Adjectives expressing the ability of a thing to be perceived using a sense are often derived from a root that differs from that of the corresponding verb. For example:
touch – tangible (not “touchable”; see below)
Something that can in principle be touched is tangible, not touchable. However, in various related meanings, “touchable” is possible (for example, if you have a tablet, there are several touchable elements on your screen). Note also that “untouchable” is used figuratively to refer to something or someone that cannot be criticized, punished, or changed.
The situation is slightly more complicated for smell and taste:
taste – gustable (but “tastable” would be more universally understood)
Both “gustable” and “tastable” are very uncommon words, so it is probably better to describe the state in a different way (e.g. instead of “it is tastable”, you can say “it has a discernible taste” or “it is possible to taste it”). Finally, the sense of hearing is probably the most common source of mistakes: