Acoustic and linguistic factors in the perception of bandpass-filtered speech

Speech can remain intelligible for listeners with normal hearing when proce ssed by narrow bandpass filters that transmit only a small fraction of the audible spectrum. Two experiments investigated the basis for the high intel ligibility of narrowband speech. Experiment I confirmed reports that everyd ay English sentences can be recognized accurately (82%-98% words correct) w hen filtered at center frequencies of 1500, 2100, and 3000 Hz. However, nar rowband low predictability (LP) sentences were less accurately recognized t han high predictability (HP) sentences (20% lower scores), and excised narr owband words were even less intelligible than LP sentences (a further 23% d rop). While experiment 1 revealed similar levels of performance for narrowb and and broadband sentences at conversational speech levels, experiment 2 s howed that speech reception thresholds were substantially (>30 dB) poorer f or narrowband sentences. One explanation for this increased disparity betwe en narrowband and broadband speech at threshold (compared to conversational speech levels) is that spectral components in the sloping transition bands of the filters provide important cues for the recognition of narrowband sp eech, but these components become inaudible as the signal level is reduced. Experiment 2 also showed that performance was degraded by the introduction of a speech masker (a single competing talker). The elevation in threshold was similar for narrowband and broadband speech (11 dB, on average), but b ecause the narrowband sentences required considerably higher sound levels t o reach their thresholds in quiet compared to broadband sentences, their ta rget-to-masker ratios were very different (+23 dB for narrowband sentences and -12 dB for broadband sentences). As in experiment 1, performance was be tter for HP than LP sentences. The LP-HP difference was larger for narrowba nd than broadband sentences, suggesting that context provides greater benef its when speech is distorted by narrow bandpass filtering. (C) 2001 Acousti cal Society of America.