Pitch and loudness are subjective aspects of sound which can be described in terms of the observed abilities of subjects to rate them on a scale from ‘low’ to ‘high’. Timbre is a subjective aspect of sound for which there is no such scale and neither qualitative nor quantitative descriptions are generally found that are widely accepted. The purpose of this paper is to shed light on some frequency domain aspects of the nature of timbre by making use of the results obtained from an analysis system which is designed to take advantage of contemporary psychoacoustical knowledge relating to human peripheral hearing. Results are presented which illustrate the relationship between contemporary psychoacoustic ideas relating to timbre and ideas first discussed by Helmholtz and later taken up by other researchers. Analyses by the system of a selection of sounds from acoustic musical instruments with clear timbral differences are also presented in order to place these discussions in a musical context.