Sound waves traveling through the air or other mediums sometimes affect the objects that they encounter. Recall that sound is caused by the molecules of a medium vibrating. Frequency refers to the number of vibrations that an individual particle makes in a specific period of time, usually a second. The frequency of a wave is different than the speed of a wave. Frequency refers to how often a wave passes through a certain point, while speed refers to how fast a wave passes through the point.
Particles vibrate at a specific frequency for each source, called its natural frequency. Steel, brass, and wood all have different natural frequencies. Occasionally, objects vibrating at their natural frequencies will cause resonance. Resonance is when objects with the same natural frequency as the vibrating source also begin to vibrate. Resonance does not happen very often and only affects object close to the vibrating source. Sometimes, the effects of resonance can be powerful. A singer can make glass vibrate enough to shatter, just by singing a note with the glasss natural frequency!
