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I have two D9 tens. they hit good but I'm hitting tops around 61 HZ. I did there recommended transmission line box. That is way to high for me!
I did the same box on a graph chart and it is spossed to hit tops at 54 HZ at +5db I believe.

So what is wrong is it my box? I built it on my own and was they frist one I did. my buddie help who has installed car audio for a while, but we didn't have a table saw for perfect straight lines. can a few mesaurements mess my HZ this bad or is this just the wrong box?

I did a box for 2 12" D9's not to long ago.

The buyer originally wanted me to go with Diamonds design but I argued about it. Its to restrictive and the tuning is good awful high.

Your best bet is to make a new box to your tastes, not what they like

thanks they are great subs and my buddie recommended we do anther box but I was sold on the time it took me to build this box. But I think I will go with his and yours recommendations. Thanks

Haven't figured out why but most factory specs for subs seem to be set to high. I imagine to save wear and tear on the subs.

a transmission line enclosure should be tuned lower, and closer to the Fs of the sub, ideally for good SQ. They are also fairly temperamental about using the right driver, and accurate measurements. Yes, being off a bit can ruin the box.
You may be better off just using a ported enclosure, tuned to about 35Hz.