I remain determined to find a way to narrow down the optimum size and opening of an IB manifold. Yet IBs seem so forgiving.
Here is an REW graph of multiple sweeps running up from 70 to 85dB(C) measured at the listening position. The manifold mouth was also progressively closed off to almost nothing with a piece of plywood:
I have moved the curve levels up and down using REW Trace Adjustment to match them all at the 12Hz peak. The variation of the curves is hardly worthy of discussion. There is no obvious clue in the frequency response to any of the changes in level or the size of the mouth opening. Which leaves testing for compression effects as the next step. To do this properly I shall need a far more sturdy shutter. I am already producing a loud rattle in the plywood mask at higher levels and would like to go to much higher SPLs.
More on this subject when I have the house to myself again tomorrow. I am presently enjoying my favourite Franck organ CD while dinner is prepared.
I thought I'd try again to narrow down where this 12Hz peak was coming from.
So I set up the Galaxy 140 and REW and took some more sweeps. I tried opening and closing the entrance hall door which is just below the stairwell. I tried blocking off the stairwell with some large pieces of thin plywood. Not a perfect seal but it should have killed any sharp port tuning effect. Finally I did a nearfield plot with the SPL meter resting on thick cushions. No changes were made to any settings during these tests.
I think it is obvious that the 12Hz peak is a room induced effect. The red nearfield curve shows only a hint of a wiggle at 10-11Hz. Look at that nice straight line of the nearfield plot! Amazing linearity before the 80Hz crossover starts to drag down the curve.
I didn't have time to prepare a really solid shutter to mask off the manifold for high SPL testing of possible compression effects.
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