Combination Tones
When two tones play at the same time, you can hear some interesting effects. This video works best with headphones. Don't listen at too loud of a volume.
In this video I play two tones simultaneously. The first tone is held at a steady 440 Hz. The second tone starts at 440 Hz and rises over the course of one minute to 880 Hz, one octave higher.
You will first notice beats. After a bit, you should hear a very faint but noticeable tone in the background. This will start at a low frequency and start to rise. It has a frequency that is the difference between the two and is called the difference frequency.
You may also hear two other tones that are special combinations of the two tones playing. (I'll be honest, I couldn't quite convince myself that I heard them. Violinists often call these Tartini's tones, as he made great use of them.
You may need to try listening a few times. You may also need to adjust the volume to hear them better.
Good luck.
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