He demonstrated that the frequency of sound was related to the perceived pitch by scraping a chisel across a brass plate causing a screech. The spacing of grooves in the scratch on the brass plate was related to the pitch. Sound is an example of a longitudinal or compressional wave which is the physicists way of saying that it is a wave that causes a disturbance parallel to the direction in which the wave is moving. By contrast light is a transverse wave in which the electromagnetic disturbance is perpendicular to the direction of travel of the wave.
Sound waves are a disturbance or vibration in a medium of solid, liquid, or gas . . .
Sound does not travel in a vacuum. The vibration, which can sometimes be seen, e.g., the vibration of a guitar string, may be amplified or modified, e.g., by the body of the guitar, and causes vibrations in the air which we pick up when they cause vibrations in the ear. The vibrating object causes alternate compressions and rarefactions in the air as it moves back and forth and these compressions and rarefactions travel through the air as a compressional wave. At ages over forty a typical human suffers a progressive decay in high frequency hearing of about 160 Hz per annum. When we hear ourselves speak we do so by air transmission of the sound from the mouth to the ear and also by bone transmission from the jaw to the ear and is the latter that is responsible for transmission of most of the low frequencies of the voice which decay more rapidly in air transmission.