Effect of temporal envelope smearing on speech reception

TLDR

The study investigated how smearing the temporal envelope affects speech‑reception thresholds and phoneme identification in normal‑hearing listeners. The authors applied low‑pass filtering to the amplitude envelope of speech split into 1/4, 1/2, or 1‑octave bands at cutoffs ranging from 0 to 64 Hz, and tested vowel and consonant recognition with nonsense syllables across 0–16 Hz cutoffs in 1/4‑octave bands. Results from 36 participants showed a severe drop in sentence intelligibility with narrow bands and low‑frequency envelope smearing (0–2 Hz), minimal benefit from modulation frequencies above 16 Hz, and SRTs that were independent of bandwidth above 4 Hz; in 24 participants, consonants were more affected than vowels, with stops most impaired and vowel errors mainly involving diphthongs and long‑short confusions.

Abstract

The effect of smearing the temporal envelope on the speech-reception threshold (SRT) for sentences in noise and on phoneme identification was investigated for normal-hearing listeners. For this purpose, the speech signal was split up into a series of frequency bands (width of 1/4, 1/2, or 1 oct) and the amplitude envelope for each band was low-pass filtered at cutoff frequencies of 0, 1/2, 1, 2, 4, 8, 16, 32, or 64 Hz. Results for 36 subjects show (1) a severe reduction in sentence intelligibility for narrow processing bands at low cutoff frequencies (0–2 Hz); and (2) a marginal contribution of modulation frequencies above 16 Hz to the intelligibility of sentences (provided that lower modulation frequencies are completely present). For cutoff frequencies above 4 Hz, the SRT appears to be independent of the frequency bandwidth upon which envelope filtering takes place. Vowel and consonant identification with nonsense syllables were studied for cutoff frequencies of 0, 2, 4, 8, or 16 Hz in 1/4-oct bands. Results for 24 subjects indicate that consonants are more affected than vowels. Errors in vowel identification mainly consist of reduced recognition of diphthongs and of confusions between long and short vowels. In case of consonant recognition, stops appear to suffer most, with confusion patterns depending on the position in the syllable (initial, medial, or final).