Movement of people, animals, machines, and other things cause pressure waves in the air. If these pressure waves are within a certain range of frequencies, our ears interpret them as sound. Pressure waves or sound waves are similar to waves created by dropping a stone in a pond of water. A large stone causes higher waves. Similarly, loud sounds are caused by larger pressure waves.
Frequency is the number of pressure waves occurring per second.
A bass violin produces low frequency sound waves, whereas, a high note of a violin produces a high frequency sound. Sound is transferred to the ear by sound waves emitted from a vibrating source such as a tractor. The outer ear funnels sound waves through the ear canal, where they strike the eardrum. The vibration of the eardrum is transmitted to the nerve of the inner ear by three small bones which make up the middle ear. The sensitive nerve of the inner ear converts the vibrations to nerve impulses.