Quickie FFT in my room. Internal iPhone mic which is calibrated but still low resolution at low frequencies. I have a hump at 50 Hz and another one at 150 Hz. I suspect it’s the thermostatically controlled cooling fan I have sitting on my amps, which are all warmed up at the moment.
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Nope, here it is with no fan. The 50 Hz peak is still there.
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But the fan knocked a noticeable dB (SPL, C-weighted full band) off the broadband noise level.
I do not hear the 50-Hz peak.
Rick “no 22.5-foot parallel walls that I see, but modes are weird” Denney