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Re: Lets plan a theater space
AAAA #415059 10/17/15 07:52 PM
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Well the QRD is almost done. It just needs time to dry a little more and then to mount behind the screen on the front wall and support with some in-situ bracing. Might be a little tricky given it is 76" wide, 37" tall and weighs roughly 60lbs. Doh! Probably going to have to remove the left channel and center channel to do it without risking a calamity.

This project started as many of mine do. Useful junk (literally) falls into my lap. This stuff was packing dunnage for insulated wall panels. They are the types of panels used for the exterior of modern food processing plants, warehouses etc. These were in abundance and headed for the dumpster. My fater in law scored 23 of them for me, so naturally the wheels began turning on how to use them in the basement. Originally, we were going to use them as two flanking diffusers on the back wall on either side of the media shelf, but once I saw how rough it was going to turn out it made sense to use it behind the screen.

I found a program called QRDude a couple of years ago and started to read up and lust after building one of these monsters.

http://www.subwoofer-builder.com/qrdude.htm

I decided the best use of all the material was to build a single diffuser rather than several repeating periods together. It made for an interesting build.....

At first I planned out the build using the software and made a cheat sheet for use in the garage.



Then I got to work cutting down the blocks with a table saw. This soon became a horribly dangerous proposition, as the foam would chatter and eventually kicked back dragging my hand perilously close to the blade..... I almost pooped.

So, Olfa knife it was! I had to cut one side along a straight edge and flip it to finish the cut. It took an hour or so, but I had all my fingers in the end.

Here's were step 1 ended up. There were more interesting things in the works (the absorber panels) so it sat like this for a month or so.



Then I had to decide on a thin but sturdy material to act as the well dividers. It had to be cheap as well. This is a bonus round item and the funds for the theater are long spent...

I ended up choosing MDF wall paneling from HD and ripped them down to 9 11/16"- the max depth of the wells. Gluing them up was a couple step process. I only had enough clamps to do them in sections, so I split it up into 3 pieces and did one at a time. I had to use left over cut off pieces to shore the very deep wells up while the glue dried. 7 Tubes of PL Premium later... This was starting to become a not so cheap project.... lol.



Once the smaller sections were glued up and set for 24 hours I got set to glue them together onto 2 backing runners I will screw to the wall.



The next morning I test sprayed a small area with flat black spray paint. Not a good result. I guessed it would happen. Spray paint and styrofoam do not play nice together. It melts. I had a tin of water based flat black left over from painting the screen goal posts so away I went. A sprayer would'a been a lot quicker, but it was a nice lazy Saturday morning putter. We'll see what happens, but this monster might get installed tomorrow. Excited to see how it "sounds" or better yet, smooths out some mid to high frequency valleys it's supposed to do. We'll see. confused


Re: Lets plan a theater space
AAAA #415062 10/17/15 08:48 PM
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Impressive amount of work and planning there, Trevor. It's funny. When you first got here I thought you were a hipster, not a nerd. You hid it well. Still impressed.


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Re: Lets plan a theater space
AAAA #415071 10/18/15 09:56 AM
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Lol. Depends who you ask I guess. I'm too fashion and fad blind to be a full fledged hipster. Because of the indie thread? I just lke off the norm music... whatever that means.

Getting this thing down the stairs will be a treat.

Re: Lets plan a theater space
AAAA #415075 10/18/15 05:45 PM
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Ok so things got "real" today. I'll explain.

It's been an exciting morning. A "wow" audio moment that rarely happens happened today. Diffusers are a game changer. I had to call in a witness to verify I wasn't imagining things. "Thats Fucking Amazing" was the witnesses input.....

The day started out with me trying to figure out how to get the QRD to fit down the stairs in one piece. I had to trim back the back rails a bit before I drug it in the house. It was around 80" long including the back rails. It made the stairs in the end. Phew!

The first stop was the 2ch listening room. Afterall, I would likely never have the opportunity to do a test like this again, and I wanted to see if the QRD was all it was cracked up to be. I was hopeful but realistic. I set it between my 2 speakers and aligned with the centerline of the room and my listening chair.




Ok, so, it's huge it turns out. grin I sat and listened to a few tracks I've heard too many times with testing and I thought something was amiss. I moved the QRD back out of the way, an awkward bastard with some chunk to it, and sat for another listen. Holy shit! The QRD makes the front wall sound like a single huge speaker in some ways. The front soundstage became huge, deep, seamless, enveloping, razor sharp and reach out and touch it right there. I thought I was imagining things. I mean this thing was a huge battleship of an eyesore, yet it produced an emotional response I have not had since I can't remember when from music. I was choked up. Crazy!

I had to measure. I had to figure out why. Here is the measurement with the QRD in place from the listening position.



Here is the same measurement with the QRD removed from the room completely.



The frequency response showed no obvious clues of the euphonic soundfield created in the room. I thought for sure some peak would have been produced right in the presence region from what I was hearing. Strange.

I was forbidden from trying to lug this thing down the stairs by myself (thank goodness I didn't try) so I made a call to the father in law, a proud owner of some Axiom speakers, to come and help out. I busied myself taking apart the screen wall downstairs and getting speakers out of the way while marking speaker positions with tape carefully.

He arrived just as I had obliterated the basement. grin I played it cool and said "Hey, tell me what you think of this." This was "garbage" he got me afterall, so he was a key shareholder. laugh I sat him down with the QRD in place and played a song he was familiar with. The song played through and he thought it sounded great. His usual "Yep, it sounds great, Trev." Then I had him stay put and drug the QRD back out of the room with my wife. Same volume, same position same everything, just no QRD. We replayed the song.....

I had resisted the urge to say anything at all. He was feeling me out for what was going to happen. I told him I didn't want to bias his opinion and let him finish listening to the track again. He didn't make it all the way through the second time. "Oh my god, that's fucking amazing. Fucking incredible." It was a success.

The difference between a recording and live event had seemed to be blurred. It was frankly astounding and beautiful.

He was super pumped to get it lugged down the stairs and chucked behind the screen. I had to slow up the process to get a gameplan going. It was going to be a snug fit afterall....

With the room apart we got started and made it down the stairs. We went in through the left side of the screen and I only had to remove one speaker. Speaker setup is a pain in the butt for me (HT neurotic perfectionist) so I only wanted to touch one channel.



Anyone who's moved a sofa around a tight corner can see what we were up against. A tilt, hook and push and we were in place. Gently threading the lunker behind the center channel. How is it you get 25% stronger while you hold your breath? lol. There's a lot less room back there now!



We sat and listened to the same track downstairs and agreed we had stumbled upon something quite special. With only 2 channels playing the entire front wall was a massive floor to ceiling, wall to wall, soundstage.

I had to try out the center channel and threw in Pink Floyd's SACD of the DSOTM. It has a pristine Stereo and 5.1Ch mix so it was great to do a quick A/B test in tracks. The center channel has came alive in a way that's hard to describe. I'm not sure if its proximity to the diffuser has something to do with it, but it was like no sound I've ever heard it produce.

I know it sounds stupid. It reads stupid. It is stupid. Stupid good somehow. It sounds like a "real" voice is behind there.

After the high fiving stopped (jk) laugh I settled in to take some new measurements of the basement with 2ch and subs. This time I wanted to see how the sweet spot improved off axis and how frequency response was affected across several listening positions. Again, like absorbers, diffusion can be magic. Here's a rough roadmap of the measured positions followed with their individual responses at each seat.



Position 1. Sweet spot.



Position 2. Chaise lounge off axis left.



Position 3. Snuggle Seat beside sweet spot.



Position 4. Mother in law chair. grin Waay off axis!


Re: Lets plan a theater space
AAAA #415076 10/18/15 05:54 PM
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I wasn't sure what to expect when messing with diffusion. The poly diffusers on the sidewalls look cool, but they were no where near as dramatic as the front wall diffuser. I have yet to watch any movies down there with it. But I can't wait to hear how dialogue in movies is affected.

Next to Dual Subs and Absorbtion, Diffusion is a must have IMO and I'd encourage anyone to try it. Very cool! I'll do a more sterile report once my excitement calms down a little. grin

Re: Lets plan a theater space
AAAA #415077 10/18/15 08:28 PM
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Absorption is better "Bang for the buck" improvement in a room, so since you started with diffusion first, I think that you will really like it when you mess with absorption. Absorption is actually easier to implement too.

A perfect solution has a combination of absorption, diffusion, and reflection of course, but you are well on your way to some really nice sound.


Farewell - June 4, 2020
Re: Lets plan a theater space
nickbuol #415078 10/18/15 08:34 PM
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A perfect solution has a combination of absorption, diffusion, and reflection of course... and Axiom speakers!


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Re: Lets plan a theater space
AAAA #415080 10/18/15 09:28 PM
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Hey Nick,

The sidewall reflections have absorbtion and front corners. I have 2 batts of Roxul left. Any suggestions on where to put them?

I have room on either side of the front diffuser if thats what you were getting at before. I am flirting with a dry room if I add much more absorbtion I fear.

It would be nice to flatten out the lower frequencies more (below 500hz) but it sounds VERY good now where its at. smile

Open to suggestions as always.

Re: Lets plan a theater space
AAAA #415081 10/18/15 10:40 PM
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Post deleted. Wrong math formula proposed.

Last edited by Serenity_Now; 10/18/15 11:22 PM.
Re: Lets plan a theater space
AAAA #415089 10/19/15 09:43 AM
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I deleted my last post but I should have left part of it.

I think the suckout on axis centered at 180hz could be due to SBIR effects. Looking at the measurements across the listening area the suckout disappears as the measurements are closer to on axis of a single speaker.

I originally posted the formula from acoustic frontiers

Sbir frequency = speed of sound/(2x(direct path-reflected path))

In my room the front wall reflection adds 38" of travel (the distance from the front baffle to the wall)

So sbir frequency = 1130/(3.2x2) = 176hz.

BUT

The vanilla formula for sbir, not taking into account the reflected path is

SBIR= speed of sound/(4xdistance to boundry) This uses the 1/4 wave distance cancellation due to phase interaction -but doesnt take into account listener angle. I think the original formula I posted is better for this instance and the 1/4 wave formula better for subwoofer modal predictions.

Since we are talking about adding absorbtion to the front wall, I'm trying to predict the best outcome and share the results with you guys. I'll try absorbtion directly behind the main speakers on the front wall. We'll see. smile

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