Thanks for the comments. I have added an example output and also added a bit more to the explanation at the beginning.

A Hz represents one cycle per second, not 10,000. I had specified that the length was in seconds but have added more explanation, I hope it's clearer. The example should make clear that each sine wave starts at 0 (you are right that I hadn't set out that assumption).

ah cool, i've never looked at that before. Thanks for the help!

I want to add randomised tests but to be honest the docs are no help. Could you give a very quick example of how they should work and I'll add some in?

Is mandatory allowed to have no arguments, or does it always have an error string? I am confused by these failing tests:

Must be the correct error. - Expected: bar, instead got: {}

I am confused about this too. It would be useful to see the test case.