Needing help with Sub placement

[dpjones]

I finally got all of my gear and got everything hooked up, and I'm not very happy with my sub. I am using a B&K Ref 50 Pre-Amp, Ref 200.7 amp, and Monitor Audio GR60s, GR Center, 2 pairs of GRFX Surrounds, and the FB212 sub. My room is basically 19'D x 16'W and I have the sub along the side wall towards the front of the room. The sub sounds really good along either side of the room, but I have a big time void running down the center of the room, which is of course where I want to sit. I have tried moving the sub down the wall, but that doesn't seem to make any difference. I don't really want the sub in the middle of the room as it would get in the way of the towers and center channel, not to mention look like crap. I have acoustical panels in the room as well. I have 3 down each side wall and 2 on the back wall. Thanks in advance for any advice!

 

[Pat D]

Start with corner placement.

My first recommendation is to put the sub in a corner or near one. It doesn't much matter which corner (I use a rear corner), but the corner should be 6 feet or more from a door, window, or other opening for best effect. In most rectangular rooms, the response will be the flattest and deepest there. Since a corner essentially horn loads it, it will also be louder there, so you will have to turn the sub's level down to compensate. Tom Nousaine did the measurements for this some years ago.

Corner placement presupposes a crossover below 80 Hz and a fairly steep crossover so that you will not hear the sub's location. If you have very small main speakers that have no real bass and require a higher crossover to the sub, then the sub should be placed between the speakers: but this involves some compromises in the smoothness of the bass response.

If corner placement does not work well, then there is another, more general method of determining the placement. First, put the sub in your listening position, and then crawl around to find where the bass is best. Then move the sub there and you will get the same sound in your listening position. This is described by Alan Lofft on the Axiom site. Look under Audio Tips:"Subwoofer Placement Tips".

http://www.axiomaudio.com/archives/index.html

 

[dpjones]

Pat,

Thanks for the reply. It is in a corner, sort of. The front wall of my room is wall to wall, floor to ceiling cabinetry. So, the wub is about 1 foot away from the cabinet and about 6" off of the side wall. I tried the placing the sub in the listening position test and didn't have any luck either. No matter where I walked along the side wall, it seemed flat.

 

[cbraver]

If you put the sub in the listening position and didn't find ANY place that that worked, you didn't crawl to all the spots in your room. Even if you have the poorest listening room in the world, I have never setup a system where I couldn't get the sub to at least be moderately well placed. Conditions can suck, but, I highly doubt there is no solution.

It was funny, in our old homes dedicated home theater the best place to put the subwoofer was slightly to the left of the room, very near the center channel. The answer can be strange!

If you find a good spot, but it doesn't work with athetics (who cares about those, right!? haha), then you might want to consider getting/trying out two smaller subwoofers (or, another one of the same one if you can afford it). The second sub can be placed to flatten out the peaks and dips in the room. If you place one in each of the two corners in a room, they will have opposite phase effects of the fundamental (half wavelength) room resonances acting in this direction. Plus, you get the benefit of roughly a 6db increase in sound pressure levels in your listening room. Which can be good for movies, but I found in dual subwoofer setups, on musical inputs the bass levels had to be left at reference or below to have good integration (that is, in the systems I have setup). With movies, go wild. My personal opinion is that, while expensive, if you can afford it, two subwoofers is really a good addition to any listening system (even if you didn't get an increase in SPLs, which you will).

Good luck and have fun with it! Crawling on the floor always makes your housemates think you are crazy, go with that.

-Chad

 

[Dr. Anthony]

Complex speaker placement is one of the most difficult things in setting up a home theater.

I think your problem might be four fold.

Problem #1 your GR60's are rated down to 28hz and if you are running them full range, they may be supplementing the bass output of the sub a causing standing waves of converging and diverging sound pressures, depending on the cutoff frequency. Cure - decrease the subs cutoff frequency or raise the main systems cutoff or both to see if anything happens. Always invest in a sound meter. It will be the best $40-50 purchase you ever made.

Problem #2 Corner loading a sub is the best way to augment SPL levels due to reflection and horn loading, however, when dealing with multiple sources of bass output, it becomes much more complex. This is why you find when placing the sub close to the main speakers, the sound improved because as you increase the central point source of the bass output, the sound overlap of the waves becomes much tighter (from my understanding) with less holes in the standing waves. Think of it as dropping water into a basin. A drop of water at two points 1 inch from each other simultaneously will yield a tight pattern of ripples. Trying this again with the drop sources on either end of the basin makes for a tremendous amount of converging and diverging areas. "Sweet Spot" location is very difficult with this setup with three sources of bass. With a dual subwoofer location of either side of the room both corner loaded is the loudest way to make bass as there is room symmetry but once again you have to move them about or play with phases to make them work for your "sweet spot" area. This works well with satellites with limited range or with the fronts set to small with most of the bass redirected to the sub. You may want to try this as well.

Problem #3 : Your room dynamics may not be conducive for good corner loading. Without symmetry, or a good "flat" wall to increase reflections, placing the sub in front of cabinetry with multiple angles and multiple places for sound absorption/dispersal may not be ideal even though it might be corner loaded. Surfaces that surround it might also be a problem such as high shag carpet, textured wall surfaces, lots of drapery that is always down, proximity to couches or behind couches with thick foam or soft surfaces, multiple paintings on the wall for sound dispersal, pitched roof, vaulted ceilings, the proximity to open doorways/ recessed windows, etc. There are many variables. If this is the case find the longest flattest surface to place the sub upon, possibly behind you if necessary. If you place the sub directly in front of those accousic panels I think is a defeated purpose if they are rather thick. Most panels work for higher frequencies and allow the lower ones to reflect. I don't know which you have.

Problem #4 - Its time to play with the phase control. This is the hardest of them all an requires time to set, a good test cd/dvd and patience and a sound meter. A small twist and listen and back and forth and back and forth.

cbraver - I love the crawling on the floor thing...too funny Effective but funny.

Tips: If you are having difficulty, try to move your front speakers forward and back to change the location of the standing waves, same with the sub. Move your sweet spot "couch or seating" location a few feet around back/forth. Place some sound absorbing foam on the surfaces behind the main speakers if possible to catch some of those reflected waves. Your acoustical panels might also be causing the problems on your side walls, not allowing the bass to propagate into the room. Panels should be placed where you do not want sound to reflect to increase point source and increase immaging from my understanding but will rob you of reflected bass sound (which corner loading is trying to increase) if they are too thick. Try removing them and see what happens.

What probably might make the most difference is to have the sub built into the cabinetry in the front in a central location. I might be wrong, but I think that will help things greatly as the sound will be enhanced by the vibration of the wood, being flush with the cabinet for more even dispersal and having a hard surface to reflect off of effectively.

A complex speaker problem is what cedia technicians are for If all else fails, time for them to make a house call.

Hope that helped somewhat.

 

[dpjones]

Thanks for the very informative response Dr. Anthony. I do have the towers playing full range and the sub crossed over at 60hz. I will cross the towers over at 60 too and see if that makes a difference.

I can't really move the towers around because then are built into the cabinetry. You may be right about the acoustical panels, but the sub sounds great on the opposite side of the room as well so I'm guessing the sound is reflecting. I don't really know what to think to be honest!

As far as the phase goes, what I've read in the past is that if the bass sounds like it blends in well with the rest of the speakers, then the phase is probably ok. To be honest, I had my wife turn the variable phase control from 0 to 180 and back while I was seated in the sweet spot and I never could tell a difference. So, I have it set at 0, but it does worry me that I couldn't tell a difference.

I probably will try to get someone to come out. I just hate paying someone if I don't get some sort of quarantee they will get it to sound better.

Thanks again!

 

[sheep]

one more trick

one thing you should always take into consideration when placing a sub is that halfway between the wall its on, and the wall across from it, there is a big void. As in a void where its like its not on.

With the corner, stuff it all the way in, and pull it out gradually until its sounds right. the 6foot thingy doesn't always work in every room. my room is 9 by 11, so thats half way down the wall.

also, don't put your sub on the side wall and face it to the other side wall, that makes it so the bass its good on the other side wall. keep it facing you and along the front or back wall.

sheep

 

[Tom Vodhanel]

Corners almost always work best---preferably corners near the key seating positions. As long as you aren't overworking the subwoofer---setting all speakers to small and letting the sub handle all the bass for the entire system usually works best too.

Tom V.
SVS

 

[dpjones]

Thanks Tom and Sheep. Yes, the void is directly in the middle between the 2 walls. I actually have my front towers and center as Large and my 4 surrounds as small. I tried what Dr. Anthony suggested and crossed my towers at 60Hz and it didn't make any difference.

One other thing, this sub is an Isobaric design so I'm not really pointing the sub any particular direction.

 

[Tom Vodhanel]

Being equidistant to two parallel boundaries will almost always cause problems like this. Can you try sliding the listening position around a bit?

Tom V.
SVS