This one goes out to our young friend Trevor who asked why frequencies played at the same time don’t combine and give us a different frequency altogether. This is a great question, and reasonable too. We all know that adding yellow and blue results in green. Likewise, adding hot to cold begets warm. Heck, if you mix sweet and sour you get Skittles…but that’s for a different article!
So why don’t frequencies combine to create a new frequency that is the sum of the original frequencies? The answer lies in what a frequency is. With color, the pigments all combine to reflect a different wavelength of light which our eyes see as a different color. With water, the energy held in the hot is released to the cold until they find balance at warm. With Skittles, well nobody really knows for sure. Frequencies, at least in the audio range, are made up of pressure waves through a medium, such as air.
Here are graph representations of a low and a high frequency:
The graph from left to right is time. If the amount of time between the beginning and the end of each graph is one second, then we would say the top one shows 1Hz, or one cycle per second. The bottom graph would then be 18Hz, or 18 cycles in the same one second. Each time the “wave” is above the line it is creating high pressure in the air, and each time the “wave” is below the line it is creating low pressure (both being relative to the air pressure in the area when no sound is being created).
In the graph below, we show high pressure areas of the 18Hz frequency with a blue line and the low-pressure areas with a red line. To simplify, we show these blue and red lines as they relate to only the high-pressure part of the 1Hz cycle, but the same would be true continued on into the low-pressure part of the cycle.
When the high-pressure of the first peek of the 18Hz frequency (1+) is combined to the high-pressure of the 1Hz cycle straight above it (meaning they are being played at the same time) they add because they are both high-pressure. However, when the low pressure of the valley following that peek (1-) is combined with the high-pressure of the 1Hz above they subtract since one is high-pressure and the other is low-pressure. Another way to say this is when you put a little bit of high-pressure from the 18Hz with the high-pressure of the 1Hz the result is the addition of both. But, when you put a little bit of low-pressure from the 18Hz with the high-pressure of the 1Hz the low-pressure wants to find equilibrium by taking some of the high-pressure away from the 1Hz. The result is the 18Hz frequency riding on the wave of the 1Hz frequency. We hear, or in this case “feel” the 18Hz frequency because it is still there…just undulating up and down with the 1Hz frequency. We also still feel the 1Hz frequency because it is what is causing the undulating of the 18Hz frequency.
If you think about it for a minute, you’ll be happy that frequencies work this way…otherwise, all frequencies would combine to a single monotone. We would have no music! We would not be able to pick out our own child’s laughter out of the drone of monotone from all of the sounds around us combining. Not even when it’s a giggle over Skittles.
So Trevor, we’ve all been using the wrong term when we say, “play combined frequencies”, since they don’t. All along we should have been saying, “play simultaneous frequencies”.