All sound, from the harsh clanging of a bell to the soothing notes of a violin, is the result of vibration. When a surface oscillates (moves back and forth), it emits waves into the surrounding air (or into whatever medium, such as water, that surrounds it). These waves are called “compression waves.” By way of analogy, they can be likened to the waves created when you toss a pebble into a pond. However, unlike water waves on the surface of a pond, which spread outward in two dimensions, compression waves travel in three dimensions. If you could see sound waves, they would look like expanding spheres moving away from the vibrating object.

One important characteristic of sound waves is their frequency, the number of waves produced by a vibrating source per second. Scientists measure frequency in units called hertz. One hertz equals one cycle (vibration, or sound wave) per second. Humans can hear sounds that are between about 20 and 20,000 hertz. If an object is vibrating at frequencies outside of this range, our ears cannot detect the sound.

Whether a sound is high-pitched like a whistle or low-pitched like a cello depends on its frequency. The frequency of sound waves is related to their wavelength, the distance between the peaks of the waves. The higher the frequency of sound waves, the shorter their wavelength, and the higher their pitch. Conversely, the lower the frequency of sound waves, the longer their wavelength, and the lower their pitch.

Two other important characteristics of sounds are their intensity and loudness. The intensity of a sound is a measure of how forcefully the object vibrates. Scientists measure the intensity of sounds in units called decibels. On the decibel scale, every increase of 10 decibels represents a tenfold increase in wave power. For example, the sound of an ordinary conversation is typically about 60 decibels; heavy city traffic can generate noises as loud as 85 decibels; the sound level in a crowded nightclub can reach 105 decibels; a typical rock concert is about 110 decibels; and a jet taking off emits about 135 decibels.

(Source: How stuff works?)