First run with REW... advice and tips please :) - Page 5

I was experimenting with Chris Whealy's spread sheet and came up with a design for an absorber that was appreciably effective at 50 Hz. 6" of 703 with a 6" air space between the 703 and the wall. Absorption @ 50 Hz is about 0.40 so it would have to be large - covering a significant part of wall(s).

I'm actually considering building one of these later on this year, as the back wall of a media booth that is currently experiencing a lot of bass build up in this range.

An alternative would be a resonant bass trap, for which a possibly useful design guide exists at
http://www.iperf.org/IPRF_ACUSES.pdf

The problem with eqing nulls is that they can often absorb impractical amounts of power, and still not be effectively dealt with.

Generally, its a well-known no-no.

if utilizing 6" absorber with 6" air-gap for LF absorption, light fibre is still the way to go. you're not gaining or losing much with such thin traps at 50hz, but the rest of the LF will see much better performance -

OC703:
(and performance is even worse if utilizing NASAs GFR values of ~28,000)





Pink Fluffy loosely filled insulation (uncompressed):




lower gas-flow-resistivity via sufficiently thick/large traps for LF absorption, as stated numerous times.

and even better as the porous trap gets thicker (spacing the insulation into areas of higher particle velocity for the lower/longer wavelengths):

Running sub boxes with IB is not a problem with the proper orientation. Often folks will say you cant mix different types of sub types (delay and mixing issues), but with the plan of attack I was suggesting, this is most certainly OK. The Geddes approach only suggests using your main and most powerful subs up front, with "filler" subs at the halfway points of each side wall and one in the rear, optimally mounted near the ceiling (pipe dream for most). all of these said "filler" subs dont need to match the performance of your main sub rig, and actually only need to play around 10db below ref, so a small capable sub is plenty. These filler subs are simply there to help tame your main modes. I will stop there but just thought it might bear a little explaining.

My 4" panels definitely took some of the ringing out of the rear of the room. The performance benefits was well worth the installation of them. If I had a graph I would post it, but I dont, sorry!!






agreed 100%, but in your graph I see absolutely no nulls in your sub response at the mlp, only two peaks that could easily be pulled down a shade and bring you right in line with your 100hz and up readings. Boosting anywhere more than 3-4 db's is a no-no and more than likely you wont run out of speaker, you'll run out of amp to do it. just think a 6db boost is QUADRUPLE the power needed to get you there over the rest of the Freq response. I learned this the hard way. I eq with cuts ONLY, if at all possible.


Phase of the entire sub system, definitely!! The individual drivers, since they are for the most part co-located wouldnt have much of an effect, and in almost all cases would be detrimental to the sound. Even if OP had the capability to adjust each sub's individual phase, they are of one system essentially and need to be treated as such. Now an additional sub in the room? This would be QUITE important to address.

I like this thread has gotten back on track, there are some excellent points being made here.

Unfortunately the OP has taken away two very useful tools. Multiple subs - and varying sub and listener positions. He also does not want anything complicated (i.e. tuned absorbers, which may not be appropriate in a small room anyway - there are just so many modes). So what we have left is thick broadband porous absorbers - but apparently no space for them.

You have "super traps" in two wall-wall corners (I'd increase the flow resistance of them, but I'll leave it up to others to determine what the optimum is). Have you considered other "corner" locations? i.e. wall-ceiling and wall-floor.
Do you need the headroom above the rear seats? If not, I'd put panels above the rear seats. Ideally 12" thick, the width of the room, and 4 feet wide into the room. Make sure you leave the face toward the screen open - you are trying mostly to absorb front to back resonances - and perhaps some top to bottom - with four subs up front side to side should not be too much of a problem.
The 4' depth front to back will really help with the primary room mode, and the large volume will help with almost all modes.

Can you put in a few porous benches? If you could find a place to put 16"x24"x4' benches along the wall/floor corner you might get some real benefit. Or even 16"x24x24" this would help - again align the long dimension front to back if possible.
Cover these big traps with a thin plastic membrane if required to reduce excessive high frequency absorption.

Are your rear seats elevated? If so, open up the front of that cavity and stuff it.
Your subs are IB - what else is up there? Can you perhaps divide the front wall space - devote some to making bass, and some to absorbing excess? What about behind your rear wall?

Are your side wall panels framed in on the sides? If so, redo them. Open the sides up so they can absorb bass.

I've never understood the appeal of air spaces - better IMO to fill all available space with absorber. The cost is not much higher, and construction is easier. And if you leave the sides open you have more surface area (and a deeper absorber) to do the job.

Basically you are going to have to be creative. And consider this a learning experience.

When you have treated the room as well as you can. Use room correction eq to optimize your favorite seats. When guests come - give them the best seats.
Anyway that is my $.02

Edit: as others have said - don't bother trying to eq a null
Edit 2: go ahead and try if you like - you might have the umph - and it might be a good learning experience

A couple points re: boosting nulls:

Whether its worth trying depends on your headroom and SPL output goals - if your output goals leave you with headroom to spare, you can put some of it to use (modest) to boost nulls. In my case, I listen at -10, I don't need reference - and I have dual THTs, so I was able to do some boosting without too much risk of blowing stuff up.

If peaks and dips are very narrow, they might not be worth touching, since program material may not expose them to any significant extent.

My experience has been peaks are easier to hear than nulls, and easier to correct. But my experience is limited. YMMV - only one way to find out - try it.

I believe that for equal sized and shaped peaks and dips, particularly those that are relatively narrow and have substantial amplitude, your statement agrees with generally accepted experience and wisdom.

Hearing a relatively narrow dip falls prey to what we know about proving a negative hypothesis. This is particularly true if we have no close correct or significantly different alternative to compare to.

How do you know that are not hearing something if you have no way to know that it exists? Answering this this question leads to a more reliable means for discerning peaks and dips, which is to compare two different things as closely as possible.

Of course, making close comparisons of speakers and particularly room treatments is very difficult. Speaker comparisons can be facilitated with quick-acting positioning devices, but room treatments tend to be quite a bit larger.

You seem to be oblivious to every post on this thread. :-(

The short answer is that you can probably address your situation with a combination of:

(1) Equalization of peaks. An approach was recommended.

(2) Sound absorbers that are actually large enough to actually be effective at these low frequencies. A detailed design was proposed.

(3) Multiple subwoofers. Their usage was proposed.

It's all about space, money and WAF. Only you really know about those.

I feel your pain. There was a silly war in this thread started by people who were overplaying the superior knowledge card for their fun at everybody else's expense.


Speaking as a person who has been solving tough technical problems professionally for over 40 years, one important rule of thumb is that you almost *always* implement the easiest solutions *first*.

For one thing, cleaning up the obvious easy problems clarifies your view of what is left over. For another thing, you get quick gratification and confidence building.

That means EQ! ;-)

Looking great! Odds are your >80hz situation is still integration issues with your mains. have you messed with the phase of your subs?

Also, give this quick article a read, Im discussing this a bit in another thread, but it is good to know what SBIR is, and if it is affecting you in regards to your speaker placement...

http://gikacoustics.com/education_sbir.html

You could also experiment with the sub distance setting on your AVR and watch what happens with the frequency response. Also, if your subs have more than two ports each - you could try plugging one port on each sub and see what that does to the frequency response as well.

I use 3 subs in my room. Placed around my room that still all puts them at the same physical distance from my main LP. I can then adjust sub distance on the AVR to fine tune phase with the speakers.

Apart from a 90hz null, I have managed to get things reasonably flat from 25hz...

Attachment 247543

yea, switching the polarity and the phase of the subs with the rest of the system are two different things as well. To adjust the polarity like you did is always going to cause one sub to cancel with the other. Adjusting the polarity of the entire sub system is what you need to look for in an eq, but delay is also a key point.

Yep there are certainly two different approaches. Geddes' approach basically states to put your big firepower up front and each "filler" sub should be operating purely to help cancel nulls in your response. This is assuming you get enough bass from your "main" subs up front.

The other approach is to just have separately located subs of all the same performance around the room with perhaps less firepower from your "main" sub. In this case all subs would be identical just not mutually located. If you are happy with the output of your main sub, sans fr issues elsewhere in the room, geddes approach will suffice. If not, start building identical doubles of your main sub and locate them where they help with the room modes the most (or where is conventient, or a combo of the two).

Do you have the ability to get the filler sub to AROUND the same distance as you are from your main sub? Just try that or try reverse polarity in other positions not equidistant to see if that helps instead.
Edited by beastaudio - 6/6/12 at 11:44am

Stop and think about what has been proposed for a moment....

If the distance between acoustical origins and the listening position are indeed identical, and if the originating signal polarity is indeed reversed (effectively 180 degrees out of phase), how will this ONLY "correct nulls"?

It will effectively restore all of the arriving energy to the ambient state, thus effectively canceling all of the energy, as all of the energy from the two sources will effectively be 180 degrees out of phase. It result will be nothing but a "null" with reference to the energy sourced by the two subs. This process is commonly referred to as 'active noise cancellation'.

A simpler 'solution', if this is imagined to be a solution: Get rid of the subs and do not stimulate the frequencies in question at all.


It is critical that folks establish a very clear grasp of the fundamental concept and behavior of superposition.

Edited for clarity, I meant try having it at the same distance from LP OR try reversing polarity in other positions, my bust, and thanks, I have a low turbidity grasp on "superposition" not exactly clear.