Radio Shack / Tandy SPL meter, should be UK£25-30ish. A search for SPL meters on your local Amazon should find you something that looks very similar to the RS one in the photo_ it’ll do the same job.

Brian

Thanks Ratbert - that's great

It does, look at the Measurements tab, right-click, View Plot. Choose Low Frequency Response.

I will concede that REW is much easier to read and can provide the relevant filters required but you can achieve decent results using MRC's built in calibration. I don't find MRC's auto filters that useful, personally, but you can tinker with them and adjust that Low Frequency Response graph in realtime.