"15 milliseconds is too short a time to do any good." That's what she said. Also, I have no idea what you are talking about. One 66hz wave takes 15 milliseconds, so I can't see how one would measure anything in such a short time. Happily however, I don't have to know anything about such things in order to use Room EQ Wizard's default settings.
Such long sweeps, in the range of seconds, start to become pretty meaningless when trying to make precise audio measurements. Listen, none of us have access to million dollar anechoic chambers and $100k+ test equipment, so we need to cheat a little to make our measurements as precise as possible. Using gated measurements is one of those methods (There seem to be three main methods for those on a budget: LMS (gated toneburst), TEF (swept sinewave with time-delayed swept bandpass filter), and MLS (Maximum Length Sequence) testing. Read more at
Measuring Loudspeakers, Part Three | Stereophile.com) When measuring for 15 seconds outdoors, just think of all the extraneous noise that is being added to your averaged measurement! All that added stuff skews your freq. response chart to being pretty much meaningless.
The main reason for a gated measurement is to remove reflections from your measurement. Even when measuring outdoors, you will get a ground reflection in your measurement. Here is a pretty good
link with steps to follow for taking semi-accurate freq. response measurements in room using a UMIK-1 microphone and REW. If you follow those steps, you should be able to make a decently accurate freq response plot without having to haul your gear outdoors. Although, if you can swing a calm day, gated measurements outdoors are the way to go.
Edit: I'm not trying to criticize your efforts. You've at least made an effort at contributing. I just wanted to give you some advice on how to obtain much more useful measurements, both for your own personal joy and maybe for the benefit of the community, assuming you haven't been soured by the responses so far and choose to continue sharing!
Double Edit: You are correct about the gated measurements. They are more accurate, but you give up frequency resolution due to the shorter sweep time. You just need to be conscious of the compromises you are making.