How to fix a lack of bass Null

If you have your speakers facing the length of the room (5.250 meters) then the Bass Null is likely to be caused by reflections off the back wall. If you cover the back wall with bookcases, wall coverings, draperies or other acoustic dampening material the Bass reflections should be corrected. Moving the speakers will not prevent the Null. It is caused by the bass wave hitting the back wall and bouncing back into the room, in effect canceling the incoming bass wave as the two meet.

Commcat's recommendation is bang on - see my virtual system for an alternative to a bookcase. The other issue is quarter wave cancellation in the bass from the wall behind the speakers. As the article explains, the only way to eliminate this entirely is to go to a soffit mount (as they almost always do in high end studios with their main monitors). An alterntive is to add plenty of bass trapping on both walls but at these frequencies you will probably need at least 4 GIK tri-traps or something of similar size before you can begin to make a difference.

Unfortunately a TACT or a PARC or an PEQ is unable to fix nulls (unless it alters the phase - which some EQ's do and which you might not want if you have phase accurate speakers to begin with). However, since a TACT or PARC allows you to EQ down the peaks then generally the "nulls" are much less masked by the peaks - so it really helps. Another trick is to have copious bass (more than you need) and then EQ the peaks down to get a desired response by attacking ONLY the peaks.

The unfortunate thing about nulls is that you cannot boost them or fix them in any other way except for speaker/listener placement and room acoustic design/treatment (if you wish to avoid playing with the phase of your bass signal which may affect how you perceive transients). The bad news is that this takes tremendous amounts of absorption to achieve - huge huge booskhelves and other large thick absorbers that work at bass frequencies. You can achieve good results with helmholtz tuned devices (like tube traps) but ONLY if you really know what you are doing - I'd suggest you hire an acoustician like Rives in this case - broadband bass and lower mid absorbers are easy for anyone to add to room corners and can really only do good....resonators require much greater expertise, IMHO.

What you have discovered is a common problem for 90%+ of all HT and full range two channel listeners. Most people focus on ever higher end more expensive gear and simply ignore this issue altogether and contend that it does not exist. These are mostly cases of extreme wishful thinking, where WAF or room aesthetic issues have a much higher priority than good sound.

From your post it would appear that you have established a fixed listening seat. If that is so, then you might eliminate the null by simply moving the listening chair back or forward somewhere between +/- a foot.

Get a test disc, a SPL meter, some graph paper (for easy visualization) and start carefully measuring actual results of speaker/listening chair positions. Programs for speaker placement, etc, are nothing more than suggestions for good starting points. There are far too many things that can change the speakers response that can't be factored into a program.

One of the benefits of the meter/test disc assessment is that you will soon learn (or if you are dumb like me, slowly learn) what nulls are inherent in the rooms dimensions which will defy correction by most any means, and nulls which are location related. The only problem with the former is you really can't, as a practical matter, change them. The only problem with the latter is that often you can change the frequency peak/null by moving stuff, but often you just move the peak/null around a bit. No matter what you do you will have to accept some form of compromise.

A personal experience. I spent some years trying to improve my frequency response, close to flat as it were except for a 6db suck out at 40 and 50hz. Never did it - I even tried using a sub with a very low cross over point (40hz) with mains running full range and when I tried to eliminate the null the sound became boomy and muddy. Wana sub? :-) I just concluded that this null was barely audible (speakers are flat at 32hz and down 3 at 25hz) something I could live with.

Shadrone's post is an excellent summary I think, except that I'm not at all sure about Commcat's comments about the surface or furnishings on the back wall having much effect on deep bass nulls. But that is just my opinion.

O