intangible, impalpable -- (incapable of being perceived by the senses especially the sense of touch; "the intangible constituent of energy"- James Jeans)
impalpable -- (imperceptible to the senses or the mind; "an impalpable cloud"; "impalpable shadows"; "impalpable distinctions"; "as impalpable as a dream")
impalpable -- (not perceptible to the touch; "an impalpable pulse")
(Sound)
