When two sine waves of slightly different frequencies (eg f1 and f2) are played on a loudspeaker, the waves will undergo interference. This means that by the time the sound reaches your ears, the waves will have physically added together and this superposition results in a wave at a frequency of (f1+f2)/2 which beats at a frequency of f2-f1.
If the two tones are played through headphones (one frequency in the left ear, another in the right ear) each ear receives a pure tone and no physical interference can occur. Amazingly, the majority of people will still perceive a beating sound which is a purely psychological effect and is known as a "binaural beat".
Use the Binaural Beat Generator below to investigate this effect yourself, and feel free to add your favourite frequency combinations to the comments section. Please note, headphones must be used for the effect to work (you will hear a beat if you use speakers but this is a physical phenomenon as opposed to a psychological one). Please remember the tone generator is currently only compatible with Firefox 4 or later.
DISCLAIMER: Binaural beats are widely available, simple in concept and published studies (see e.g. Helané Wahbeh, Carlo Calabrese, and Heather Zwickey. The Journal of Alternative and Complementary Medicine. January/February 2007, 13(1): 25-32) have shown that they can reduce levels of anxiety. However, binaural beats are the auditory equivalent of strobe lights and for certain beat frequencies they may affect the behaviour of brainwaves - a process called "entrainment". A well known example of entrainment is the practice of gently rocking a baby to sleep, and many people use entrainment for relaxation/meditation purposes. However, we recommend that you seek professional medical advice before use. You should not use the Binaural Beat Generator if you are under 18, pregnant, prone to seizures or suffer from a mental illness. OnlineToneGenerator.com accepts no liability for any adverse consequences that arise from using the Binaural Beat Generator and by pressing play you are agreeing to accept this disclaimer. If you are unsure or do not agree to this disclaimer, do not press play.