Which of the following is not a sound wave characteristic?

The frequency of a sound wave is the parameter that the ear interprets as pitch. A higher frequency sound has a higher pitch, and a lower frequency sound has a lower pitch. The amplitude of a sound wave determines its loudness or volume. A larger amplitude indicates a louder sound, while a smaller amplitude indicates a softer sound. It is the differences that allow us to tell them apart. Timbre is associated with the shape of sound waves or waveforms produced by them.

Sound wave characteristics include amplitude, wavelength, and frequency. The brightness of a sound wave is not a characteristic. As a result, option 4 is correct.