Sonority has to do with how much acoustic energy a given speech sound has. Linguists divide speech sounds into three broad categories, vowels, consonants, and glides, according to their sonority.
Remember that there are three steps involved in producing speech sounds. The process starts with respiration as air flows up from the lungs. Phonation occurs at the larynx, where the vocal folds may or may not vibrate to produce voicing, and then we use our mouth, jaw, lips, teeth and tongue to shape the sound, which is called articulation.
In phonetics, we classify sounds according to how they’re produced, and also according to the acoustic properties of the sounds. The primary acoustic property that we’re interested in is called sonority. Sonority has to do with the amount of acoustic energy that a sound has. A simple example of this is that a loud sound is more sonorous and a quiet sound is less sonorous. But sonority is not just about loudness. Sounds that are made with lots of airflow from the lungs, and with vocal folds vibrating, are sonorous sounds. Sounds that have less airflow or don’t have voicing from the vocal folds have less sonority. Those two pieces of information, sonority and articulation, allow us to group sounds into three broad categories
We produce vowels with the vocal tract quite open and usually with our vocal folds vibrating so vowels have a lot of acoustic energy: they’re sonorous. Vowel sounds can go on for a long time: if you’re singing, when you hold the note, you hold it on the vowels. Make some vowel sounds and notice how you can hold them for a long time: “aaaaa iiiii uuuuu”.
The sounds that we call consonants are ones where we use our articulators to obstruct the vocal tract, either partially or completely. Because the vocal tract is somewhat obstructed, less air flows from the lungs, so these sounds have less energy, they’re less sonorous, and they’re usually shorter than vowels. Consonant sounds can be voiced or voiceless.
There’s also an intermediate category called glides that have some of the properties of vowels and some of the consonants. The vocal tract is unobstructed for glides, like for vowels, but they are shorter and less sonorous than vowels. We’ll learn more about glides when we take a closer look at vowels.
This acoustic notion of sonority plays a role in every language of the world because spoken words are organized around the property of sonority. Every single spoken word is made up of one or more syllables. You probably know that a syllable is like a beat in the rhythm of the word, so you know that ball has one syllable, basket has two syllables, and bicycle has three.
But what is a syllable, in phonetic terms? A syllable is a peak of sonority that is surrounded by less sonorous sounds. What that means is that a syllable is made up of a vowel, or some other very sonorous sound, with some sounds before it and after it that are less sonorous, usually glides and consonants. The most sonorous sound, the peak of sonority, is called the nucleus of a syllable.
Looking back at those words, we can see that the word ball contains the sonorous vowel sound [ɑ], with two less-sonorous consonants, [b] and [l] on each side of it. Likewise, basket has two vowel sounds [æ] and [ɪ], with the consonants [b] before the first syllable, [sk] between the two vowels, and [t] after the second vowel. Can you figure out what the vowel and consonant sounds are in the word bicycle? Remember that written letters don’t necessarily map directly onto speech sounds!