Acoustic structure of consonants

1. Revision of basic concepts

a) The source-filter model; quasi-periodic and aperiodic (noise) sources.

b) The filtering effect of the vocal tract, acting as a variable resonator.

c) The characterisation of vowel sounds by formants.

d) Other resonant sounds are also characterised by formants: sonorant consonants i.e. nasals, medial and lateral approximants. Obstruents - stops, fricatives and affricates - are characterised by a combination of intervals of noise, silence, and changing formant transitions.

2. Stops

a) In articulation, stops consist of:

b) Because of (ii), all stops have a period of silence. If the stop is fully voiced (e.g. some intervocalic voiced stops in English), there will be a low level of quasiperiodic energy during the closure.

c) The burst is very brief, with energy across the spectrum. Unreleased stops do not show a burst, of course.

d) Aspiration is a longer interval of noise, with a broad-band frequency distribution that varies with the place of articulation of the stop.

e) Prevocalic stops have a formant frequency pattern which changes in time for an interval after the release of the stop, during the early part of the vowel - the transition.

f) Postvocalic stops have similar (but 'mirror-image') formant frequency transitions at the end of the preceding vowel, as the stop closure is formed.

For the most part, (b-d) are acoustic realizations of the manner of articulation; (e-f) realize the place of articulation.

2.1. Acoustic details

a) Voicing. A low frequency voicing buzz is present in phonetically voiced sounds, seen on spectrograms as 'voice bar'. Also, in English, presence vs. absence of aspiration cues voicelessness vs. voicing, respectively.

b) Place of articulation. There are three major acoustic correlates:

c) Post-vocalic stops: In post-vocalic position (VC sequences) the trajectories are the opposite of those in pre-vocalic stops, although these may be less clearly visible spectrograms in the case of voiceless stops.

3. Fricatives and affricates

a) Main features:

b) Voicing. Again, phonetically voiced fricatives have low level voicing along with frication noise. Phonemically voiceless fricatives may have short period of aspiration before voicing onset in some languages.

c) Place of articulation. Two major acoustic features carry place distinctions:

d) [h] is acoustically a voiceless vowel. It has a weak formant structure appropriate to following vowel, but with noise excitation rather than voicing.

e) Affricates have frication portions similar to the corresponding fricatives, preceded by stop-like 'silent' portion.

4. Nasals

a) As a class nasals share certain features. They are nearly always voiced, and they show a sudden change in formant structure from/to adjacent vowel (a 'fault transition').


b) Place of articulation. Combination of formant, antiformant and transition information are all used in recognition, but transition information is apparently the most important.

5. Liquids

a) Liquids are normally voiced (though they are sometimes devoiced or realised as voiceless fricatives e.g. following a voiceless obstruent) - hence clear formant structure, though with less energy than vowels.

b) Laterals have antiformants, though usually less strong than in nasals. For example, in /l/ there is one around 2500 Hz, between the apparent F2 and F3.

c) In both [ɹ] and [l], F1 and F2 are lower than in adjacent vowels: in [l], and laterals in general, transitions very sudden (cf. nasals); in [ɹ], less so.

d) In [ɹ], F3 also falls from adjacent vowels; in [l] this is much less pronounced.

e) Clear and dark /l/ differ in their formant structure; F2 is somewhat higher - about 1500 Hz - for a clear /l/ and lower for a dark /l/.

f) Trills are characterised acoustically by a 'pattern of pulses of closures and openings' (Lindau, 1986). Their spectral structures vary considerably.

Further reading

Fry, D.B. The Physics of Speech, esp. Chapters 10 & 11.

Johnson, K. (1997) Acoustic and Auditory Phonetics. Chapters 6-8.

Ladefoged, P. (993) A Course in Phonetics. Chapter 8

Lindau, M. (1985) The story of /r/. In V. Fromkin, ed. Phonetic Linguistics

Olive, Greenwood and Coleman (1993) Acoustics of American English Speech