By far, the simplest way for an electronic circuit to produce an audible sound is by generating a square wave. As you saw in the previous chapter, a square wave is the infinite sum of odd harmonics. However, as far as Arduino is concerned, it is achieved by putting a pin high and then a short time after, putting it low. This will generate a signal that can be turned into a sound by putting it through a loud speaker. There are many ways of generating this signal, some are more efficient than others .