In linguistics, a surface filter is a type of sound change that operates not at a particular point in time but over a longer period.
That example, however, still illustrates the importance of surface filters in preserving the phonological structure of words within the language.
One very common example of a surface filter is final-obstruent devoicing in which a voiced obstruent at the end of a word is automatically converted to their unvoiced counterpart.
If that were a regular sound change, the devoicing would occur only at a particular point in time, and any new words that entered the language later might end in voiced obstruents.
The more-faithful rendering *putijaz was not permitted since the short vowel u was followed by a single consonant t. The Germanic spirant law affected combinations of an obstruent that was followed by -t-.
Sometimes, sound changes occur that directly violate a surface filter, which may cause it to cease operating.