Pitch drift in 'a cappella' choral singing

Seaton, Richard (2019). Pitch drift in 'a cappella' choral singing. PhD thesis The Open University.


Choral singing is a very popular activity in the United Kingdom (UK) with an estimated 2.14 million people, most of whom are likely to be amateur singers, regularly taking part in ensemble singing in over 40,000 choirs across the country. Despite the popularity of choral singing, most of the research into pitch drift (i.e. any difference between the pitch of the initial and final pitch of chords of the song) when performing unaccompanied choral music (i.e. 'a cappella') has concentrated on the internal effects of the music rather than external influences on the ensemble of singers.
Whilst singing regularly in amateur choirs for the past thirty years the author had observed that the performances of 'a cappella' works varied considerably from rehearsal to rehearsal with respect to the degree to which the pitch drifted. At some rehearsals the pitch drift would be insignificant, at others it would be intolerable.
This thesis opens with an introduction to the research question and background theory. This is followed by a review of the academic literature on choral singing. The results of an international survey of choral practitioners plus a series of interviews and correspondence with professional musicians and composers will be presented. The outcomes of this survey informed the development of a subsequent series of experiments. These experiments were implemented to determine the effects specific external variables might have on the measured pitch drift of a cohort of eleven amateur choirs recruited to the research. These choirs performed and recorded the same two pieces of music at every rehearsal over an extended period. The data collected included:
• recordings of 192 rehearsal performances of each of two songs,
• the temperature, humidity and atmospheric pressure at each rehearsal,
• the attendance pattern of each singer over the period,
• an acoustic analysis of the rehearsal venues.
Additionally, every singer was invited to take part in a survey to determine their ability to discriminate between two closely spaced pitches.
The results from the measurements made from recordings of the a cappella performances confirm that the degree of pitch drift does vary irregularly from rehearsal to rehearsal. There was some variation between the choirs with several only drifting flat during performances whereas others tended to drift sharp on some occasions and flat on others. There was no correlation between the number of singers in each choir and the tendency to drift in pitch. Furthermore, choirs that auditioned their singers did not necessarily enjoy a lesser degree of pitch drift.
There was evidence from some choirs that the variation in attendance experienced at rehearsals may affect pitch drift. However, the changes in the environmental factors from rehearsal to rehearsal, such as temperature, humidity and barometric pressure, did not appear to be contributory factors to irregular pitch drift. Moreover, no correlation could be determined between the degree of pitch drift experienced by a choir and the acoustic parameters of their rehearsal space.
Whilst the pitch discrimination abilities of the singers taking part in the survey showed them to be no better or worse than those of the general population, an inconsistency in their responses between sharpened and flattened tones was discovered and has yet to be explained.

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