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.