a foghorn producing a low-pitched, penetrating signal of two tones.
2.
Phonetics.
a.
a phoneme in one dialect corresponding to a similar but phonetically different phoneme in a related dialect.
b.
a group of sounds comprising all the phonetically different dialectal variants of a given phoneme in a language: The broad a and flat a of “half” are members of a single diaphone.
[dahy-uh-fohn] 
əˌfoʊn