 This is a 440 Hz tone. This is a minus 440 Hz tone. They sound the same, don't they? That's because they are, at least when you think of frequency as how many times something occurs per second. However, if you look at frequency as angular velocity, then the concept of negative frequency becomes a little easier to understand. Negative frequency simply means a rotation in the opposite direction. Signals like sound signals that we tend to meet in our everyday lives are all real signals. That is to say their amplitude at any given moment in time can be described by a single number. Because of the complex nature of sine and cosine waves, sound signals are made up of positive and negative frequencies. The negative frequency is the complex conjugate of the positive frequency. To find out more, go to howthetransformerx.com slash negative frequency.