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I've taken some measurements of an in-wall speaker I built. The measurements are taken at about 5" from the speaker. The speaker and the microphone are about 18" away from a side wall. The speaker measurements consistently have a hump in them from about 300hz to about 500hz, peaking at 400hz, with a magnitude of about 5dB. I did some calculations and it seems like the the wavelength@375Hz is ~36" (2 * 18"). Is this causing the peak in the measurement?
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Easy test - assuming you can't move speaker and wall - put a large sheet of MDF or something reflective at 2/3rd the distance from the speaker (1/3rd distance in from the reflecting wall) and see if the hump moves up a bit (as the reflective distance will have decreased).
What software are you measuring with? Most measurement software requires you to gate out the reflections - which limits how low in frequency you can measure. The bigger the space - the longer the dimension of first reflection = the lower the measurement frequency.
The other option is to do a nearfield low frequency measurement and splice with a high frequency gated farfield measurement to see the overall.
Sounds like you are getting quarter space loading to me.
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