Wells, A study of the formants of the pure vowels of British English
In Anthony's words , "it is very often not possible to derive the formant frequencies with any degree of accuracy by the Sectioning method used in the Sound Spectrograph."
It is hoped that in the near future it may be possible to carry out much more accurate studies on formants by means of a new technique called variously inverse filtering, anti-filtering, zeroing, or spectrum matching [27; 2; 12 p.8ff].
In this technique, first suggested by Dunn, a set of filters is used which have a frequency characteristic which is the inverse of that of the vocal cavities. These filters can be adjusted as regards frequency and bandwidth so as to cancel out the effects on the larynx tone of the supraglottal cavities.
An interesting by-product of this technique is that the residual signal, after the formants have been zeroed out, may give information about the characteristics of the vocal source which would otherwise be impossible to acquire.