Please refer to the animated graph below that is labeled "Step Test Signal". Here you can see the very simple test signal that, when applied to a speaker, reveals all. Basically, we ask the speaker instantly to go from 'rest' to compression - and that's it!. Press the start button on the animation and watch the time cursor proceed. An easy way of looking at this simple test signal is that this is exactly the same thing as connecting a battery to the speaker - the input is merely a sudden DC current. You can see why the signal is called a 'step' - it looks like a 'step'.