Alternatives to moments for characterizing fricatives: Reconsidering Forrest et al. (1988)

Means of characterizing acoustic signals of fricatives with a few parameters have long been sought. When Forrest, Weismer, Milenkovic, and Dougall [(1988) J. Acoust. Soc. Am. 84, 115-123] described their system of treating spectra as probability density functions and computing the first four spectral moments, others quickly adopted their clearly described method, although it did not distinguish /f/ and /θ/. Various problems with their method are described, including the lack of spectral averaging, the necessity of normalizing the amplitude, and correlation between pairs of moments. Even when these issues are rectified by alternative methods, the fact remains that moments are not ideal descriptors because they can only describe departures from the shape of a normal Gaussian distribution. Fricative spectra, particularly of non-sibilants, are often quite dissimilar in shape from Gaussians. Furthermore, shape descriptors do not lend themselves to direct inferences about the production variables that caused the acoustic effects. Here, alternative parameters are defined, it is shown how to adapt them to specific experimental conditions, and tests of efficacy are proposed. These parameters are strongly linked to the articulatory and aerodynamic variables that underlie fricative production.