oPhysics: Interactive Physics Simulations

Sound Waves

Sound waves in air are longitudinal waves. The velocity of all waves depends on the medium, so for sound waves in air the speed depends on characteristics of the air. In dry air at 20 degrees Celsius, the speed of sound is 343 meters per second. At higher temperatures sound travels faster and at lower temperatures it is slower.

Sound waves can move through air and other gases, but sound waves can also move through solids and liquids. Sound actually moves much faster through solids and liquids than it does through gases. This is due to the fact that liquids and solids are much more dense, allowing the longitudinal wave to pass from particle to particle much more quickly.

Animals can detect sounds with a wide range of frequencies. Young humans typically are capable of detecting sounds with frequencies from about 20 Hz to around 20,000 Hz. This can vary somewhat from person to person. As people age they generally become less sensitive to the high end of this range. The word pitch is used to describe the perceived frequency of a sound wave. High pitch means high frequency, low pitch means low frequency. Some animals can hear higher frequencies than humans (like those produced by a dog whistle that humans can not detect), while others, like elephants, communicate with frequencies below the human hearing range. Sound frequencies above 20,000 Hz are referred to as ultrasonic, while those below 20 Hz are called infrasonic.

The amplitude of wave is closely related to the energy associated with the wave. For sound, large amplitudes mean high intensity sounds, which our ears perceive as being loud. The source of a sound wave puts out a particular amount of power, the rate of energy released. For an ideal source, this energy spreads equally in all directions. This energy is spread over a larger and larger area as the sound moves away from the source. The intensity of a sound wave at any point is the amount of power delivered per amount of area (unit: watt/meter2). Sound intensity follows an inverse square relationship. If one person is twice as far from a sound source as another the closer person will receive sound with four times the intensity.

Loudness is a perception and is not directly related to sound intensity. The decibel is a unit used to measure sound level, which in turn is related to perceived loudness. The decibel scale is logarithmic. Zero decibels is defined as the quietest sound that a typical human can detect. Every increase of ten decibels (one bel) is equal to a tenfold increase in sound intensity.

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