Bookshelves behind speakers for absorption/diffusion?

sleepysurf · Oct 31, 2008

As seen in my system pics, I have recently added acoustic panels (and large silk trees for diffusion) behind my Summits. Overall, the bass, soundstage, and imaging were much improved (vs. plain walls), but the music now lacks what I call presence (not as live sounding). I'm pretty sure that's from the front wall being a little too "dead" now.

Turns out, I'm about to re-do my home office, and the cabinetmaker says he can build me 2' wide open bookcases (8" deep) for the space behind my Summits. He would design it to match my center console cabinet. The bottom-most section would be 24" x 24", to accommodate the lower acoustic panels of that same size (which would remain angled for additional bass trapping). See attached draft. I'd then fill the rest of the shelves with scattered books, and cut-up portions from my upper acoustic panels. I would get rid of the silk trees. This would allow me to add and subtract books/absorption and "tune" the front wall to get the exact sound I want. The only potential downside is that I'd have 8" less space from the stat panels to the front wall (would be 37" vs. 45" that I currently have). Unfortunately, I really can't move the speakers farther out, as that would block the lateral view to my HDTV from side chairs.

As I recall, Roberto has bookshelves behind his ML's, and perhaps others do as well.

What do y'all think?

sleepysurf · Nov 1, 2008

Ethan Winer replied to my same post on Audiocircle, and referred me to one of his videos that actually compares various diffusors to a bookcase. The Diffusor comparison video is in the middle of this link...
http://www.realtraps.com/videos.htm

The QRD diffusor design is best, but I think I can get pretty close to it using the bookcase. The advantage of the custom bookcase is that it still incorporates the bottom absorption panel, provides functional storage, and will match my center console = acoustic functionality with high WAF!

68sting · Nov 1, 2008

I read this on the Gik site the other day.

There are a great may myths about ‘home brew’ ways to provide diffusion. Most do not work at all and many work poorly or only over a very narrow range of frequencies. Let’s take a look at one – a bookcase with books set at randomly varying depths.

First of all, books, if anything will be more absorbtive than reflective at all but the highest frequencies. Second, random depths do not generate random reflections over a predictable and controllable area. The width, height, spacing, and pattern of the wells of a diffuser are carefully calculated to make sure they generate a smooth and even scattering of the waves over optimally a 180 degree angle. Third, without the careful calculated spacing, we can cause frequency related aberrations due to constructive and destructive wave interactions from various reflections. We’re in effect getting very little of the benefit of a diffuser while causing more issues in the frequency response.

Another myth is using CD/DVD cases or LPs in the same sort of random manner. In addition to the issues presented above, the cases themselves are so narrow that the frequencies that would be affected would be only in the highest ranges. Again, the width, depth, and pattern of the wells and peaks of a diffuser not only generate the proper diffuse pattern, but also determine the frequency range over which a diffuser is effective.

VanDaRo · Nov 1, 2008

Ethan Winer said once in a post that "a treated room is something of an acquired taste". I couldn't agree with him more. That lack of presence( high treble) you hear is a normal thing. Most rooms ring like hell and you become used to it. When you treat the room, that nasty effect largely disappears and many people think they've lost something... until you get used to it and realize how much cleaner the sound is and how much louder you can crank your system before harshness sets in.

BTW, my room is well treated with products from GIK and I am very happy with it today. But it did take some getting used to for the first day or so.

~VDR

sleepysurf · Nov 1, 2008

With the 2' wide (inside diameter) bookcases, in a "worst case scenario" if I am NOT happy with the acoustic outcome, I can fill in the spaces with more absorption panels or GIK Acoustic diffusors (2' width). In the end, I'd merely end up with a framed acoustic treatment, that matches my furniture.

The other part of my design (which I hadn't mentioned) is that the bookcases will be 78" tall, and will support a decorative cross-shelf that frames out the top of my HDTV. The shelf will have a facade that could eventually hide a drop-down screen for use with a front projector. The plot thickens!

bpape · Nov 1, 2008

Unfortunately, bookshelves with books in them will still be more absorptive than diffusive. Books themselves are absorptive - which is in direct conflict with diffusion. If you want diffusion behind, then real diffusers are the way to go - sorry.

Bryan

kwr · Nov 1, 2008

We had years of books overflowing off shelves, spread over every flat surface stacked against the walls etc, etc. Long story short to house most of our books we ended up purchasing metal shelving that you usually see in libraries. One of the prime walls in the house was the wall behind the speakers in our dynamic, valve setup. Fortuitously it all worked with an improvement in image depth and width compared to the previous bare wall. Noticeably easier to follow musical lines in complex pieces. No change in tonality of the system to our ears.

I am not suggesting for a second that it would be the same as a professional solution but we had a worthwhile improvement over a bare wall.

Kevin

Equipment is Audio Aero cdp into Consonance preamp to Audio Aero monoblocs to Osborn (Australian with Focal drivers) speakers.

bpape · Nov 2, 2008

Ah! A lot of what I see in those shelves are not books though. Looks like a lot of HARD CD cases which will allow a little bit of skewing of the reflections - much moreso than a book.

I did see one person who spent the time to actually build the back of his CD racks to the exact specs of a proper QRD diffuser. So, he stored the CD's on it but the fronts of the CD's then replicated a proper QRD sequence. Form and function can coexist - it's just a hassle

Bryan

sleepysurf · Nov 2, 2008

I was also planning to store some CD's on the shelves, and could easily stagger them as part of a QRD pattern. In essence, the bookshelves will function as a universal frame for various diffusion/absorption options. Should provide years of tweaking options for me!

BTW, anybody know of a website that describes the precise pattern/ratio for a true QRD diffusor??

Ethan Winer · Nov 2, 2008

sleepysurf said:
anybody know of a website that describes the precise pattern/ratio for a true QRD diffusor??

www.mhsoft.nl

Acoustics tab, then Acoustic Calculator.

--Ethan

JohnA · Nov 3, 2008

sleepysurf said:
... but the music now lacks what I call presence (not as live sounding). I'm pretty sure that's from the front wall being a little too "dead" now. ..

In my experience, this 'lack of presence' is real, and it is due to lack of dynamics.
If you think about it, this is unavoidable, since you are now reducing the 'reverberant field' and loudness at the listening seat goes down (at the same settings)

After creating decent absorbtion behind my panels, I ended up needing almost twice as much power for the same volume.
It was the price to pay for the increased clarity. No free lunch.

JonFo · Nov 3, 2008

Alan, I'll come out again on the side of absorption is better than diffusion behind the Front speakers.

Please it a few days, re-run the Audyssey process and I think you will find that the lack of 'presence' is actually the absence of additional ringing.

Yes, there is also a loss of some side-wall reflected information, but that generally causes destructive comb-filtering, and since your system location is asymmetrical in the room, it never was balanced in terms of side-wall reflections.

Bernard · Nov 3, 2008

JohnA said:
After creating decent absorbtion behind my panels, I ended up needing almost twice as much power for the same volume.
It was the price to pay for the increased clarity. No free lunch.

Interesting. I had never thought about that!

JohnA · Nov 3, 2008

yep, I don't think a straight A/B test will be fair if you want to decide if it is worth going for room treatments, especially if they involve a lot of absorbtion

You'd have to adjust the volume levels between the two states, otherwise the untreated will be louder (hence prone to be perceived as 'better'

)
...and rumping up the preamp by 3db might not be 100% fair either, if the power amps are now forced to operate nearer their clipping region.

Ah, the joys of audiophilia...

:music:

sleepysurf · Nov 3, 2008

JonFo said:
Alan, I'll come out again on the side of absorption is better than diffusion behind the Front speakers.

Please it a few days, re-run the Audyssey process and I think you will find that the lack of 'presence' is actually the absence of additional ringing...

I'm trying to buy the XTZ Analyzer (if they'd only reply to my email <g>), which should provide objective data!

bpape · Nov 3, 2008

Even without a remeasure, live with the absorption for a while - maybe 2-4 weeks. Then pull it out of the room again and listen again.

Let us know your impressions.

Bryan

JohnA · Nov 4, 2008

sleepysurf said:
I'm trying to buy the XTZ Analyzer (if they'd only reply to my email <g>), which should provide objective data!

Why not use REW?
It does the job, and it's free!
We had a relevant thread a few weeks back.

sleepysurf · Nov 4, 2008

JohnA said:
yep, I don't think a straight A/B test will be fair if you want to decide if it is worth going for room treatments, especially if they involve a lot of absorbtion

You'd have to adjust the volume levels between the two states, otherwise the untreated will be louder (hence prone to be perceived as 'better')
...and rumping up the preamp by 3db might not be 100% fair either, if the power amps are now forced to operate nearer their clipping region.

Ah, the joys of audiophilia...:music:

Good point! That's why I want the XTZ analyzer, I believe it let's you calibrate the test tone db output before measurements, so you're comparing apples to apples.

sleepysurf · Dec 15, 2008

I recently got the XTZ Room Analyzer, but haven't had time yet for comparative measurements with/without the absorption. Will post results ASAP.

Also, I was perusing the Spire owners manual, and they actually mention that using bookshelves behind the speaker (for diffusion) may yield a cleaner sound than absorption! I think I'm on the right track here, and proceeding with the bookshelves. The design will also have a top cross shelf that will allow the future addition of a manual pull-down front projection screen!

twich54 · Dec 15, 2008

sleepysurf said:
using bookshelves behind the speaker (for diffusion) may yield a cleaner sound than absorption! !

Alan, that is the belief that I subscribe to as well. With the stipulation being distance, for greater distance from speaker to wall diffusion comes into play, and closer absorption makes more sense. Now is there a hard fast number for what that distance is ?? I would suspect it to be 5-6' would yield better result with diffusion, no doubt those with 3' or less absortion needs to be in the equation. So 4-5' a combination of the two ??

Bookshelves behind speakers for absorption/diffusion?

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