Spin a toothed wheel against a flexible card. At low speeds, individual impacts sound like separate clicks. As the wheel speeds up, the clicks join together and create a musical pitch.
Around 1830, French physicist Félix Savart refined the toothed-wheel experiment to investigate how people perceive sound. He demonstrated that slowly repeated impacts are heard as separate clicks, but when the impacts occur rapidly and regularly, they blend into a continuous musical pitch.
Savart's experiments showed that pitch depends on the number of vibrations or impacts produced each second. Increasing the wheel speed or using a wheel with more teeth raises the frequency and therefore raises the pitch.