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!. 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'.