Porting an IB... looking for room gain...

[Chrisbee]

An open stairwell ~5' x 3' lies about 4' in front of my listening position.

My boarded walls slope at 45 degrees with only a 6' wide boarded ceiling. The room is about 27' long.

There is a large window on the far gable end too.

The IB is in the wall to the left with another 7 feet of enclosure room not shown in plain view.




My first two graphs are nearfield.

Using the latest analogue RS SPL meter 6" from dustcap of lowest driver in my array.

The first graph is without and the second graph with the rear louvered panel in place.


IB fullrange no CX no BFD nearfield @ 6" no rear louvered panel.



Nearfield with the rear louvered panel in place.



Fullrange no CX or BFD at the listening position no louvered panel.



Fullrange at listening position no CX no BFD but with louvered panel in place:



The IB now with CX at the listening position no BFD but with louvered panel (REW recalibrated).



IB listening position with CX and BFD with max boost @ 20Hz with louvered panel.


My conclusions are that:

My louvered panel helps to lift, smooth and extend the LF response.

There are no hidden high pass filters in my CX or BFD.

The BFD soaks up about 4-5dB in boost mode.

I seem to have no useful room gain at the listening position.

Here's my SVS response curve. It is sitting in exactly the same room at exactly the same distance from my chair as the IB. Look! No BFD!

[brucek]

Why is the microphone cal different in the SVS graph?

[Sonnie]

The IB graph corrections look like the ECM8000 mic cal file.

[brucek]

The IB graph corrections look like the ECM8000 mic cal file.

But why is the level of it down at ~65dB rather than tracking the target level?

[SteveCallas]

Yeah, that hurts, you're down ~35db in the low end naturally It doesn't seem like the 600 cubic feet is having much effect on the FR with those drivers, as that FR could be achieved in a small sealed box. How widespread was that Fs issue you mentioned? Thanks for sharing the numerous measurements, I was getting very curious.

[Chrisbee]

I'm afraid I published the SVS graph using my old RS SPL meter. We are comparing apples with oranges.

Having noticed the error I have wasted an hour ploughing through my images looking for a graph using the latest analogue meter. (without success)

I shall just have to run REW through the SVS again and post the results later. My NSD driver is now installed.

Going back to the IB I think you will agree that apart from a tiny bit of gain around 40Hz there is no real room gain to speak of.

ThomasW has already pointed out on the IB Cult that I have accidental bass traps in the doors, the open stairs, the large windows, the boarded floor, walls and ceilings. Even the baffle wall is too flexible with antique double doors right beside the array.

I have tried running WinISD Pro and Beta to try and lift the extreme VLF for a bit of a house curve to compensate for my natural bass traps. Results are confusing and inconclusive with regards to porting a 600cu ft enclosure with 4 x IB15s. I can get a hump around 10Hz but port size and length remain a mystery.

I have a fixed (but removable) window in my IB baffle wall which could be used as a test port in REW. This window is about five feet from the array on the other side of the double doors. The window could easily be replaced with an opening one of similar size (presently 20"wide x 35" tall) if it proved useful as a reflex port.

At this stage I am treading water hoping for a free VLF lifesaver to be tossed into my sinking IB response.

[SteveCallas]

Well again, if you port it, you wouldn't want the enclosure volume to be the full 600 cubic feet.

[Chrisbee]

You may be right, Steve, but that isn't an option.

My room is just a great big bass sponge.

Thomas has made some useful suggestions on the IB Cult forum.

http://ibsubwoofers.proboards51.com/...8731620&page=1