Perception of complex tone pairs mistuned from unison.

Periodic sounds mistuned from unison may interact to produce pitch glides: When a broad-spectrum complex tone having a fundamental frequency of 400 Hz or less and containing several harmonics above the 8th is mixed with itself after a slight change in the waveform repetition frequency (1 Hz or less), listeners hear a rising glissando when corresponding portions of the waveforms approach alignment and a falling glissando as they recede from alignment. Glissandi are unimpaired if harmonics below the 8th are absent, but if, instead, harmonics above the 8th are removed, only amplitude fluctuations are heard (not glissandi). When two broad-spectrum complex tones with independent, randomly derived phase spectra are mistuned slightly from unison and mixed, complex repeated patterns other than glissandi are heard. These observations, along with others involving a variety of periodic sounds mistuned from unison, provide information concerning the nature of frequency domain and time domain mechanisms employed for the perception of iterated acoustic patterns.

Illusory continuity of interrupted speech: speech rate determines durational limits.

Deleted segments of speech can be restored perceptually if they are replaced by a louder noise. An earlier study of this "phonemic restoration effect" found that, when recorded discourse was interrupted periodically by noise, the durational limit for illusory continuity corresponded to the average word duration. The present study employed a different passage of discourse recorded by a different speaker. Durational limits for apparent continuity of discourse interrupted by noise were measured at the normal (original) playback speed, as well as at rates that were 15% greater and 15% less. At the normal playback rate, once again the limit of continuity approximated the average word duration--but of especial interest was the finding that changes in playback rate produced proportional changes in continuity limits. These results, together with other evidence, suggest that phonemic restorations represent a special linguistic application of a general auditory mechanism (auditory induction) producing appropriate syntheses of obliterated sounds, and that for discourse the limits of illusory continuity correspond to a fixed amount of verbal information, and not a fixed temporal value.

Auditory induction: reciprocal changes in alternating sounds.

When portions of a sound are replaced by a potential masker, the missing fragments may be perceptually restored, resulting in apparent continuity of the interrupted signal. This phenomenon has been examined extensively by using pulsation threshold, auditory induction, and phonemic restoration paradigms in which two sounds, the inducer and the inducee, are alternated (ABABA...), and the conditions required for apparent continuity of the lower amplitude inducee are determined. Previous studies have generally neglected to examine concomitant changes produced in the inducing sound. Results from the present experiments have demonstrated decreases in the loudness of inducers using inducer/inducee pairs consisting of tone/tone and noise/noise, as well as the noise/speech pairs associated with phonemic restorations. Interestingly, reductions in inducer loudness occurred even when the inducee was heard as discontinuous, and these decreases in loudness were accompanied by graded increases in apparent duration of the inducee, contrary to the conventional view of auditory induction as an all-or-none phenomenon. Under some conditions, the reduced loudness of the inducer was coupled with a marked alteration in its timbre. Especially profound changes in the inducer quality occurred when the alternating stimuli were tones having the same frequency and differing only in intensity--it seems that following subtraction of components corresponding to the inducee, an anomalous auditory residue remained that did not correspond to the representation of a tone.

Detection of acoustic repetition for very long stochastic patterns.

Guttman and Julesz (1963) employed recycling frozen noise segments (RFNs) as model stimuli in their classic study of the lower limits for periodicity detection and short-term auditory memory. They reported that listeners can hear iteration of these stochastic signals effortlessly as "motorboating" for repetition periods ranging from 50 to 250 msec and as "whooshing" from 250 msec to 1 sec. Both motorboating and whooshing RFNs are global percepts encompassing the entire period, as are RFNs in the pitch range (repetition periods shorter than 50 msec). However, with continued listening to whooshing (but not motorboating) RFNs, individuals hear recurrent brief components such as clanks and thumps that are characteristic of the particular waveform. Experiment 1 of the present study describes a cross-modal cuing procedure that enables listeners to store and then recognize the recurrence of portions of frozen noise waveforms that are repeated after intervals of 10 sec or more. Experiment 2 compares the relative saliencies of different spectral regions in enabling listeners to detect repetition of these long-period patterns. Special difficulty was encountered with the 6-kHz band of RFNs, possibly due to the lack of fine-structure phase locking at this frequency range. In addition, a similarity is noted between the organizational principles operating over particular durational ranges of stochastic patterns and the characteristics of traditional hierarchical units of speech having corresponding durations.

Spectral redundancy: intelligibility of sentences heard through narrow spectral slits.

The intelligibility of word lists subjected to various types of spectral filtering has been studied extensively. Although words used for communication are usually present in sentences rather than lists, there has been no systematic report of the intelligibility of lexical components of narrowband sentences. In the present study, we found that surprisingly little spectral information is required to identify component words when sentences are heard through narrow spectral slits. Four hundred twenty listeners (21 groups of 20 subjects) were each presented with 100 bandpass filtered CID ("everyday speech") sentences; separate groups received center frequencies of 370, 530, 750, 1100, 1500, 2100, 3000, 4200, and 6000 Hz at 70 dBA SPL. In Experiment 1, intelligibility of single 1/3-octave bands with steep filter slopes (96 dB/octave) averaged more than 95% for sentences centered at 1100, 1500, and 2100 Hz. In Experiment 2, we used the same center frequencies with extremely narrow bands (slopes of 115 dB/octave intersecting at the center frequency, resulting in a nominal bandwidth of 1/20 octave). Despite the severe spectral tilt for all frequencies of this impoverished spectrum, intelligibility remained relatively high for most bands, with the greatest intelligibility (77%) at 1500 Hz. In Experiments 1 and 2, the bands centered at 370 and 6000 Hz provided little useful information when presented individually, but in each experiment they interacted synergistically when combined. The present findings demonstrate the adaptive flexibility of mechanisms used for speech perception and are discussed in the context of the LAME model of opportunistic multilevel processing.

Spectral restoration of speech: intelligibility is increased by inserting noise in spectral gaps.

In order to function effectively as a means of communication, speech must be intelligible under the noisy conditions encountered in everyday life. Two types of perceptual synthesis have been reported that can reduce or cancel the effects of masking by extraneous sounds: Phonemic restoration can enhance intelligibility when segments are replaced or masked by noise, and contralateral induction can prevent mislateralization by effectively restoring speech masked at one ear when it is heard in the other. The present study reports a third type of perceptual synthesis induced by noise: enhancement of intelligibility produced by adding noise to spectral gaps. In most of the experiments, the speech stimuli consisted of two widely separated narrow bands of speech (center frequencies of 370 and 6,000 Hz, each band having high-pass and low-pass slopes of 115 dB/octave meeting at the center frequency). These very narrow bands effectively reduced the available information to frequency-limited patterns of amplitude fluctuation lacking information concerning formant structure and frequency transitions. When stochastic noise was introduced into the gap separating the two speech bands, intelligibility increased for "everyday" sentences, for sentences that varied in the transitional probability of keywords, and for monosyllabic word lists. Effects produced by systematically varying noise amplitude and noise bandwidth are reported, and the implications of some of the novel effects observed are discussed.