[Housteau]

An ideal listening room would be mostly reflective and diffusive with higher frequencies while being absorptive of the lower ones.  The actual ideal would have that room unable to reflect those lower frequencies back into itself.  Often the best we can do is try and mitigate those reflections using various construction techniques and acoustic treatments.  Bass tends to build up the most in certain areas of the room such as in the corners and often at the mid-wall points front and back.  My attempt at control was to build large triangular corner traps for my space.  They had to be large to do any real good and they did certainly help, but not as much as I was hoping at the time.

I was able to greatly improve the bass performance of my room by changing it's dimensions.  It already had a theoretically good ratio, but I jumped in and added 10 more feet to the length.  I was able to do that easily by taking over one car spot of a two car garage.  This turned the former long wall, where the speakers were against, into the short wall.  I did this several years ago, but was just recently thinking about something.  Since adding to my room made such a positive change, I wondered if a person had the chance to build a new room, or modify an existing one, could that person build in a good bass trapping design fairly easily and cost effectively?



 

I was thinking it would not be difficult to take an entire wall, lets say the front wall behind the speakers, and make it more absorptive of bass yet still reflective of the higher frequencies by design.  Lets look at a room that is too long and narrow.  It wouldn't take much to shorten that room and improve the dimensions by building a new wall three feet in front of the existing one.  This new wall would not meet the two adjacent walls leaving a gap 18 inches to 2 feet across on each side, floor to ceiling. 

In effect the front corners of that wall are about to become an infinite bass trap.  The openings in the wall get covered by an acoustic screen, or an actual bass trap.  The bass energy passes through those traps where some gets absorbed.  Instead of getting reflected back into the room it enters that new space where it becomes trapped and further absorbed by any number of inexpensive acoustic options.  Other holes could also be made in that wall at strategic locations, like right in the middle where a standard bass trap might go.  Those wall / hole screen, or bass trap covers could be made reflective and diffusive of the higher frequencies if needed.  What was done with the front wall could also be done for the back. 

Cost is all very relative in this hobby.  Considering what some of us spend on cables and such, the cost of building a new wall would be negligible.  This isn't something that my room needs, but if I had to start over I would consider it in combination with a smaller overall space.  The real question is would it be worthwhile?  Does something like this have the potential to work as intended?

[JLM]

You're on the right path, the room is the second most important element in the overall audio system IMO (after speakers).  Bigger is better, but shape is important too.  Smaller rooms have more primary echo frequencies.  Poorly shaped rooms reinforce those echos or begin to sound like a tunnel if too long/skinny.  And of course having the room properly insulated lowers the noise floor so dynamic range (and the life of your ears) can be extended while still deriving full enjoyment.

But all residentially sized rooms have bass peaks/dips (up to 30 dB).  Read Floyd Toole's "Sound Reproduction" 3rd edition, the consummate audiophile primer on how speakers and subwoofers behave in-room.  The first solution to bass peak/dips is the use of 3 or 4 carefully placed subwoofers to even out the response.  Proper absorption is the second step, avoiding over-damping and focusing on bass.  This is hard to do as most materials/designs are ineffective.  Final icing on the cake is room correction.  Earl Geddes says treat physical issues by physical means (subs/absorption, not room correction). 

Realize that sound must go somewhere in the room, like a balloon the air must stay inside.  Room correction just pushes the air around, so corrections in one spot "comes out" (or is emphasized) somewhere else.  And trying to boost dips can be dangerous when you realize that a say 20 dB dip would require 100 times the wattage to balance out (watts versus dB is a logarithmic relationship) and 100 times the power could easily clip the amp and cause speaker damage. 

[Housteau]

The first solution to bass peak/dips is the use of 3 or 4 carefully placed subwoofers to even out the response. 

Realize that sound must go somewhere in the room, like a balloon the air must stay inside. 

I have always found the whole multiple sub idea as very interesting and if I was starting over from scratch, room and all, I would certainly have that in mind for my short list of set up ideas.  Currently my room while not huge is large enough now with that 10 ft. addition that the bass issues are not nearly as severe as they were originally.  Careful placement of the bass towers along with some DSP have reduced them to near zero at my listening position.  I got lucky. 

I recently played around with a third sub in that room.  I thought that with adding that the response would smooth out even more and I could reduce the need for the DSP I was using.  I tried everything with that third sub and nothing I did with placement, volume, or phase made any sort of worthwhile change.  That surprised me.

As to your second point, do you think that my idea of a semi bass porous wall would allow some of that air out of the balloon, assuming once it left it could be properly trapped and dissipated within that new space? 

[John Casler]

Hi Dave,

Hope you are well.

I tend to think that "sound shaping" your room is a lesson in listening preferences, and psychoacoustics.

First and foremost, IF we are looking at attempting to reproduce the recorded event. . ANY reflection, echo, wave, or sound NOT in the original recording is a distortion to the original signal.  You might remember our VMPS versus Live events, where we clearly found that recording the room, and then playing back in the same room doubled the room sound.  Most clearly evident when we heard the air conditioner rumble.

That said, I have explored an incredible number of acoustic adjustments and learned from each.

As far as bass, you might remember at one time I had 7 SUBWOOFERS.

Bass is a difficult one, because as you mentioned, the room boundaries and reflections affect the "original" first wave energy, with reflected energies crashing into them.

And those energies are "directed" into various intersecting walls, with the greatest impact (loading) in the corners.

All interesting and complicated, so the idea is to make the bass waves at your listening position as close to the recording as possible.  NOT an easy task since the bass in the center of the room is weak compared to that which is directed/collected by room boundaries.

The multiple sub concept is primarily to attempt to cause a multi-positional launch of bass energies to "smooth" the reactive/reflected peaks and valleys of just one or two subs.  I started using it decades ago, so have had a lot of experience.

I found the 7 sub solution to be quite good, but there are several issues to consider.  Since most have their subs on the floor, the floor to ceiling modes are an issue.  I ended up having a couple subs 5-6 feet off the floor, and actually having some at ceiling level would be even better. (but unpractical unless you ceiling/attic mount)  Your SUB TOWERS might help you in that regard.

The other issue wave launch and phase.  If u use multiple subs, they will be muddy, wooly, or distorted if care is not taken to place them equidistant from your listening position and in exact phase with your mains.

Then comes the other enhancement Brian Cheney told me about years ago, and that is "push/pull".  To get the air in the center of the room at you listening position to have the air movement and power you want you may look into one of two methods:

1) Place a sub or two directly behind the listening seat, running "in phase" with the front subs.  This will increase the bass energies at the listening position to a much higher power.

2) Place your rear subs facing your listening position and run "out of phase" so they are pulling while the front subs are pushing.

These take some fine tuning and placements just like all multiple sub placements, but the rewards are INCREDIBLE.

Obviously Your room is your sonic work of art, and not everyone has the same goals and preferences.  In fact many room set ups DO NOT intend to reproduce the original event (as recorded)

Some want to reproduce a specific sonic that "includes" the room sound, which can be quite pleasant and the goal. 

In the end, THAT is part of our hobby, and what makes us "audiophiles" searching for the SOUND we want.

[Housteau]

Hello John.  That was a well worded and thought out post.  They usually are.  Yep, I remember very well the double room sound effect when were making those recordings.  I miss those days.

I also remember reading your posts back in the day when you were doing the push pull sub thing.  All of these solutions using multiple subs have to deal with working with fixed room bass issues.  My thought and original question was more to, what if we could alter the physics of the bass reflections within that room by eliminating some primary reflections by having them pass through the wall instead of being reflected back into the room?

In this way we would be preventing part of the problem from even existing in the first place.

[John Casler]

I agree, and in my "mental sonic exercises" envisioned the following:

1) Listening in a rather large TENT so that bass energies were not trapped and released in the space with little reflection/loading

2) Fixing a large Horn (much like some outdoor concert venues) to direct the initial base energies to you.

Both of these, would direct the bass to you and allow it to escape and not create the modes and nodes.

But this, obviously, is impractical.

Lots of room to play around with real, and "mind" experiments.  You might even draw from any Sound reinforcement experiences from OUTDOOR concerts where you have no walls or directive/reflective barriers.