intensity
A friend whispers to you in class in a voice so soft that you have to lean very close to hear what hes saying. Later that day, your friend shouts to you across the football field. Now his voice is loud enough for you to hear him clearly even though hes many meters away. Obviously, sounds can vary in loudness. Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of sound. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel dB .