For humans, hearing is normally limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz), although these limits are not definite. The upper limit generally decreases with age.

In a human, sound waves funnel into the ear via the external ear canal and hit the eardrum (tympanic membrane). Consequently, the compression and rarefaction of the wave set this thin membrane in motion, causing the middle ear bones (the ossicles: malleus, incus and stapes) to move. The vibrations of the ossicular chain displace the basilar fluid in the cochlea, causing the hairs within it, called stereocilia, to vibrate. Hairs line the cochlea from base to apex, and the part stimulated and the intensity of stimulation gives an indication of the nature of the sound. Information gathered from the hair cells is sent via the auditory nerve for processing in the brain.
The number of sound pressure level vibrations (sonic waves) per second denotes the frequency. Infrasonic (below hearing), sonic (aural), and ultrasonic (above hearing) frequencies are measured in Hertz (Hz); one Hertz is one cycle wave (or singular pressure wave in audionics) per second. Humans have a maximum aural range that begins as low as 12 Hz under ideal laboratory conditions,[3] to 20 kHz[note 1] in most children and some adults. The range shrinks during life, usually beginning at around age of eight with the upper frequency limit being reduced. Women typically experience a lesser degree of hearing loss than men, with a later onset. Men have approximately 5 to 10 dB greater loss in the upper frequencies by age 40.[4][5] Inaudible infrasonic sound waves can be felt by humans through physical body vibration in the range of 4 to 16 Hz.

OFF-TOPIC: You can get the sound waves to resonate with the fluids in your eyeballs, and distort vision and cause blindness and stuff.

So 20,000 is as high as you want to go, as NeXT-Generation said.