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Sound waves cause our eardrums, tiny bones within our ears, and the cochlea to vibrate. Specialised cells in the cochlea sense vibrations and cause electrical signals to be sent to the brain along a nerve. The brain creates the sensation of hearing a sound when it receives signals from the ear through a nerve. Human ears can detect sound waves with frequencies between 20 Hz and 20 kHz, being most sensitive to 2–5 kHz waves. Sound waves below 20 Hz are called infrasound; above 20 kHz, ultrasound. They are not detected by human ears.