If you have an ipad I would highly recommend using Audiotools by Studio Six digital.

No external mic or interface needed (but optional and better for high frequency accuracy).

If you buy the extra plugin for transfer function you can do some pretty clever things.

Another optional plugin is Smaart. Industry standard.

Comes standard with RTA, FFT and RT60 measurement tools. Extremely useful.

I tried REW after using this. Nah!