Bass Boost

[UNDERSTANDIN]

I want to model a sub with an amp that has bass boost of +5db at 35hz. Should I model the vented box to be -5db at 35hz for a flat response? The box will also be smaller and more room friendly.

[SY]

Isn't this a bit backward? Normally, you start with the box, then determine what eq you need to flatten the response.

In any case, if there's a specific reason for designing to that boost (like you have a plate amp with that built in and you don't want to take it out), yes, drop the box tuning frequency to get the driver to roll off enough that it's down 5 dB at 35Hz.

[Collo]

I have built a few subs using amps that have Bass boost. I use WinISD pro which has a section that lets you add filters including a Parametric equaliser. For 6dB boost, I reduce power to 25% of the normal rating, so the output at the boosted frequency will be 100%. For your situation of +5dB, it will be slightly different.

Here's a link to my setup in WinISD
http://www.users.bigpond.com/bcolliso/WinISD-boost.htm

I find that with careful selection of tuning frequency, using a boosted amp in a ported design can save you needing an infrasonic filter to control excursion.

regards
Collo

[rcw]

This type of filter is used on the QB5 class I type of reflex alignment. This is much used in professional designs as it comes close to an optimum size/extension/output solution.
I recently published an article on Rod Elliots website that outlines an easy way to design such enclosures.
Basically if your driver has a standard B4 alignment that requires a tuning frequency that is 35Hz./0.7. then tuning it to 35Hz. gives a flat output when used with this type of filter, as well as greatly improving the power handling and providing a l.f. filter.
By compliance scaling you can fit a wider variety of drivers to these alignments.