What is the rule for using 'a' or 'an' before words starting with 'b'?

What is the rule for using 'a' or 'an' before words starting with 'b'?

The rule for using 'a' or 'an' before words starting with 'b' is to use 'a' before words starting with a consonant sound and 'an' before words starting with a vowel sound.