Several folks have asked how to model the increase in SPL that's occurs in the low frequencies (so-called "room gain" or "cabin gain") in WinISD.
Each room is different and there is a lot more going on in rooms than what is typically referred to as room gain (e.g., reflections, modes, etc.), but having a basic model may be helpful, so here goes...
I'll use the example of a Full Marty, which is roughly a 10 cubic footer tuned to around 16Hz with a UM18 driver.
Full Marty (UM18) raw response with 1100w:
As a ported cab, it is always best to put a protective high pass filter on.
A 2nd order Butterworth (12dB/oct) protective high pass at 16Hz is used. "N=2" means second order.
Full Marty (UM18) with protective high pass:
**Uncheck** the protective high pass filter before proceeding.
The next step is to "force a flat response" so the room gain can be viewed easily. Switch the view at the top to Transfer Function Magnitude and check the box on the Advanced tab to Force Flat Response. The result should look like this:
Then a room gain profile has to be created. A medium size home room will typically have some lift beginning around 30Hz or so, approximately 6dB or so by 20Hz and somewhat more under that. A large room will typically have its gain starting lower in frequency and a smaller room will typically start higher up. [BTW, room gain in cars frequently starts up around 60Hz or higher, which is why small subs can make such enormous bass in cars, but not so much in home-size rooms.]
For a starting point, a Linkwitz Transform with F0=25Hz, Q0=0.707, F1=7Hz, Qp=0.5 is a decent starting point. For small rooms, increase F0 to something like 30Hz and for very large rooms, lower F0 to something like 20Hz.
Again, be sure to **uncheck** the protective high pass filter (and any other filters that may have been created) in order to see the room gain profile only.
A simulated room gain profile for a medium-large room
The final step is to turn the protective high pass filter back on and turn "force flat response" off.
The final in-room response is a combination of the raw response of the sub, the protective high pass filter, and the room gain.
In this case, it is pretty flat to around 14Hz, has an F3 of about 13Hz, and an F6 of about 12Hz.
FINAL RESULT: Full Marty (UM18) with protective high pass and room gain
Note: Be sure to turn **off** the Linkwitz-Transform when checking air speed, cone excursion, amplifier power, etc. because room gain is "free". It is not actual EQ that is being applied to the subwoofer.
Enjoy,
John
