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I am wondering if that is because I split the signal from the computer to the left and right channels
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Splitting a line-level signal has no effect on its level.
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I wonder if your sub isn't compressing at the low end, thereby flattening out with the more signal you give it... That's what it looks like to me -- as you increase power, the signal gets flatter, but not louder. I'd be interested to see more plots, displayed on one set of axes, with increasing preamp levels, while the SPL meter is in an appropriate range.
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I thought of that myself when he first displayed the graphs, but decided that it doesn't seem likely at this low level (~75dBSPL). Compression shouldn't occur until the sub is up to ~> 90dB.
But it certainly is worth a try.
Greg, if you're going to take a set of readings at 75dB, 80dB, 85dB, 90dB, 95dB etc, then before each test you would do a complete setup (as I described in detail earlier in the post), except set all the 75dB targets to a new value each test. Also be sure to increase the SPL selector wheel on the RS SPL meter when needed (i.e. for 75dB and 80dB use 80dB position, then move up to 90dB position etc...)
Post all the graphs on a single All Measured display for comparison...
brucek