Grouping

Grouping of Components of Sound Mixtures

One issue that has been addressed by researchers concerns the relationships between the components of a sound mixture that cause listeners to fuse them together so as to perceive a single tone and those that cause listeners to perceive two or more simultaneously sounding tones. One cue that listeners employ is termed harmonicity. When a note is played on an instrument such as a violin or a flute, or when a note is sung, many frequency components are produced. These are termed harmonics, and they are whole number multiples of the lowest frequency component, which is termed the fundamental frequency. In line with this, listeners fuse simultaneous tones that stand in harmonic relation into a single tone, and they perceive a pitch that corresponds to the fundamental frequency. Furthermore, the simpler the ratio between the frequencies that combine to form the complex tone, the more likely it is that listeners will fuse them perceptually.

Another cue that listeners employ concerns the relative onsets of the components of a sound mixture. Components that begin at the same time are [Page 523]likely to have arisen from the same source, and those that begin at different times are likely to have arisen from different sources. Accordingly, listeners tend to fuse together components of a sound mixture whose onsets are synchronous, and to separate out those that begin at different times.

Composers have exploited these grouping principles. For example, in polyphonic music different melodic lines are played in parallel, and it is important that each line be clearly heard. Interestingly, Johann Sebastian Bach in his polyphonic keyboard works tended to avoid simultaneous tones that stand in simple harmonic relation, and to avoid tones with simultaneous onsets. Other composers such as Claude Debussy and Maurice Ravel have produced impressionistic effects in their orchestral pieces by creating combinations of tones that are likely to fuse perceptually.

Grouping of Sequences of Tones

Researchers have also explored how listeners group tones that are presented in sequence. One cue that is used here is pitch proximity. Sounds that are close in pitch are more likely to be coming from the same source than are sounds in different pitch ranges. Accordingly, listeners tend to link together tones that are close in pitch and to separate out those that are further apart. This effect is particularly strong when tones are presented at a rapid tempo. Instead of hearing a single stream of connected tones, listeners hear two melodic lines in parallel, one corresponding to the lower tones and the other to the higher tones. This perceptual effect, termed pseudopolyphony, was exploited by composers of the baroque era, such as Bach and Georg Philipp Telemann, and excellent examples occur in Bach's cello suites. The effect is also exploited in classical and romantic guitar music, such as composed by Francisco Tarrega and Augustin Barrios.

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