Human hearing is limited to a range of between 20 to 20,000 cycles per second. Sound at a vibration rate of greater than 20,000 cycles per second is called ultrasound. Other mammals can hear ultrasound, some such as whales approach 100,000 cycles per second. A sound is characterized by its frequency and intensity. The frequency of a sound contributes to its pitch and is measured by counting the number of cycles per second in the vibration.
Intensity is a measure of loudness.
The pressure waves of sound are represented by the advancing concentric lines radiating away from the vibrating source. Middle C has a frequency of 256 cycles per second, while upper C has a frequency of 512 cycles per second. The intensity of a sound is a measure of its loudness and reflects how tightly packed the molecules of air become during the compression phase of a sound wave. The ear can detect sounds where the vibration of the air at the ear drum is less than the diameter of a hydrogen molecule. The ear has the ability to discriminate intensities over a 100,000 fold difference in energy.