We investigated the extent to which durational cues to word boundaries are present in spontaneous speech. Spontaneous speech of North American English was elicited in a production experiment, with target phrases embedded in articles provided to participants. Each pair of target phrases only differed in the placement of word boundaries, e.g. beef#eater vs. bee#feeder. We examined the duration of: (1) the pivot consonant at the juncture (e.g. [f] in [bi:fiɾɚ]), (2) the pre-juncture section (e.g. [bi:] in [bi:fiɾɚ]), and (3) the post-juncture section (e.g. [iɾɚ] in [bi:fiɾɚ]), to see how these durations can signal word boundaries. We found no evidence for word-final lengthening in our study. However, similar to boundary-related lengthening found in laboratory read speech, word-initial lengthening was found in spontaneous speech, which could potentially serve as an important cue to word segmentation.

Studies of lab speech have identified several acoustic variables that listeners use to identify word boundaries (e.g. allophonic variation, segment duration), however spontaneous speech may not contain the same acoustic signals. Using data from a recent study we tested whether disambiguating information is also available to listeners under more typical conditions. 15 pairs of target phrases only differing in the placement of word boundaries, e.g., grey#day vs. grade#A were elicited under read (146 tokens) and spontaneous speech conditions (316 tokens) from multiple talkers who were not aware of the ambiguities. Phrases were played in isolation to 30 Native English listeners in a 2AFC segmentation task (e.g. grey day vs. grade A). Accuracy was above chance for both read (80.9%) and spontaneous speech (73.1%). Accuracy varied according to the critical consonant type and was highest for voiceless stops followed by nasals, voiced stops, fricatives and clusters. Only two items (pierced ears vs. peer steers and beef eater vs. bee feeder) were at chance for spontaneous speech. Measures of word-initial lengthening were good predictors of performance but listeners outperformed predictions from these measures indicating they weren’t the only cues.