The Rhythm of Lexical Stress in Prose

The Rhythm of Lexical Stress in Prose Doug Beeferman School of Computer Science Carnegie Mellon University Pittsburgh, PA 15213, USA dougb+@cs, cmu. edu Abstract "Prose rhythm" is a widely observed but scarcely quantified phenomenon. We de- scribe an information-theoretic model for measuring the regularity of lexical stress in English texts, and use it in combination with trigram language models to demon- strate a relationship between the probabil- ity of word sequences in English and the amount of rhythm present in them. We find that the stream of lexical stress in text from the Wall Street Journal has an en- tropy rate of less than 0.75 bits per sylla- ble for common sentences. We observe that the average number of syllables per word is greater for rarer word sequences, and to normalize for this effect we run control ex- periments to show that the choice of word order contributes significantly to stress reg- ularity, and increasingly with lexical prob- ability. 1 Introduction Rhythm inheres in creative output, asserting itself as the meter in music, the iambs and trochees of poetry, and the uniformity in distances between objects in art and architecture. More subtly there is widely be- lieved to be rhythm in English prose, reflecting the arrangement of words, whether deliberate or sub- conscious, to enhance the perceived acoustic signal or reduce the burden of remembrance for the reader or author. In this paper we describe an information-theoretic model based on lexical stress that substantiates this common perception and relates stress regularity in written speech (which we shall equate with the in- tuitive notion of "rhythm") to the probability of the text itself. By computing the stress entropy rate for both a set of Wall Street Journal sentences and a ver- sion of the corpus with randomized intra-sentential word order, we also find that word order contributes significantly to rhythm, particularly within highly probable sentences. We regard this as a first step in quantifying the extent to which metrical properties influence syntactic choice in writing. 1.1 Basics In speech production, syllables are emitted as pulses of sound synchronized with movements of the mus- culature in the rib cage. Degrees of stress arise from variations in the amount of energy expended by the speaker to contract these muscles, and from other factors such as intonation. Perceptually stress is more abstractly defined, and it is often associated with "peaks of prominence" in some representation of the acoustic input signal (Ochsner, 1989). Stress as a lexical property, the primary concern of this paper, is a function that maps a word to a sequence of discrete levels of physical stress, approx- imating the relative emphasis given each syllable when the word is pronounced. Phonologists distin- guish between three levels of lexical stress in English: primary, secondary, and what we shall call weak for lack of a better substitute for unstressed. For the purposes of this paper we shall regard stresses as symbols fused serially in time by the writer or speaker, with words acting as …

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