Acoustics of consonants

Fricatives

While the spectrum of a fricative will be unpredictable for any given instant, if we look at the average amplitudes over time, we find patterns that allow us to tell the fricatives apart.

• the strident fricatives, [s] and [], have higher overall amplitudes than non-strident fricatives like [f] and [].
• the average spectrum for [s] has more amplitude at higher frequences than [] has.

The average spectrum is often used to give visual feed-back during the treatment of lisping.

Oral stops

Since the closure phases of all voiceless oral stops are identical, in order to tell vowels apart on a spectrogram we have to pay attention to the properties of the approach phase and the release phase (just like listeners have to).

There are two main audible cues for the stop's place of articulation:

1. the release burst: there is a split-second of turbulence after a stop is released. The average spectrum of this release burst looks like that of the fricative with the same place of articulation. For example, [t] on a spectrogram will appear as a blank space followed by a very brief burst of static concentrated at higher frequencies just like [s].
2. formant transitions: as the vocal tract moves from its position for the consonant to its position for a following vowel, there are very brief influences on the formants at the beginning of the vowel. There are also influences on the formants at the end of a preceding vowel.

This is why unreleased stops are harder to identify. Since they have no release burst, the listener must rely only on the subtle influence of the consonant on the formants of the preceding vowel.

Voiced oral stops will look much like voiceless stops on spectrograms, except that:

• because there is some vocal cord vibration, there will some visible darkness in the low frequencies at the bottom of the spectrogram (often called a "voice bar")
• the release burst will be less pronounced