Continuing yesterday’s theme of sounds that can be challenging to pronounce, today we look at the voiceless dental fricative /θ/. This sound is usually written th in English and appears in such words as three [θriː], thought [θɔːt] and thin [θɪn]. In the Spanish of Spain it’s written c (when followed by i or e), as in cien [θien], and z, as in Zaragoza [θaragoθa]. It’s also used in a number of other languages, including Greek – the Greek letter θ (Θήτα/theta) represents this sound in the IPA (International Phonetic Alphabet).
The voiced version of this sound is represented in the IPA by the letter ð (eth), which comes from Old English. This sound is written th in English and appears in the [ðə] and though [ðoʊ]. In Welsh it’s written dd; in Icelandic ð; in Albanian, Cornish and Swahili dh, and in Greek δ (Δέλτα/delta). In Spanish d can be pronounced /ð/ when it comes between two vowels, as in nada ['naða].
These sounds are fairly rare among the world’s languages and can be tricky for speakers of languages which don’t use them. They also tend to be the last sounds acquired by native speakers of English, according to this page.
There’s an explanation of how to pronounce the voiceless dental fricative here. You do the same and just let vocal folds vibrate to pronounce the voiced version.
I didn’t learn how to pronounce the voiceless dental fricative properly until the age of 23, when a Taiwanese friend who was studying English at university explained it to me. Before that, I wasn’t aware that there was a difference between three and free – I pronounced both with /f/ at the beginning. I still have to make a conscious effort to pronounce this sound sometimes and tend to slip back to the /f/ sound when not concentrating.
I often pronounce the voiced dental fricative as /v/ when it’s not at the beginning of a word, for example in brother. However, since starting to learn Welsh, which makes quite a lot of use of dental fricatives, my ability to pronounce them has improved. Combinations of /ð/ and /v/ can also trip me up, as in swyddfa [sʊɨðva].