Chirp Signals

There are four sounds to study. First listen to each of them without knowing what they are.

Did you notice that they all had a distinct movement of frequency vs. time.

Let's look at one of them plotted in the time domain (i.e. x(t) vs. t). Plot Linearly Varying Chirp in the Time Domain

Another way to study the sounds is to view them in the time-frequency domain. This is done by breaking the signal into many parts and computing the spectrum of each part. The spectra are plotted with a dark component representing a strong spectral component and white meaning no energy.

Did you notice that all of spectra changed with time? The movement of frequency, however, is simple enough that you can describe it with a simple mathematical equation such as a straight line or a sine wave. The concept of instantaneous frequency provides the correct way to derive the frequency versus time behavior for a chirp.

Go up a level then to "real sounds" to see some more complicated sounds that do change with time.