I am developing my aural skills and in Ron Gorow's Hearing and Writing music it is suggested that learning to sing an interval will help internalise it.
Is there an iOS app I can sing into which will tell me what interval I just sang or played?
If you know your musical alphabet well enough to know what note a 3rd/4th/5th/etc above/below your starting note is, then a simple tuner will do. Sing your first note and then try to sing an interval and see how you do.
A tuner will also demonstrate how close you are to a note and how much you wobble around.
A free piano app (basically a touch-screen keyboard) can be used too. Sing your first note from a key, then sing and hold the second note and see what note on the keyboard it matches.
If you really insist in using an app for that, I suppose you could use a guitar tuning app, at least to some extent. Adjacent strings of a guitar have intervals of perfect fourth and major third, depending on the strings.
But personally I think a stringed acoustic instrument would work well, if not the best (Gorow suggests acoustic instruments, too, for the drills he suggests in his book).
A couple of possible reasons I can think of are:
And some extra points in favor of guitar, against a piano (in my experience):
There is an app that does exactly this. It is called Singers Studio by Erol Studio.
Check out the interval singing app here: https://www.risingsoftware.com/apps/