwidth, modulator bandwidth, and signal duration. In experiment 1, thresholds were measured for a 400-ms, 2000-Hz signal masked by continuous noise varying in bandwidth from 50-3200 Hz in 1-oct steps. In one condition, using random noise maskers, thresholds increased with increasing bandwidth up to 400 Hz and then remained approximately constant. In another set of conditions, the masker was multiplied (amplitude modulated) by a low-pass noise (bandwidth varied from 12.5-400 Hz in 1-oct steps). This produced correlated envelope fluctuations across frequency. Thresholds were generally lower than for random noise maskers with the same bandwidth. For maskers less than one critical band wide, the release from masking was largest (about 5 dB) for maskers with low rates of modulation (12.5-Hz-wide low-pass modulator). It is argued that this release from masking is not a "true" CMR but results from a within-channel cue. For broadband maskers (greater than 400 Hz), the release from masking increased with increasing masker bandwidth and decreasing modulator bandwidth, reaching an asymptote of 12 dB for a masker bandwidth of 800 Hz and a modulator bandwidth of 50 Hz. Most of this release from masking can be attributed to a CMR. In experiment 2, the modulator bandwidth was fixed at 12.5 Hz and the signal duration was varied. For masker bandwidths greater than 400 Hz, the CMR decreased from 12 to 5 dB as the signal duration was decreased from 400 to 25 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Comodulation masking release (CMR) as a function of masker bandwidth, signal duration, and modulator bandwidth

Thresholds were measured for a 2000‐Hz signal masked by continuous noise varying in bandwidth from 50 to 3200 Hz in 1‐oct steps. For random noise maskers, thresholds increased with increasing bandwidth up to 400 Hz and then remained approximately constant. When the masker was amplitude modulated by a low‐pass noise, so as to produce coherent envelope fluctuations across frequency, thresholds decreased as the masker bandwidth was increased beyond 200 Hz, giving a CMR. For a 400‐ms signal duration, the CMR for masker bandwidths greater than 400 Hz increased from 2.4 to 12.3 dB as the modulator bandwidth was decreased from 400 to 12.5 Hz in 1‐oct steps. For modulator bandwidths of 50 Hz or less, a release from masking of 3.5 to 7.3 dB occurred even for maskers with bandwidths of 50 and 100 Hz, less than the critical bandwidth at 2000 Hz. For a modulator bandwidth of 12.5 Hz, the CMR decreased from 12.3 to 5.3 dB as the signal duration was decreased from 400 to 25 ms in 1‐oct steps. When the signal duration was less than 100 ms, there was no release from masking for masker bandwidths less than 400 Hz. The results suggest that, for maskers with fluctuating envelopes, across‐channel comparisons can reduce signal thresholds even for short signals, but an extra within‐channel process can produce a release from masking for long signals. This second process may reflect the ability of subjects to detect a change in the statistical properties of the envelope of the stimulus when the signal is added to the masker.

The effect of masker bandwidth on signal detectability

The effect of masker bandwidth on the detectability of narrow‐ and wideband signals was investigated. The signals were pure tones and noise bands. The noise bands varied from 50–792 Hz, center frequency being 1500 Hz. Stimuli were computer generated and the noise bands had nearly rectangular spectra. Log‐log plots of threshold as a function of masker bandwidth (Fletcher plots) were found to be well fitted by two line segments. An iterative least‐squares procedure was employed to determine slopes and intercepts of the line segments. These parameters were found to depend on the bandwidth of the signal. An energy detection model [Green, J. Acoust. Soc. Am. 32, 121–131 (1960)] was modified by the addition of internal variability that is bandwidth‐dependent. This modified energy‐detection model was able to predict the results obtained with narrow bandwidth signals. The relative magnitude of internal variability was estimated and the ratio of internal‐to‐external variability agreed with estimates presented by Raab and Goldberg [J. Acoust. Soc. Am. 57, 437–447 (1975)]. In contrast, the results obtained with wideband signals were not predicted by the simple model. They may reflect the operation of a mechanism for the enhancement of spectral contours.

Monaural and binaural auditory frequency resolution measured using bandlimited noise and notched-noise masking.

Several studies using bandlimited masking noise have indicated that NOSO frequency resolution is better than that for NOS pi. The present study examined NOSO and NOS pi frequency resolution with two different masking methods: bandlimited noise and notched noise. Noise spectrum levels of 10, 30, and 50 dB/Hz were used. Thresholds were determined for a 500-Hz signal, using a three-alternative forced-choice adaptive procedure, as a function of masker bandwidth and notchwidth. For NOSO presentation, 3-dB down points were comparable for the notched-noise and bandlimiting methods. For NOS pi presentation, 3-dB down points were generally greater for the bandlimiting method than the notched noise method. Furthermore, for NOS pi presentation, the 3-dB down estimate increased as noise level increased for the bandlimiting method, but stayed constant for the notched-noise method. It is suggested that the two masking methods measured different aspects of binaural processing.

Detection of binaural tones as a function of masker bandwidth.

Several experiments have shown that the difference in masked threshold between signals in the S0M0 and SπM0 conditions, the masking-level difference (MLD), increases markedly with reduction in masker bandwidth. However, there are two inconsistencies this result. First, apparently similar experiments conducted by different laboratories have produced quite different results. Second, the MLD-bandwidth relation appears to be nonmonotonic; with very narrow-band maskers (e.g., a pure tone), the MLD is often small. The results of the experiments reported here suggest that these problems arise from anomalies in the S0M0 data alone. It appears that in some narrow-band masking conditions the SπM0 threshold is spuriously low, resulting in deceptively small MLDs. The proposed explanation is that observers detect signal energy which falls outside the masker band, for when this energy is attenuated by filtering the signal, the S0M0 threshold rises dramatically. The SπM0 threshold is unaffected by this manipulation. With specially filtered signals, the nonmonotonicity of the MLD-bandwidth relation is considerably reduced, and MLDs with continuous pure-tone maskers are observed which are nearly as large as the largest narrow-band noise MLDs ever reported (28 dB).