What is the difference in pronunciation between the ‘th’ sounds, theta and ð, at the end of words?
Answered by Peggy Tharpe
They are the same sound basically. The only difference is the airiness or soft hissing that comes with ‘theta’ (‘th’ represented by an oval with a line across the middle), and the voicing that comes with ð (called ‘delta’). (My software and website won’t allow me to use the ‘theta’ symbol so I’ll be writing out that word).
Strictly speaking, theta should be accompanied by a flow of air that exits through the lips around the tongue tip, making it a softer sound than ð. However, American English speakers often don’t bother with the air. They often substitute the ð sound for the airy theta sound—and there’s a reason for this.
American English has a default setting preferential to voicing–vocal vibrations in the throat. ð has voicing, theta does not, so American speakers use ð, whenever they can.
Vowels are voiced. Most consonants are voiced. And voicing is the tool we use to form liaisons, the links between words that help create English rhythm. So American English speakers like voicing. and therefore may substitute ð for theta when possible.
A bit more info: there is a small difference in how we hold our tongues for theta and ð. Since the theta sound requires space for air to flow around the tongue and through the teeth, we keep our tongue tip looser and don’t touch our teeth with it, which makes a larger gap for air to escape. However for ð, no air management is required. We touch the back of our upper teeth with the tongue tip to make the sound.