2.1 Sibilants

/θ/ and /s/ are called sibilant fricatives because of the hissing noise that characterises them. Although they are contrastive in some varieties of Spanish, distinguishing words like casa (house) – caza (hunt) and sien (temple) – cien (hundred), it is not worth insisting that learners of Spanish also make the distinction as most of the Spanish-speaking world doesn’t either. Where the distinction is made, the actual pronunciation of /s/ is something like the English ‘sh’ sound (she), while in most other areas it is very similar to an English ‘s’ (see). 

In much of the Hispanic world, /s/ in syllable coda weakens and is pronounced as an [h] sound or simply deleted; esto[ehto] (this). In many places this is more characteristic of colloquial speech than formal speech. Areas where /s/-weakening is absent are the north of Spain, the highlands of Central America and the Andean region. 

In those varieties where /s/ in syllable coda is preserved, it often assimilates in voice to the following voiced consonant. This happens within the word: mismo [ˈmizmo] (same), esbelto [ezˈβelto] (thin) and across a word-boundary: los niños [lozˈniɲos] (the children). Note that [z] in these cases might be less “strong” and less voiced than zed in English. Vowels, although they are fully voiced, do not trigger voicing assimilation except in Spanish in Catalonia and some varieties of Ecuadorian Spanish.