parametric question: 2 channels on one null?

EricH

I've just complete my home addition and have hooked up the new PB2+. I'm trying to flatten out my bass response using a Rane pe-17. I have a huge null at 40hz and can't move the sub to another location. So far, I've been able to reduce the null somewhat, but have a question. Is it o.k. to use two channels of the eq to try to fix a null? I know this isn't optimum, but the sub simply cannot be moved.

Thanks in adavance, Eric.

gatemplin

Hi Eric,
Try adjusting the crossovers and the phase on the sub (I try 0,45,90,135,180 degrees). If that doesn't work, you can either boost, or build some bass traps. The name is deceptive because a bass trap usually increases the output by eliminating some of the cancellations according to what I have read.

Traps are better than boosting because if you boost too much with the PE, the amp will start clipping the signal.

http://www.ethanwiner.com/basstrap.html
http://www.customaudiodesigns.co.uk/articles/membranes.html
http://www.geocities.com/jonrisch/a4.htm

Tour2ma

Are you running the rest of your speakers on small? If not, too many bass sources can compound bass management issues. Likely that your surrounds aren't an issue, but at least mains should be.

If they are set to small, gatemplin pretty well nailed it. I'd only add that if you can tolerate rotating the sub in its current location, try that as well.

Good luck....

EricH

Hello Graham,

Yes, 7.1 set-up on small with all speakers crossed at 80hz. I think I will build some bass traps. The previous foam examples that I have seen would not score high on the WAF, but the ones on the links are very nice! Thank you both for your help.

Dr. Spec

Not all dips in the response are true nulls. The only way to tell is by trying to boost it.

The Rane PE-17 (I've got one too) can be used as you described, if you use bands 3&4 which are wired in series (all other bands are wired in parallel and cannot be combined).

But if you need two bands at the same frequency to boost a low, then it probably IS a null, and boosting a true null is not possible and will (as already stated) waste amp power and abuse the woofers.

Try phase manipulation first. Although be aware 40 Hz is at/near the lower end of what the phase control will affect; the mains have signed off by that point (when set to small at 80 Hz), and the bass waves are getting long enough to develop standing waves in most mid size rooms.

Also try mapping out the size of the null and adjusting the listening position, if possible. Some nulls are fairly small and can be almost totally avoided by moving your head 1-2 feet in any given direction.

If the null area is huge and unavoidable, definitely try bass traps.

EricH

Thanks Doc. The null appears to start at 34hz and drops like a stone to 40hz. Then it raises back to "normal" around 45hz. I realize now that I was running my test WAY louder than necessary. I was trying for a flat 105db response, and it dropped to 72db at 40hz. I'll try for a flat 85db response tonight. I'll also move the SPL around to different seating positions. I had it pointed up above the TV, is this correct?

Also, my south wall has 8 3' x 5' windows. Will the 2" wood blinds that I will be putting on them help at all with this issue?

Thanks, Eric.

gatemplin

'll also move the SPL around to different seating positions. I had it pointed up above the TV, is this correct?

Sounds fine, I use a small camera tripod to put the SPL meter right at my head level on the couch.

Also, my south wall has 8 3' x 5' windows. Will the 2" wood blinds that I will be putting on them help at all with this issue?

I dont think the blinds will help from what I have read.

Here is an example that I am thinking of making:
http://www.geocities.com/jonrisch/basstrap.htm