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Auralex Room Analysis Plus Initial Assessment and Fixes

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Plus_room.JPGI have an irregularly shaped room… Actually, it pretty much is a nightmare acoustically. It is open on two sides with the openings offset making the side walls non-symmetrical. The room opens up to the rest of the house which isn't helping the acoustics any. The DIY acoustical treatments I built many moons ago were helping (subjectively) but I knew I needed more. I've done some tests with the Velodyne SMS-1 with the treatments in and out of the room and tested a lot of different positions. I felt I had done everything I could within the limits of spousal tolerance and my own sense of aesthetics. Since this room is right next to the front door I really couldn't do anything too extreme as it is literally the first room people see when they enter the house. On top of that, I have grand plans for a new home or an addition to my current home which will include a dedicated home theater. This means I'm not really willing to do anything too permanent to the room.

The thing to remember is that most people have non-optimal rooms. Auralex was very excited to test out their service in a room such as mine as it really does represent most consumers. In any room, Auralex suggests measurements from every listening position. The way it works is you hook up one speaker to a source that can play the test tones. Auralex suggests the most direct route possible which probably means just setting your receiver to "source direct" and unplugging your second main speaker. In a rectangular enclosed room you can imagine that the modeling would be fairly easy to flip for the second speaker provided your speakers are fairly symmetrically placed in the room.

Yeah, that sounds likely.

OK, even if your speakers aren't symmetrically placed the real key to "easy" modeling is an enclosed room of rectangular shape. When you have openings or weird shapes (like octagons) you're introducing problems that give the poor acoustical engineers migraines. And that's if they are in the room much less when they are trying to fix your problems via email. Auralex would like to see a measurement from each seat in your home theater. My suggestions is at a minimum in an enclosed rectangular room do a single measurement at the prime (i.e. where you sit) listening position from either speaker. In a non-optimal room, at least two measurements - prime listening position one with each speaker. Save the rest of the measurements for the sub and some sort of post (after changes) measurement. If it comes out good enough, print the two graphs out and have them framed. I would.

You may be wondering what is the deal with the single speaker? I wondered the same thing. Why don't they use a speaker and the sub? The bass is where all the problems lie, right? Well, if you did the Room Analysis Pro (where they have an Auralex representative come out and measure for you), they'd do just that. But over the phone (so to speak), they need a single source. If you included the sub or a second speaker it would be two sources and they wouldn't be able to tell from the measurement which source was causing the issue. That being said, you can add a separate measurement of just the sub which Auralex can add into your analysis and report.

I decided to do a number of measurements (much more than you would ever possibly want to do). I have three listening positions - left, right, and center. With two main speakers and a measurement a piece, that gave six separate measurements. Plus I wanted to see what the difference would be between the treated and untreated versions of the room would look like. That doubled the number of measurements. The full report can be viewed here. This report is the EXACT report I received. It has not been edited in any way.

For the tests, I burned the test tones to a CD and played them back through an Oppo 970HD into a Sherbourn 2/75B amp plugged directly into a Salk SongTower QWT speaker. They asked for a direct path, I gave them one. The Sherbourn has a volume control on it. I used the omnidirectional mic included in my Sencore SP395A FFT Audio Analyzer package and the recording equipment from my AV Rant podcasting setup to record directly onto my laptop with Audacity. I then exported each of the recordings as a wav file (about 2 megs a piece) and sent them to Auralex. It took about a week to get the results. You can probably expect a quicker turnaround since you'll more likely be sending in one or two measurements and not 12.

The preliminary results suggest a few things. First, I have bass problems. This was no surprise as that was my concern from the beginning. Not only that, but the bass problems were exactly where I expected them to be from my tests with the SMS-1. The room treatments in general made improvements but mostly above 100Hz and mostly subtle. Does that mean that subjectively the differences were minor? No. But with this type of measurement we can see that not only do the effects change based on your seated location but also between the speakers. What is missing is a summative report which is really the downfall of this type of service. With a representative on site with specialized equipment, they could do that but with a single measurement it becomes much more complicated (if not impossible).

Plus_br-treatvuntreat.JPG

Dark - With Treatment, Light - Without Treatment 

What was encouraging overall was the flatness of the frequency response of the room above 100Hz. While not the graph I was hoping for, it could have been a lot worse. The report from Auralex also shows that the bass problems don't just stay in the bass region but have harmonics higher up. Fix the bass and watch the line flatten out even more. Note - Part of the drop in the bass region is also the response of the speaker falling off so it's hard to interpret (other than grossly) exactly what is going on in the region. The multiple measurements do really nail down that I have a main suckout between 75Hz and 90Hz depending on the measurement. From my experiences with the SMS-1 and other measurement equipment, this was exactly what I was afraid of. In fact, when I approached Sean Bowman at the beginning of this process, I was asking how to build a resonator right in that range!

Author's Note - What I have (and what most people employ) for room treatments are what is referred to as broadband absorption. These are panels, usually constructed out of fiberglass or rockwool insulation, which absorb a multitude of different frequencies to varying degrees of success. A resonator is a very targeted room treatment that vibrates (or resonates) at a specific frequency. Think tuning fork. A 90Hz resonator will take the bass energy at that frequency and use it to "power" the vibration which in turn stops that energy from reflecting around the room. When I asked Auralex about building my own resonators for some of these bass issues, they were hesitant to give me plans (though they said they had them). Why? Tolerances on resonators are very tight. If you are off even a little it will resonate at the wrong frequency and instead of removing sound, will actually create more. And that would be bad.

The Fix

Now that I have a report of my problems the question becomes what to do about them. Most of the suggestions on the report include increased bass trapping in the corners. Had I not contacted Auralex, I would have thought this to be the case as well. In addition, they suggested closing off the room or at least putting up thick curtains across the openings. Additional absorption on the back wall seemed to be warranted and a suggestion was made about adding some diffusion panels on the ceiling between the listening position and the speakers. All good suggestions and all (I'm sure) would make a big difference in the sound of my room. Let's take them one at a time.

Closing off the room
As you can imagine, this requires the most money, time, and convincing of the significant other. Before I ever started working for Audioholics I considered this option and rejected it as costing too much money, destroying the "flow" of the house, and generally reducing the resale value of what is probably not the last home I'll ever own. The one way I thought I could get this done is with pocket doors but the wiring/venting system in that room pretty much makes this an impossibility. Good idea though.

Curtains across openings
I've suggested this idea to the wife before and met with skepticism and reluctance. I've let it drop… until… one night she's complaining about not having a TV in the bedroom. Well, contrary to popular belief, I don't have piles of extra TVs to put back there. What I do have is a bunch of projector companies that would love for me to review their latest projector. I can't though because I have no light control in my main room. But if we had curtains… Sometimes, a suggestion that is met with reluctance needs only good timing to become a plan. We are now in the planning stage. Score one for the AV Geek!

Increased absorption on the back wall
This one is really not a big problem and was quickly implemented. Unlike most people, I do just happen to have extra insulation just lying around. Part of the perks of the job I suspect. Now, instead of having a 4" think panel with 2" of insulation and 2" of air gap I have a full 4" of insulation. In addition, I removed the two 2' by 2' panels from the front wall and filled one with 4" of insulation and added it to the back wall and added the second below the absorber on the left wall to help catch first reflections. I considered putting up larger (than 2' x 2') panels in the back of the room but nixed it for a couple of reasons. First, the wife just agreed to curtains (eventually) and I didn't want to push my luck. Also, and in some ways more importantly, I like the diamonds on the back wall. If I switched them out for rectangles, I'd miss the diamonds and the room would lose the one element that isn't rectangular. To me, the diamonds make the room almost acceptable to guests and I didn't want to mess that up.

Diffusion panels on the ceiling
Umm… no. This is an 8 foot high room and hanging panels from the ceiling is something only a madman or a bachelor would consider. I'm sure it would help with the sound but my popcorn ceiling will have to be good enough diffusion.

Addition bass trapping in all the corners
If you read through the recommendations, the single common theme was additional corner trapping. I have two 1' wide, 8' tall panels straddling the rear corners now. In many of the measurements, they are already making a difference. The reason I went with the 1' wide panels is clearly detailed in my DIY articles but to recap - didn't think 2' wide would fit, worried about interfering with the rear/back speakers, and I have outlets/vents nearby that I didn't want to cover. Fast forward a few years and I've moved slightly the side speakers and removed completely the rears (just didn't add enough for the size of the room and I thought actually messed up the rear soundstage as compared to a 5.1 setup). I've also removed the foil from the insulation as I felt it was resonating and I added 2" of insulation to the corner panels to beef them up a little. Auralex's suggestion was to add a LOT more absorption and to all the corners.

Enter GiK Acoustics. GiK makes a Tri-Trap which is essentially a triangular shaped trap that is made for corner placement. They extend about 17" from the corner and can be stacked. While the stats say they are 4' tall, they are really just shy of 47". This is perfect as my 8' ceilings are really a little less than that once you factor in the popcorn ceiling and the carpet. A perfect fit.

 

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Recent Forum Posts:

bsaxman posts on July 10, 2009 19:00
Anyone try this product?

Anyone try this product yet? I might try the free version without the mic. The thought of having to record the sweep is daunting (I don't have any recording devices except an ancient circa 1985 Sony tape deck) so I anticipate it will be frustrating for me.

My issue is that I have an almost perfectly square room, concrete floors, and the ceiling is dropped down to about 7 ft across the front and along one side (a basement room with bulkheads for HVAC and plumbing) so I'm thinking the room will definitely need to be tamed.

Does anyone know if the microphone can be plugged into the ‘mic in’ on a laptop and use Sound Recorder or something like that to record the sweep?

Heck does Windows Vista even still have Sound Recorder? ….well if not, I'm sure there's freeware somewhere and Google might rescue me (again!)

Comments?
fredk posts on April 10, 2009 11:57
An article on the thing that most influences sound quality (after the speaker) and… silence…
Weasel9992 posts on April 09, 2009 11:24
I'd kinda have to agree with Fred here. I mean, it's a good thing to know the specific issues in any room, but it's not terribly relevant in most cases because the solution will be the same in all but the most unusual cases. After all the basics are covered in terms of room treatments, then measurements become useful in terms of finding the nagging problems, and avoiding over treating one particular area or another.

Frank
fredk posts on April 07, 2009 20:49
Bass trapping in the corner: as much as you can. Absorbtion/diffusion at first reflection points.

This is the standard advice you see in most posts asking for room treatment help. Every room will benefit from these basic treatments.

In fact, if you go onto forums for people setting up in home recording studios, the advice is much the same.

As intriguing an idea as this service is, if you are going to spend money, spend it on treatments.

If you are inclined to see what your rooms acoustic profile, download REW (freeware), pick up the mic etc. and measure AND graph to your hearts content. Spend some of that $250 on your significant other so she is more likely to tolerate your obsession.
gene posts on April 07, 2009 15:27
Its fixed now. Check it again. thx.
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