Wordnet 3.0
ADJECTIVE (1)
1. 
 impossible or difficult to perceive by the mind or senses; 
- Example: "an imperceptible drop in temperature"- Example: "an imperceptible nod"- Example: "color is unperceivable to the touch"[syn: imperceptible, unperceivable]
The Collaborative International Dictionary of English v.0.48:
Unperceivable \Unperceivable\
   See perceivable.
WordNet (r) 3.0 (2006):
unperceivable
    adj 1: impossible or difficult to perceive by the mind or
           senses; "an imperceptible drop in temperature"; "an
           imperceptible nod"; "color is unperceivable to the touch"
           [syn: imperceptible, unperceivable] [ant:
           perceptible]