Within-speaker variability; Between-language differences

Problems of definition

While most people seem to believe that they can judge how quickly someone is speaking, it is generally said that subjective judgements and opinions cannot serve as scientific evidence for statements about speech tempo; J. Laver has written that analyzing tempo can be "dangerously open to subjective bias ... listeners' judgements rapidly begin to lose objectivity when the utterance concerned comes either from an unfamiliar accent or ... from an unfamiliar language". Scientific observation depends on accurate segmenting of recorded speech along the time course of an utterance, usually using one of the acoustic analysis software tools available on the internet such as  or, specifically for speech research,  Speech Analyzer  or SFS.

Measurements of speech tempo can be strongly affected by pauses and hesitations. For this reason, it is usual to distinguish between speech tempo including pauses and hesitations and speech tempo excluding them. The former is called speaking rate and the latter articulation rate.

Various units of speech have been used as a basis for measurement. The traditional measure of speed in typing and Morse code transmission has been words per minute (wpm). However, in the study of speech the word is not well defined (being primarily a unit of grammar), and speech is not usually temporally stable over a period as long as a minute. Many studies have used the measure of syllables per second, but this is not completely reliable because, although the syllable as a phonological unit of a given language is well-defined, it is not always possible to get agreement on the phonetic syllable. For example, the English word 'particularly' in the form in which it occurs in dictionaries is, phonologically speaking, composed of five syllables /pə.tɪk.jə.lə.li/. Phonetic realizations of the word, however, may be heard as comprising five [pə.tɪk.jə.lə.li], four [pə.tɪk.jə.li], three [pə.tɪk.li] or even two syllables [ptɪk.li], and listeners are likely to have different opinions about the number of syllables heard.

An alternative measure that has been proposed is that of sounds per second. One study found rates varying from an average of 9.4 sounds per second for poetry reading to 13.83 per second for sports commentary. The problem with this approach is that the researcher must be clear as to whether the "sounds" s/he is counting are phonemes or physically observable phonetic units (sometimes called "phones"). As an example, the utterance 'Don't forget to record it' might in slow, careful speech be pronounced /dəʊnt fəget tə rɪkɔːd ɪt/, with 19 phonemes, each of which is phonetically realized. When the sentence is said at high speed it might be pronounced as [də̃ʊ̃ʔ fɡeʔtrɪkɔːd ɪt], with 16 units. If we are counting only units that can be observed and measured, it is clear that at faster speeds of utterance the number of sounds produced per second does not necessarily increase.