In real-world acoustic environments, constructive and destructive interference occurs constantly due to room acoustics and other factors. In fact, interference between the sound source and reflected waves is key to producing standing waves—good for instrument sound production, bad for listening environments. Sometimes taking a small step in any direction may completely change the timbral characteristic of a sound because it alters the phase relationship of the source and its reflections. Stereo microphone pairs that are improperly placed can inadvertently lead to unwanted phase cancellations at certain frequencies.
Two sounds with a small difference in frequency, say two piano strings of the same pitch, may be perceived as a single sound, but as the waves evolve, they move slightly in and out of phase with each other. The resulting constructive and destructive interference produces a pulsation of amplitude. This pulsation is known as beating. The rate of pulsation, or beat frequency, is the difference in frequencies, so frequencybeats = absolute (frequency1 - frequency2). A string tuned to 440 Hz and another tuned to 441 Hz will produce a pulsation once per second (pictured below).
Use the interactive example below to create beats for yourself. Turn on the 440 Hz sine wave, then turn on the 441 Hz tone. This will produce one beat per second (441 - 440 = 1). Turn off the 441 Hz tone and click on the 442 Hz tone. This will produce two beats per second (442 - 400 = 2). What happens when you activate all three tones? NB: As of this writing, the HTML5 audio player may produce clicks as it loops.
For further study, see Hyperphysics->Interference, Hyperphysics->Beats