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R A Lutfi

Publications and source records attributed to R A Lutfi.

35 records · Page 2Linked to original sources
PubMed1990

Informational processing of complex sound. II. Cross-dimensional analysis.

A series of experiments investigated listeners' ability simultaneously to process information across different acoustic dimensions. On each trial, the listener heard a pair of brief n-tone sequences (n = 1 to 12). The frequency, intensity, and duration of each tone in the sequence varied randomly from trial to trial. On average, the values of these three parameters were greater for one sequence, the target, than the other, the nontarget. The listener's task was to identify the target on each trial. For an ideal observer in this task, d' performance grows as the square root n. Obtained d' grew at a rate slightly less than the square root of n. Close to cube root of n growth was observed when the average difference occurred in only one of the three tone parameter values within a block of trials. Although performance fell short of ideal, optimum weights were consistently given to each tone and each parameter. The results are consistent with a model in which performance depends predominantly on the information content of the sounds regardless of how the information is "packaged" in the stimulus. Transmitted information is estimated to be 0.9-2.0 bits within a single acoustic dimension, 2.1-3.0 bits when distributed across dimensions.

Adult↗
PubMed1989

Informational processing of complex sound. I: Intensity discrimination.

This paper reports on some initial experiments using the sample discrimination paradigm to investigate normal-hearing listeners' ability to process information in complex, nonspeech sounds. An important feature of the sample discrimination experiment is that the value of the difference to be discriminated randomly varies from trial to trial. It is this variation that yields potential information. In the present study, listeners heard a pair of multitone complexes (or sequences) on each trial. The individual levels of the tones were drawn from two normal distributions differing only in mean. The listener's task was to identify the sound having the higher mean tone level. For an ideal observer in these experiments, performance in d' grows as the square root n, where n is the number of tones. Obtained d' grew more nearly as the cube root of n regardless of whether the tones were played sequentially or simultaneously or whether they were increased in number from high frequencies to low or from low frequencies to high. A preliminary model is proposed in which discrimination performance depends predominantly on the information content of the sounds and is largely independent of the physical dimensions along which the sounds vary. Information content is defined in terms of the variance of the underlying stimulus distributions and a stimulus equivocation factor that is derived from the data. Based on this model, transmitted information is estimated to be between 1.0 and 2.6 bits.

Acoustic Stimulation↗
PubMed1988

Complex interactions between pairs of forward maskers.

The present study was conducted to determine to what extent the combined effects of two forward maskers can be predicted from addition of their individual effects. The maskers were 50-Hz wide noise bands with center frequencies ranging from 1.8 to 2.2 kHz. The signal was a brief, 2.0-kHz tone burst. When the maskers were gated on and off together, the combination produced sometimes more and sometimes less masking than predicted depending on the particular pair and the relative amounts of masking produced by the individual members of the pair. The greatest discrepancy occurred, however, when the masker pair was presented simultaneously with the signal or when the forward maskers were presented in sequence. In the latter case, the obtained threshold exceeded the predicted threshold by as much as 34 dB.

Adolescent↗
PubMed1988

Interpreting measures of frequency selectivity: is forward masking special?

In a previous article [Lutfi, J. Acoust. Soc. Am. 76, 1045-1050 (1984)], the following relation was used to predict measures of frequency selectivity obtained in forward masking from measures obtained in simultaneous masking: F(g) = G + H(g) - H(0), where, for a given masker level, F is the amount of forward masking (in dB) as a function of signal-masker frequency separation (g), H is the amount of simultaneous masking, and G is the amount of forward masking for g = 0. In the present study, the relation was tested for a wider range of signal and masker frequencies, masker levels, and signal delays. The relation described thresholds from all conditions well with the inclusion of one free parameter lambda corresponding to a constant frequency increment, F(g) = G + H(g + lambda) - H(lambda). The parameter lambda was required to account for observed shifts in the frequency of maximum forward masking. It is argued that a single tuning mechanism can account for commonly observed differences between simultaneous- and forward-masked measures of frequency selectivity.

Adult↗
PubMed1987

Masking additivity in the hearing-impaired.

The purpose of the present study was to investigate how the effects of maskers combine in hearing-impaired listeners. The levels of two narrow bands of noise separated in frequency were selected so that each band individually produced the same amount of masking of a sinusoidal signal. Thresholds for the signal were then measured in the presence of both bands combined. For normal-hearing listeners, the effect of combining maskers was nonlinear, but, additive. A cube-root transformation of the masking produced by the individual and combined maskers preserved an additive relation among the effects of these maskers. This type of nonlinear additivity also characterized the results from the hearing-impaired listeners when the maskers were restricted to a region of normal sensitivity. In contrast, the effect of combining maskers was linear, additive without transformation, when the maskers were restricted to a region of significant loss. In many instances, the auditory filter model of masking may describe the detection performance of hearing-impaired listeners better than that of normal-hearing listeners.

Adult↗
PubMed1986

Two- versus four-tone masking, revisited.

Canahl [J. Acoust. Soc. Am. 50, 471-474 (1971)] measured thresholds for a 1.0-kHz sinusoid masked either by two or by four surrounding tones. He reported four-tone masked thresholds that exceeded, by 5-7.5 dB, the energy sum of the masking produced by the individual tone pairs. The present paper reports on a series of experiments investigating the effects of several factors on this 5-7.5 dB "excess" masking. In each experiment, thresholds for a 1.0-kHz 250-ms sinusoid were measured as a function of the overall level of two or four equal amplitude sinusoids with frequencies arithmetically centered around 1.0 kHz. For conditions similar to those of the Canahl experiment, 5-6 dB of excess masking was obtained independent of the level of the masking tones. Randomly varying overall level across presentations had no effect on the excess masking. The excess masking was reduced or eliminated when the masking tones were generated using an amplitude modulation technique, when they were gated on and off with the signal, or when their waveshapes were fixed across trials. Canahl's result may reflect listeners' ability to detect the signal as a change in the waveshape of the multitone masker.

Attention↗
PubMed1985

A power-law transformation predicting masking by sounds with complex spectra.

In a previous paper [R. Lutfi, J. Acoust. Soc. Am. 73, 262-267 (1983)], the following rule was proposed for predicting masking by pairs of simultaneous maskers; Xab = [XPa + XPb]1/P, where in units of power, Xa and Xb are the individual masking effects of the maskers, Xab is the combined effect, and 0.20 less than or equal to p less than or equal to 0.33. In this paper, the rule is used to predict the results of studies in the literature that have measured masking by sounds with various other complex spectra. In most of these studies, the individual maskers comprising the complex have nominally nonoverlapping power spectra. A single value of p = 0.33 yields predictions in good agreement with the data of these studies. For a study in which the component maskers overlap more appreciably, a larger value of p = 0.50 produces equally accurate predictions. The rule also predicts some general features of the results of studies in which the individual effects of the maskers in the complex are not known but can be estimated. It is suggested that the general applicability of the rule reflects a conjoint analysis by the auditory system of two or more waveform statistics.

Acoustic Stimulation↗
PubMed1984

On the growth of masking asymmetry with stimulus intensity.

Masking asymmetry was investigated over a wide range of stimulus intensities for two signal frequencies, fo = 1.0 and 4.0 kHz, using both fixed-masker and fixed-signal paradigms. The masker was a notched noise with the upper and lower edges of the notch, fu and fl, respectively, placed asymmetrically about fo. For various notch widths, the asymmetry of masking was measured as the difference between the masked threshold obtained when fl was nearer fo and that obtained when fu was nearer fo. For maskers with wide notches, (fu - fl)/fo greater than 0.15, masking asymmetry changed with stimulus level; at the highest level, masked threshold was greatest when fl was nearer fo, and, at the lowest level the asymmetry reversed slightly for fo = 1.0 kHz so that masked threshold was actually greater when fu was nearer fo. Nonparallel growth of masking functions reveal changes in masking asymmetry with signal level as well as with masker level. It is concluded that the nonlinear growth of masking with level is due primarily to changes in the auditory filter, rather than changes in the detector following the filter.

Auditory Threshold↗
PubMed1984

Predicting frequency selectivity in forward masking from simultaneous masking.

Measures of frequency selectivity from forward masking suggest sharper tuning than those from simultaneous masking. To account for this result, various interpretations involving additional tuning mechanisms have been proposed. In the present study, it is shown that a simple multiplicative relation between on-frequency forward and off-frequency simultaneous masking predicts this result quite well. The relation assumes that changes in masking produced by separating the masker from the signal in frequency, in time, and in both frequency and time are related to one another by Weber's law. The accuracy of the predictions suggests that the limits of auditory frequency selectivity are already established in simultaneous masking and that special interpretations involving additional tuning mechanisms are not required to account for the difference between simultaneous and forward measures. Implications for a dB scale of masking are discussed.

Adolescent↗
PubMed1983

Additivity of simultaneous masking.

Simultaneous masking functions (signal level at threshold versus masker level) were obtained for equally intense maskers presented individually and in pairs. The signal was a 2.0-kHz sinusoid. The pairs of maskers were (1) two sinusoids with frequencies 1.9 and 2.1 kHz, (2) two narrow bands of noise (50 Hz wide) centered at 1.9 and 2.1 kHz, (3) two narrow bands of noise (50 Hz wide) centered at 1.8 and 1.9 kHz, and (4) the 1.9-kHz sinusoid combined with the narrow band of noise centered at 2.1 kHz. The pairs of maskers produced anywhere from 10 to 17 dB of masking beyond that predicted from the simple sum of the masking produced by the individual maskers. The amount of this "additional masking" was independent of masker level. Adding a continuous low level background noise reduced the amount of additional masking only slightly (approximately 5 dB). The data are consistent with a model in which the effects of the maskers are summed after undergoing independent compressive transformations.

Acoustic Stimulation↗
PubMed1983

Simultaneous masking and unmasking with bandlimited noise.

This study examines how simultaneous masking of a tone by bandlimited noise may be affected by nonlinear interactions among spectral components of the noise. Simultaneous masking patterns (signal threshold versus signal frequency) were obtained with three types of maskers: (A) a narrow-band noise, 50 Hz wide with variable center frequency fv, (B) pairs of narrow-band noises, each band 50 Hz wide with center frequencies fl and fu, and (C) wide-band noise formed by filling the spectral gap between the two bands of (B). The variable frequency fv was set to 1.0, 1.1, 1.2, and 1.3 kHz: fl was fixed at 1.0 kHz, and fu had values of 1.1, 1.2, and 1.3 kHz. In most conditions, the two-band maskers and the wideband maskers produced more masking than would be predicted from the masking produced by the single narrow-band maskers. For certain signal frequencies below the maskers, adding noise to fill the spectral gap of the two-band masker actually resulted in a 3- to 15-dB release from masking. These results reveal factors that may operate to confound modern measures of frequency selectivity.

Acoustic Stimulation↗
PubMed1983

Detecting a repeated tone burst in repeated noise.

Samples of wideband noise 0.05, 0.1, 0.2, or 0.4 s in duration were digitized and then replayed cyclically to produce repeated-noise maskers. The signal was a repeating tone burst (0.4 or 1.6 kHz). It was half the duration of the noise sample, centered in the noise temporally, and it was repeated at the same point in each repetition of the noise. In the antiphasic conditions of the experiment, either the noise sample or the tone burst was inverted in alternate repetitions of the masker; in the homophasic conditions both the tone burst and noise, or neither, were inverted in alternative repetitions. If the auditory system were capable of storing detailed waveforms of sufficient length, alternate repetitions could be added or subtracted and we might expect a release from masking in the antiphasic conditions. The results show a small but significant advantage for the antiphasic conditions when the signal frequency was 0.4 kHz, but no difference with the 1.6-kHz signal.

Adult↗
PubMed1982

The phase angle of addition in temporal masking for diotic and dichotic listening conditions.

The phase angle, alpha, between a tonal signal and a tonal masker was varied from 0 degrees to 135 degrees in simultaneous masking, forward masking, and pulsation-threshold paradigms. In all conditions the frequency of the signal and masker was 500 Hz. For forward masking both diotic (MOSO) and dichotic (MOS pi) listening conditions were investigated. Only the dichotic case was studied using the pulsation threshold method. In simultaneous masking, thresholds varied as a function of alpha in both diotic and dichotic conditions. Thresholds in the diotic conditions were consistently different from those in the dichotic conditions -- i.e., there were masking-level differences (MLDs) at most values of alpha tested. In forward masking and pulsation-threshold, however, thresholds were independent of alpha in the dichotic conditions; and thresholds were independent of alpha in the diotic, forward masking conditions. Nevertheless, for forward masking the dichotic thresholds remained below the diotic thresholds, yielding MLDs of 3-6 dB. Thus, in nonsimultaneous masking, there is a clear effect of the interaural signal phase, but not of the masker-signal phase relationship, on signal detectability. These results imply that masker-signal phase information is either not preserved or not used by subjects in nonsimultaneous tone-on-tone masking experiments.

Auditory Perception↗
PubMed1982

Off-frequency listening and masker uncertainty.

This experiment was designed to determine if masker uncertainty can reduce the effects of off-frequency listening on psychophysical tuning curves (PTCs). PTCs were determined in simultaneous masking with a 1-kHZ signal at 12 dV SPL, using an adaptive two-alternative forced-choice procedure. In a given run the center frequency on the narrow-band noise masker was either fixed, or varied randomly between up to eight values distributed equally above and below the signal frequency. In a control condition off-frequency listening was restricted using a "notched" broadband noise added to fixed-frequency masker. The notch was 200 HZ wide, centered at the signal frequency, and the spectrum level in the passband was - 8 dB SPL/Hz. The "restricted listening" PTCs were considerably broader than the "traditional" PTCs, but uncertainty about masker frequency had little effect. It appears that prior knowledge of masker frequency is not necessary for off-frequency listening to be effective.

Auditory Threshold↗
PubMed1980

Two-tone unmasking in the forward-masking procedure: suppression or perceptual cueing?

Unmasking in the forward-masking procedure was investigated in two experiments. In the first, signal frequency (fs) was held equal to the frequency of the primary masker component (f1) while the frequency of a secondary masker component f2 (where f2 > f1) was varied. In the second experiment, f1 and f2 were held constant while signal frequency varied. The results are discussed in terms of the effects potential perceptual cues may have on unmasking in the forward-masking procedure.

Cues↗