A few years ago I wrote a post about the nature of letters b and v in Spanish. The post explains in general terms, the nature of the sounds of these two letters in Spanish in relation with the positions in which they are found in a word. It also refers, in general terms, to the symbols employed, using the International Phonetic Alphabet to represent their sounds. However, that post needs to spell out other aspects such as: the listing in more detail of the particular situations in oral Spanish, in which letters b and v change in sound; the particular way they are pronounced in those situations; as well as providing a brief analysis of how the nature of the written Spanish structures affect the choice of pronunciation patterns.
So in order to not to repeat the main points explained in the referred post, I quote them here:
Using the International Phonetic Alphabet symbols, the Spanish letters b (‘be’) and v (‘uve’) are both represented by either /b/ or /B/. Therefore you must notice that the letters b or v can sound either /b/ or /B/ depending on where they are found in a word. As a result of this, neither of the two letters can exclusively be represented by only one of these two phonetic symbols.
The letters b or v, are pronounced /b/ in words with structures as those found in vino, beso or banco. But when these two letters are found in words such as oveja, cabra or abeja, their pronunciation uses /B/. This sound doesn’t have a direct equivalent in English, so the student very often requires expert help from a language professional in order to achieve acceptable pronunciation ability.
In other words, what I’m saying in the paragraphs above can be also explained as follows:
1. The Spanish letters b and v can’t be represented by an exclusive phonetic symbol of the International Phonetic Alphabet, because neither b nor v, have a fixed or only sound – as explained in this post.
2. The sound of /b/ is like b used in English words such as bank, box or base. (For Spanish examples, see the words used above).
3. Generally speaking, letters b or v in initial positions are generally pronounced as b (represented phonetically as /b/), like in the examples just given in number 2. However, it must be said here, that because of the nature of the Spanish word structure, a word that in isolation can be pronounced with /b/, can change into a /B/, Most commonly when such a word is accompanied by articles: For example the word boca in isolation is usually pronounced as /’boKa/, but if we add the articles la, or una, i.e., la boca, una boca, they are pronounced (including the articles) as /la’Boka/, /una’Boka/, respectively, because now they follow the structure vowel+consonant+vowel, as explained in 4, below.
4. The sound of /B/ (which you must remember, can be applied in Spanish to either letters b or v) is approximately like a very soft /v/ – as pronounced in English. In general terms, in Spanish, when either letters b or v are placed in a cluster vowel+consonant+vowel, like in abanico or avión, b and/or v should be pronounced as /B/. When b and/or v, appear in combinations br+vowel, and bl+vowel, the sound used is also generally /B/: examples, brasa /’Brasa/, brincar /Brin’kar/.
5. Used in oral language /B/ can be so soft to the point that sometimes it may be quite difficult to be heard by a learner of Spanish as a second language. Also I must add that as I have repeatedly told my students, proper pronunciation of /B/ may be achieved by constant exposure to the way in which any native Spanish speaker utters it in daily natural language usage.
I must also add that a fuller understanding of the nature of the sounds to which I’m referring to here, cannot be achieved without a basic understanding of phonetics i.e., of the phonetic symbols contained in the International Phonetic Alphabet as applied to Spanish and maybe also to English.