Acoustic Impedence

Acoustic Impedance is an idea about how sound is transmitted from one medium to another. The more closely matched to objects are in their impedences the more sound will be transmitted through. A medium's acoustic impedance is measure by the density and the speed which sound travels through it. The human ear membrane also has an acoustic impedence. I wonder how much this limits our hearing? How many sound waves hit only to bouce away, never to trigger the auditory nerve?