To answer your dilemma, you set your speakers, treat room, then tweak your positions if need be. It's really not as hyper dynamic as it seems. The speaker response direct/off axis doesn't change, just reflections. Since your mains are omni directional I'm not sure treating the front wall is a good idea with anything but diffusion (if at all.) I have seen pics of a few dipole setups with large diffusers on the front wall but I'm not sure if it was done as a matter of performance rather than indulgence.
Alan would be best to comment on what to do there.
I could not eliminate the peak/null at around 60hz in my test room by moving subs around. I tried several locations. The response didn't change enough to warrant more graph pics. -just a differing level of nullness by a few db or so. The dual setup pictured was the best response overall. What was interesting is how the bass hump changed shape as I moved things around. It seemed to remain broadest at the 1/3 locations and become more peaky as I moved away.
My seating location must move to improve things I would say. Not worth it to smooth a 7db null. In content I couldn't ever reliably tell a null that size was present anyway. But it was an interesting test.
"We only started "fixing" rooms when we started measuring rooms." - Floyd Toole
I have been left wondering about diffusers, base traps, and absorption panels.
In laying out a room, with too many variables it makes me wonder how would you know what, where and how many? You have your reflection points, but getting rid of them will effect the sound stage and bring your lively Axiom speakers to sound like a studio. But too much and you loose the definition of what you want to hear in garble.
I hear about these base traps, but do I need one in each corner of the room, or can I put one into a corner where I have some protruding piping that I'd like to hide anyway?
if you diffuse a wall, does it need to be the back wall? Or if you did a side wall, would it be negative if you didn't do the opposite wall too?
That is before you start throwing back into the mix, optimal speaker placement as that might change as soon as you hang your room treatment up. The room starts spinning and it's time to get off.
I've been following the acoustical treatment thread over at AVS for a few years and it is, honestly, VERY complicated when throwing everything into the mix. There are definitely guidelines that help to tame every room, but getting those to be optimal is difficult for the DIYer. That is why a number of people focus just on the "biggest bang for your buck" approach while doing the most simple application of treatments that they can. That is just absorption. Corner bass traps are large absorption pieces, front wall treatments are pretty much just really tall/wide absorption pieces, and then stopping the first reflection points with absorption panels, and you get (my made up number) 80% there. WAY better than nothing in the room, but still room for improvement. Then the old 80/20 rule kicks in...
You can spend 20% of your time/money/effort to get 80% of the desired result, but to get that last 20% of the desired result, will cost you 80% of your effort. In other words, to get any further will take 4 times the time/money/effort than the other pieces. That has a lot to do with getting it right and not just slapping things up, and getting it right really takes someone that knows what they are doing (money), and for the diffusion panels it takes more money to buy them, or time and effort to make them.
I have my entire front wall treated (behind my screen and false wall). Floor to ceiling bass traps, and then 3.5" of absorption everywhere else on that wall. I have a thick carpet and pad (counts towards high frequency absorption more so than a thick rug on a hard floor), and I have 6 large (24"x48") acoustical (absorption) panels on my walls - 3 on each wall, plus an extra thick 24"x48" panel on my back wall above my rear seats. I didn't want to make the rear wall completely dead like the front as most advise that you have a dead-live setup with the front wall dead (absorption) and the rear wall live (reflective or maybe diffusion) but I was able to really get some good taming of lower frequencies when I added that 5.5" thick panel to the back wall.
I've been in a home theater that was fully tested and designed by an expert acoustician, and it was absolutely amazing, but that room cost $800 for the room analysis and acoustical plan (actually cheap, but because the expert had the homeowner take the measurements with equipment that they shipped him to save on flying the expert out to Iowa), and then they worked up a ton of DIY acoustical panels specifically designed for his room. It took the guy about a month, he said, of working every night after work and a bit on weekends to get all of those done (again, lots of time). It was a small price to pay for him though. I mean, he put in a separate $15,000 or so HVAC unit just for his home theater so that he could pump in cold air in winter when the rest of the house was getting heat, and so that in summer, the rest of the house wasn't an ice box just to keep the theater cool.
For more info on it, and even a 21 minute video done by the acoustician in post #5, go here (16 minutes 40 seconds into the video shows the left side wall with the fabric panels removed and the many, but not all, DIY treatments that were designed specifically for that room):
In that same post (#5) it shows the slides that David (the homeowner) used for the get together that I was at. There are some slides showing the plans for the acoustical treatments as well as general construction.
Oh, and to accomplish this, he lost something like 2 feet of space for each side wall and the rear wall, the front screen is something like 4 feet from the actual real wall.
After being in this business for over 18 years now, and being a lifelong "audiophile", I have set up hundreds of systems in all types of rooms. I have been involved in and present for the evaluation of numerous new room builds, ranging from simple to extremely complex in terms of acoustic treatment. Based on this wide range of information, these are the rules I live by and recommend to customers:
1. If you are building a new construction room or home and have the option, the most important thing you can do is to build the room following "golden ratio" dimensions.
2. If you are building a new room from the ground up and think that lossey channels, staggered studs, double drywall, etc. will result in a "better" sounding room you might be disappointed by the results. I would suggest actually finding a room built in this manner and doing some listening.
3. There is nothing wrong with adding room treatments to an existing room, but don't think you will be able to make a "bad" speaker into a "good" one.
4. Whatever room treatments you think you need, make sure that they can be easily added and removed. In most cases I have found that proper placement combined with the addition of simple items such as area rugs, drapes, and a bookcase or two can fix the majority of issues in a typical room...assuming you are starting with a "good" speaker.
5. Always trust your ears over whatever your measurement system might be showing you. If it sounds good, it is good, regardless of what the measurements tell you.
Hey Andrew, thanks for your advice. Can you say if HT use is different than music listening rooms in terms of acoustics in your experience? It would be great to have more input from you as this rolls along. Can you elaborate on how sound isolation construction strategies can make a room sound bad?
Would be great to have a speaker designer give their impressions as this thread evolves into something hopefully useful to someone.
It isn't that room isolation will make things sound bad, per se, but they are for soundproofing, which is different than acoustical treatments.
I will say this. Soundproofing (ok, nothing will truly be "sound proof" unless you bury yourself in a concrete bunker underground and have no door to escape) can help with sound quality though, but not like you think. It is a nice improvement from preventing outside sound from contaminating your room's audio performance. Most people focus on keeping sound IN their room, but dropping the audio level of their "sound floor" by blocking exterior noise helps too. Just keep in mind that without treatments inside the room itself, it won't really matter. That is what, I believe, Andrew is getting at.
Think of it this way. Lets say that you have a great sounding audio system in your car. It obviously sounds better than some cheap-o system in another car. Maybe it is tuned better, has better components, etc, but if you put the windows down (no more "soundproofing" then the audio is completely contaminated with ambient and wind noise and neither will sound great.
Same with audio speakers and gear, and the same with a room. A well treated room can sound bad if you hear the HVAC running next door, or a TV on an adjacent wall.
Any single piece will effect the other. Bad vs good speakers and gear (#1 priority), treated vs. untreated room (should be #2 priority), and isolated vs. non-isolated room (#3 most important, unless you live by train tracks or some other loud environment).
That's exactly what I thought and expected. I just wasn't sure if Andrew meant the use of soundproof strategies was detrimental in some way. I chose to do it so my furnace and laundry nearby wouldn't be a bother.
Ok, so here is a bit of a before after (and after part 2) summary. It looks like in this room the "ideal" front wall sub location is somewhere between 1/3 and 1/4 to center of driver from sidewall length. I was not able to test on exactly 1/3 or 1/4 because this footprint is occupied by the speaker stands. I would love to hear peoples input on where their R/L speakers landed in their room. Same general 1/4-1/3 footprint perhaps? I am confident we can assume somewhere in this general region would work well for subs in this room.
No subs. Baseline measurement.
Test 1. Subs inside speakers just larger than a 1/3 from sidewall measurement. (by a few inches)
Test 2. Subs outside speakers just smaller than a 1/4 from sidewall measurement. (by a few inches) I was messing around with louder tests so the the graph Y axis is taller but the range is still in 10 db intervals. Notice the gross low frequency output is higher as they move closer to the corners.
I will do a similar test in the HT eventually to determine where an "ideal" front wall placement outcome exists, but I can hypothesize from this test 1/3 and 1/4 placement is a fantastic place to start.
In this room, with the bookshelf speakers, the measured overall system response is better with the subs located outside of the speakers near 1/4 width. My overall listening preference is with the subs just inside of the speakers as they seem to blend better and decay faster somehow near 1/3 width. A tighter sound. Perhaps this is due to less reinforcement from the corners as the subs move from the 1/4 to 1/3 position.
The Subs lateral move was about 16" for this test. The net sub change from test 2 to test 3 was widening ~32" The difference in output and timing was not subtle.
I learned a couple of things from part 2 of this test:
1. A better measured result does not always mean a superior listening experience.
2. Our ability to hear subtle changes in bass output or nulls is not very good at all. An SPL meter should be used to help set the sub volume where possible. In this test, with 2 listeners, the bass was initially set far too hot before measuring. Almost 10db!