Human ears are capable of perceiving an extraordinarily wide range of changes in loudness, the tiniest audible sound being about 1 trillion times less intense than a sound loud enough to cause the ear pain. The loudness or intensity of a noise is measured in a unit called the decibel. The softest audible sound to humans is 0 decibels, while painful sounds are those that rise above 140 decibels.

 Besides loudness, the human ear can detect a sound’s pitch, which is related to a sound’s vibration frequency, or the number of sound waves passing into the ear in a given period. The greater the frequency, the higher the pitch. The maximum range of human hearing includes sound frequencies from about 15 to about 18,000 waves, or cycles, per second.

 Because the human ear cannot hear very low frequencies, the sound of one’s own heartbeat is inaudible. At the other end of the scale, a highly pitched whistle producing 30,000 cycles per second is not audible to the human ear, but a dog can hear it.

 The third characteristic of sound detected by the human ear is tone. The ability to recognize tone enables humans to distinguish a violin from a clarinet when both instruments are playing the same note. The least noticeable change in tone that can be picked up by the ear varies with pitch and loudness.

 Another sonic phenomenon, known as masking, occurs because lower-pitched sounds tend to deafen the ear to higher-pitched sounds. To overcome the effects of masking in noisy places, people are forced to raise their voices.