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Any thoughts on why more sweeps would give rise to large comb-filter like effects? In the attached png, the flat(er) curve shows a single sweep, and the wacky curves show 2,4 and 8 sweeps. Exactly the same measurements otherwise. I can't imagine that the first 10 second sweep is having such a large effect on 2nd 10 s sweep. Am I missing something obvious?
 

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Only thing that comes to mind is that part of the room structure or something in the room is sent into a symapethic resonance, acting like a Helmholtz resonator at those frequencies. First sweep, nothing is vibrating, subsequent sweeps whatever it is is in motion. They all appear to be harmonically related (multiples of the same frequency).

I could be wrong, but at least it bumps your thread. :innocent:
 

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Some soundcards/systems do not maintain sync between sweeps, which causes problems for multiple sweeps - typically shows up on the impulse response plot as multiple peaks (one per sweep) all about the same level and closely spaced. Stick to using a single sweep.
 
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