Physiology of the Ear

The pinna; the depression at the entrance to the auditory canal is the concha. Check Google for images.
The auditory canal is approx. 25mm to 35mm long from the concha to the tympanic membrane.
The pinna helps us to locate sounds, and also enhances certain frequencies. The concha acts as a resonant cavity.
The auditory canal has a resonant frequency of around 4kHz.
The tympanic membrane is a thin, highly elastic structure. It converts acoustic pressure variations into mechanical vibrations to the middle ear. Google has some revolting photos…

The mechanical vibrations of the tympanic membrane are transmitted to three small bones called the ossicles; the malleus, incus and staples (otherwise known as the hammer, anvil and stirrup).
One end of the staples (the staples footplate) is connected to the oval window of the cochlea; the oval window is the start of the inner ear.
The ossicles transfer mechanical movement from the tympanic membrane to the cochlea.
The ossicles act as a mechanical 'impedance converter' due to the lever effect of the malleus and incus, and the area difference between the tympanic membrane and the staples footplate. Check Google for images.
The middle ear protects the inner ear from loud sounds. Two muscles (the tensor tympani and the stapedius muscle) contract in response to sounds greater than approximately 75 dB (SPL). This increases the impedance of the ossicular chain and protects the inner ear to a certain extent. Frequencies below 1kHz can be attenuated by between 12 to 14 dB. This is the acoustic reflex. The reflex takes between 60ms to 120ms; this means that loud impulses such as a gunshot are too fast to be attenuated by the acoustic reflex.

The inner ear is the cochlea (have a look on Google). It's function is to convert mechanical vibrations into nerve firings.
The cochlea is a tube coiled into a spiral. The oval and round windows are at the base; the other end is the apex.

The scala vestibule and scala tympani are filled with perilymph, an incompressible fluid.
Movement at the staples footplate and oval window moves the perilymph within the cochlea. This displaces the round window and causes movement of both Reissner's membrane and the basilar membrane.
The basilar membrane carries out frequency analysis of sounds. It is narrow and thin at the base end, becoming wider and thicker by the apex. The narrow and thin end responds best to high frequencies, moving to lower frequencies along its length.
The basilar membrane has a number of hair cells along its length called the organ of corti. These hair cells trigger nerve firings when displaced by input sounds. The nerves from the hair cells form a spiral bundle known as the auditory nerve. Again, Google has some diagrams.