Upon being launched into the scala vestibuli by the oval window, sound waves travel to the end of the cochlea, and return through the scala timpani, where they cause vibrations of the round window (see Figure 3). In the process, however, they also cause the basilar membrane to vibrate in a specific way: sound energy is said to be organized tonotopically along the basilar membrane. This means that energy occupying a given spectral range is said to cause displacement of a corresponding region of the BM. In other words, a sinusoidal sound of a given frequency will cause a corresponding peak vibration at a unique place on the BM. The effect of different BM vibrations for sounds of various spectral contents is demonstrated by an online animation.
