Pronunciation of "have" in "I don't have to" [do something]

Normally when I say "I don't have to do that" (meaning I'm not obliged to), I find that as well as putting heavy stress on the word "have", I pronounce if haff.

Is this common? If so, why does the trailing consonant change from "v" to "f"? Are there any other contexts where this or a similar change occurs?

Edit: It's starting to look like have to in the sense of "must" is almost a completely different verb to the standard have. That allows it to conjugate differently, which explains why in "I had to to that", I often say hat.

Edit2: We seem to have identified have, use, and suppose as verbs that can undergo significant changes in pronunciation when coupled with 'to' to indicate a special meaning (the required, habitual, and requested/ordered senses). Per @PPL's comments, there's also the ought/owe pair, and various (often dialectal) changes with some forms of go, want and get. Any more?

Solution 1:

This is very common — it’s almost universal in standard English. For instance, Merriam-Webster gives the pronunciation of have as

\ˈhav, (h)əv, v; in “have to” meaning “must” usually ˈhaf\

It’s an example of the general phenomenon of voicing/devoicing. Many consonant sounds come in “voiced/unvoiced” pairs: (z / s), (d / t), (v / f), and so on. In English (as in many other languages), there are a lot of situations where a consonant which would normally be voiced is not, or vice versa. For instance:

• d in missed is pronounced as its unvoiced counterpart t;
• s in dogs is pronounced as its voiced counterpart z;
• v in have to is pronounced as its unvoiced counterpart f.

In all these cases, the change in voicing comes from the consonant immediately preceding or following the one in question. In the case of have to, this is slightly more surprising, since as written, the vt is not a consonant cluster but split across two separate words; roughly, the reason for the devoicing here is that have to acts (both grammatically and in pronunciation) almost like a single word, hafta. See the links below for more on this point.

The detailed rules governing voicing assimilation (and related phenomena) are ridiculously complex and fascinating (as ever: English pronunciation is a mess!), but Wikipedia is of course a good starting point.

Edit: what I said above applies when have to is being used in the sense of must, like in the OP’s example. In other senses, eg in I will give everything I have to charity, the /v/ of have would not usually be devoiced, or at least not nearly so strongly.

Edit again: There’s an excellent discussion of this particular case, hafta, here (by John Lawler). There are also a few mentions in passing Language Log.

Solution 2:

This is an example of assimilation, where one sound modifies a bit to be more like a nearby sound.

Another example (from wikipedia) is 'handbag' which in non-careful speech is pronounced [hæmbæg], the 'n' changing to 'm' to be more like the 'b'.