Absorbers as bass traps - question regarding absorber's backwall.

[Martin1703]

Hi,
I've built some rockwool filled absorbers that are approx 65 x 100 x 15 cm (respectively 26'' x 40'' x 4.7'').
I've built more than I need and would like to re-purpose them as bass traps by stacking them, i.e. having pairs of them in front of each other.. mounted with a little distance to the wall. One such pair would then have the dimensions of approximately 65 cm x 100 cm x 30 cm (respectively 26'' x 40'' x 9,4'').

I'm uncertain, though, whether the backpanels of each absorber would have any detrimental effect on the absorbers functioning well as bass traps as the backpanels might reflect the bass frequencies instead of letting them penetrate through the first absorber, the second absorber, get reflected by the wall, etc.. and by and by get reduced in energy.

The mentioned backpanels of my absorbers are 4mm (1/16'') birch plywood and they are glued (airtight) to the sidewalls of the absorber.
The front of the absorber is covered with Molton only, no airtight seal or any kind of membrane there.
If I had not glued the rockwool to these plywood backpanels, I'd be much more open to just cut the backpanels open..

Curious if anybody has experience, opinions, or suggestions for me.. thank you!

 

[ddude003]

It all depends on the frequencies you are trying to reflect and absorb... May I suggest you find a copy of
the Master Handbook of Acoustics F. Alton Everest & Ken C. Pohlmann on the internet, google is your frenemy... There are a few chapters on building various types of absorbers and some charts on the frequencies that various designs and materials absorb and/or reflect... Get a little of that under your belt and you will figure out how best to mod or redesign your absorbers... I am thinking you might just need to drill a pattern of various size holes into the back panels...

 

[Martin1703]

Thanks, that's great advice! Matter of fact, I do have a copy of that book. Bought it when I was planning the listening room (whose dimensions I could—within limitations—choose relatively freely) when I was segmenting off a space within our basement) and I was seeking information on room ratios etc.

After that I didn't think about the book much more and turned to other sources of info.. but, hey, I'll check it out carefully for all I can learn about absorption.

And just starting drilling a couple of various sized holes and keeping measuring if things develop in the right direction is a good way to try things out by trial and error.

 

[ddude003]

Have a look at chapter 12 on Absorption... I have the 5th Edition so chapters may be a little different... The towards end of chapter 12 shows the various types of absorbers and how to calculate the size of the holes... Have fun!!!

 

[Martin1703]

Thanks for your help! I've got the 6th Edition but chapter 12 is on absorption, so that seems right. I'm wondering, though, the holes are relevant for Helmholtz and resonant absorberts where there's an enclosed cavity with one or more holes. In my case, I believe what I'm dealing with is just a porous absorber: Out of the six surfaces/planes that are making up the cuboid/box, one, the frontside, is totally open (other than the non-airtight molton textile that's covering it. (To use an analogy, it's like an open shoebox... without its lid). Based on that I would assume it's not a Helmholtz resonator.

I'll study the book further but might eventually go for your pragmatic approach to just drill a pattern of various sized holes so that the thin backpanels have even less chance to obstruct the low frequencies.

 

[ddude003]

If you are stacking two absorbers together it would be like two shoeboxes without lids so the bottom of the front/top shoebox would have the holes and stack on top of a more or less sealed shoe box... Or am I miss-reading/understanding your design? Pretty sure the goal is to absorb/trap low frequency waves... Voilà filled Helmholtz hybrid design... You could even put some butcher paper in front of the front/top absorber and/or in-between the two absorbers to reflect high and mid range... Aka a strike plate...

 

[Martin1703]

I totally follow what you're saying and it makes perfect sense. Then again, I might have misled you by omitting important information beforehand.
So let me add some of the missing pieces of info and upload a few photos as well.

The backside of the absorbers is ugly, so I want to keep it out of sight.
The third photo conceptually represents what my idea is.. to put two absorbers in front of each other, maybe with a few inches between them and have them act a porous absorbers for low frequencies, i.e., basstraps.

If that turns out to not work well I can still build a new big filled Helmholtz absorber for the front wall.

Lastly, here's a schematic drawing of the listening room.
Dark grey = concrete walls
Red = drywalls
Purple = speakes (position is not reflected correctly.. just a very rough approximation)
Dark green = Bass traps floor to ceiling
Light green = Absorbers
Light green with dashed red lines around them: This would be the re-purposed spare absorbers I could put in front of the absorbers that are already there.. so the subject of this thread.

 

[ddude003]

It looks like you have things well in hand... Your conceptual photo looks fine to me... You may be able to fine tune by varying the gap distance between the two absorbers... I was just suggesting you perforate the back of the front absorber (light green with red dashed lines) as a method of tuning also... Looks like your room is coming along nicely... How does it sound to you?

 

[BenToronto]

Not sure I have a clear picture of your situation or absorbers. Generally speaking, first, absorbents (and I mean padded furnishings, carpets, curtains, a lot of drywall....) anywhere is the room kind of adds together, even stuff under a couch and behind pictures on walls.

Second, the air space behind an absorber (that is, hanging the absorber spaced from the wall) increases the effect, maybe not as much as stuffing that space would, but a lot.

Not simple to do the Helmholtz design. But a thin panel may be fairly transparent to sound and making a few holes in a panel lets through a large amount of sound (you can't control sound transmission with a room wall with even a few holes).

Nearly every photo of a music room I see in these forums is bare enough to make the sound truly hideous. Buy carpets.

B.

 

[Sonnie Parker]

You can use REW to measure response and reflections at your listening position (before and after) to see what changes are made. Whether you can hear them might be a different story, but once you see it, then you might hear it by default.

 

[ddude003]

With the Master Handbook of Acoustics F. by Alton Everest & Ken C. Pohlmann in hand and most of his room already filled with corner bass absorbers and other wide band absorbers, I think that @Martin1703 has things pretty much covered... And in another thread he is getting his kit set to use REW... Stay tuned and frosty my friends...

 

[Martin1703]

It's fascinating to get all this feedback (after having thought I'd be alone in all of this).. so thanks all very much!

Hi @BenToronto,
I think I have a lot of absorbents (as also rightly pointed out by ddude003) but I will add a few photos (to the chat, not the room ) to better show what the room looks like.
Keep in mind, though, please(!), that this is a basement room that's work in progress.. so it's untidy, unfinished, if I forget to lower the pics' resolution you might notice dustbunnies, and the plant's condition is nothing to be proud of either.. sheesh. After a couple of Hifi-iterations, I'll squeeze in one to bring the room's visuals into better shape.

Acoustically, my listeining (and first measurement impressions) are that the room is on the dry side..except—surprise—modes in the low end.

Yes, I'll play around with the spacing between the "stacked" absorbers and observe the effects.. both by listening and measuring.

The Helmholtz might be the last resort.. if nothing else helps. Although, last resort may very well just mean: latest step, i.e., from a timing perspective. Thanks for the warning that they're difficult to get right.. that confirms what I've been wondering and reading about so far.

Hi @Sonnie,
Yupp,... that's the plan. And I fear y'all to to bear with me as I'll start raising tons of REW related questions. And the more I'll see in the graphs the more I'll start to believe hearing, I guess. Thanks for that warning, too.

Hi @ddude003,
Yes, I'll definitely play around with the gap distances and I will definitely perforate the back of the front absorber (maybe also the one at the back so I can make better use of the gap between the absorber that's closer to the wall, and the wall.

How does it sound to me?
Overall, I'm very happy.. things sound very very good to me and I'm whining on a high level. Then again, it's a hobby and I'm curious to find out, how much better things can sound.
- High and mid frequencies sound great in my mind, playing around with toe-in might all that's needed.
- Stereo image is good between the speakers but doesn't go outside of them. Although, I seem to be quite agnoscent of a good holographic stereo picture even if it's there. In my years as musician in a brass quintet, I'd trained myself rather to hear each of the five instruments "in one" as opposed to separating them. I remember when we recorded a CD (in a live environment) and the engineer called us to the monitoring room, played us back the music.. and the stereo picture was so extreme that I felt I could not judge any longer whether our attacks had been simultaneous, whether I was playing in tune, etc. As an active musician, I didn't feel that advantageous. Now.. years later I feel to the contrary and do desire a stronger stereo image, I just don't know if the room, the kit, or my brain/ears is the limiting factor.
- Low frequencies: There are clearly audible boomy notes in the room, at least for some kind of music. The sub I've bought doesn't help yet—I yet need to acquire the skill to position and integrate it. But I'm optimisitic it's going to happen.

Examples of where and how clearly I notice booming bass:
- Mahler 3, San Fran, Michael Tilson Thomas: Great recording, no issues with the bass. I just love the sound, happiest camper in the world.
- Barbra Streisand, West Side Story / Something's Coming from 'The Broadway Album': The bass (starting at about 0:44) is purring along very nicely if I keep the level at comfortable/normal and slightly above volume level; if I raise the volume further, the lower end gets muddier.
- Marcus Miller, B's River from 'Afrodeezia': The low notes, e.g.. at around 0:26 are very boomy, that's clearly audible and does not sound very good.

So.. here's the approach I intend on taking over the next weeks. Could you all do me a huge favour and let me know what you think about the approach?
I'm basically following the Hifizine Bass Integration Guide, which John Mulcahy recommends also in this forum.

• Done
  • I've studied the article
  • I've got REW setup and - in principle - working
  • I use a Behring ECM8000 microphone (not calibrated, but supposedly very flat)
• Coming Up
  • Ensure that I'm using REW the right way, so that I don't have to radically change the way I use it later on and make comparisons futile.
  • Double check listening position using REW
  • Measure existing bass performance of the room (Hifizine guide Step 2)
  • Start planning and implementing the acoustic treatment (while always double checking the effects by measuring and listening).
  • All along I'll continue to study the Everest/Pohlmann Handbook of Acoustics and whatever else I'll find I need on my quest to Audio Nirvana.
• Later
  • Speaker Placement / System Calibration / EQ

 

[BenToronto]

Ah, the secret problem may be your basement. Which often means lotsa concrete and cinderblocks and very challenging reverb. Which is why I previously mentioned dry-wall construction which actually does some amount of absorption esp, I suppose, into an adjacent room. And "cabin pressure boost" or whatever is the current name of that old urban myth; while rarely happening outside well-sealed car audios, might happen in your basement.

REW has a nice reverberation time display. If meaningful, it might be your yardstick. If I recall, the aim varies with freq but is around 300 ms.

Mahler's 3rd, great big side drum heard a lot in that piece. Good test. But truly hard to control freq that low.*

Brass musician, eh. Hope you wore musician's ear plugs. Or else.....

Good luck.

B.
* there's a clothes closet in my music room opposite wall to the speakers. Holds my heavy Canadian winter motorcycle clothing, bedding, etc.. Clear difference in REW freq run when I slide the door open.

 

[Martin1703]

Hey Ben,
yes.. the fact this is all in the basement is certainly a challenge.

I've tried to contain it by taking following steps:

• Two of the room's sidewalls are concrete. In front of those there's a drywall construction .. in German that's called "Vorsatzschale".. according to the online translator that should translate to "facing formwork" or "flexible shell". Basically there's a 4 inch cavity filled with glasswool and two layers of gypsum plaster boards on the usual metal construction.
• Two of the other walls have no concrete behind them, so they are stand alone drywalls.
• The ceiling is concrete but I've installed a suspended ceiling.. suspended by about 8 inches and filled with glasswool.
• The flooring is complex.. at least from a translation perspective, I hope the screenshot will provide some clarity
  • 

Here's another view of the entire basement's floor plan. So since not all of the sic delimiting planes are concrete I hope I'll have less of that "cabin pressure boost".

Grey = concrete / Red = drywall

 

[Martin1703]

Quick question to everbody.. please. Could you take a look at the attached measurements and let me know if there are any obvious flaws with how I've taken the measurements, as far as can be told from the graphs. At this stage I'm mostly interested in the process of taking the measurements. If it seems legit, I'll go ahead and start taking measurements from various positions in the search of the best listening spot.
I assume I would do that with the subwoofer off, and probably just one measurement (L+R) for each position, or am I wrong here?

As always, thanks for any light you can shed on this!

 

[ddude003]

Everything looks pretty good at this point Martin... Some things I would suggest... Try aiming your mic 90 degrees (straight up) when taking measurements... Measure the Subwoofer separately... Do the "Subwoofer Crawl"... By the way, how do you have your crossover set for the Sub? Full range on the Focals and blend the Sub?

 

[BenToronto]

What are you XO points and slopes?

Looking at your final "sub" trace, you have a prominent boost 80-130 Hz. Once you have that tamed, the trace south of that should be made to curve upward from the level in order to sound right.

Not saying this applies to you whatsoever, but very common for folks will think a loud bass guitar booming at 80 Hz in their car is super bass.

B.

 

[Martin1703]

Thank you both!

I'm going to aim the mic straight up and will measure the sub separately!
The sub XO is a tricky point.. I haven't spent too much time with that yet and with the little time I've spent I've not made very much progress yet.

I have no digital (or analog) external crossover, so I'm running the Kantas full-range and use the sub's following two controls:
1) The XO frequency knob

- It has 39 notches that let one dial in the frequency between 20 and 120 Hz.​

- For the measurement I've had it at the third notch, so probably at approx 28 Hz​

2) The volume know

- It has 39 notches from min to max.​

- I've had it at 5 .. so quite low.​

According to the Hifizine Bass Integration guide a digital XO would be great; would you agree?
My best guess is that a DEQX or Trinnov based solution will be out of my financial reach in the near future (and maybe mid-term as well). How good the miniDSP works in a 2 channel (or rather 2.1 or 2.2) I don't know.. yet. They Suggest a Behringer DCX2496. However, the article is from June 2011 so there may be tons of new and better options around?

Regarding the 80-130 Hz boost. Yes, in a first round I'll see how much I can do about it with the bass traps I want to construct out of my spare absorbers. In a second step, I'll need to think about EQ/XO/DSQ I guess.. right?

Please note, though, that it's not the sub driving the boost.. with and without sub both look quite the same in that range:

All of the above steps I'll address when I get back home .. at the moment I'm on a business trip in Salzburg for a couple of days. Walked past the house where Mozart was born yesterday. Nice.. but listening to his music beats lookin' at an ole house hands down.

Will keep you posted!

 

[BenToronto]

No DSP? Seems inconceivable to me not to have DSP and be bi-amped today ("today" is for me after 65 years struggling to add more "fi" to my hifi).

Smart move is to get the Behringer DCX2496. You'll soon ask how you lived without one heretofore. Ignore the critics.

B.

 

[ddude003]

@Martin1703, you should be able to run your Focals on full range and get your sub to match up (blend) somewhere between 30Hz and 80Hz... As for DSP, don't you have Roon for that??? Both Parametric EQ and Convolution EQ there for your use...

 

[BenToronto]

A basic rule is not to stress a driver outside its useful passband. That's esp true not forcing a mid-range driver to handle the vastly larger excursions demanded playing bass.

My impression is that folks are increasingly opting for steep slopes in crossovers (now that DSP makes that, at last, feasible). Which means each driver can be massaged into good performance without too much interaction between drivers.

B.

 

[ddude003]

There are two woofers in each Kanta No.2, which Focal claims extends down to 29Hz, –6dB... No "forcing" a mid-range driver here... And I am sure that Focal knows what they are doing when it comes to designing this 3 way speaker and its internal crossovers... This is more than sufficient for musical content in 2.0 / 2.1 stereo, which it appears is @Martin1703 intent...

 

[Martin1703]

Hi .. I'm back home and have spent two half days taking measurements.. but before I go there, let me see if I understand your DSP guidance right:

DSP Question

• One option is to use the DCX2496 between the preamp and amp, which is an additional A/D/A conversion but allows to set delays, phase, and cross-over for the sub and the speakers separately.
• Another option is to use the Nucleus, which offers very fine Parametric and Convolution EQ, and does not require any addtional A/D/A conversion; I'd run the speakers full range and use the subwoofers built-in crossover.

I might go for the Nucleus option first, as I've got all the required equipment already.. and play around with the DCX2496 option later on.

Forum Etiquette Question
Btw, quick question.. as I'm haven't been active in forums yet. Should I keep all my posts in this thread ("Absorbers as bass traps..") or split it up into different topics as I go along?

REW Measurements - Subwoofer
Coming up soon.

REW Measurements - Main Listening Position
I've taken 59 measurements, mostly in 1'' distances and found that the waterfall graphs from the middle of the room look best to me.
The filenames indicate the distance from the wall behind the listening position (as opposed to the wall behind the speakers). The ones from 270 to 280 cm look best to me, at least from a low frequency decay perspective.. but there are some dB peaks that will need to be tamed, e.g. around 215Hz.

Pretty soon, I'll start trying out if the stacked absorbers will function well as bass traps to see if the help with the mentioned peaks.

Here's a visualization of where in the room these positions are (and some color coding based on my belief which measurements/positions are better and worse than others) and I'll also upload a number of exemplary files (but not the 59 measurements I've taken, can't do that to you).

I'd me most curious about your feedback in general but especially pertaining to file 277.5.mdat.... thank you!
This position would be a good MLP also from a stereo triangle perspective.

 

[BenToronto]

REW will average (that is, essentially sum) readings. I measure where my head is and 6 inches or so on both sides and then average the three. You want to see each driver of interest plotted individually and later with XO operating.

Might as well settle on a reference like your head location (or maybe also the three-some measurements) and go from there.

REW RT (reverb time) might be interesting to see.

Direction of pointing mic not important except for highest freq - and then, as mentioned earlier - broadside to the mic.

I bought a vintage DAC at the scrap store for $4 some years ago and it measured more perfectly than my means of testing could show. Some of us now use all digital sources; so we use the DCX2496 with only the exit DAC operating. The DCX will give you astonishing powers of all sorts and adjustable instantly (once you get adept as using it), esp important since you are a discerning music listener in a hard-walled room.

Go to bi-amp'ed - second-hand amps subsequent to 1980 all sound identical.

B.

 

[ddude003]

Hi Martin, welcome back...

DSP Question

• One option is to use the DCX2496 between the preamp and amp, which is an additional A/D/A conversion but allows to set delays, phase, and cross-over for the sub and the speakers separately.
• Another option is to use the Nucleus, which offers very fine Parametric and Convolution EQ, and does not require any addtional A/D/A conversion; I'd run the speakers full range and use the subwoofers built-in crossover.

I might go for the Nucleus option first, as I've got all the required equipment already.. and play around with the DCX2496 option later on.

Was there a question in there? If you were to have several other speakers or multiple subwoofers you might look for speaker management hardware... Personally I would not put a ~$300 A/D/A in between my gear... I guess if you had horns and bass bins to manage, or some other speaker array(s), this might be the way to go... Your not planning on any DIY mods to your Focals are you? Like deleting the internal crossovers and controlling each driver separately... So bi/multi-amping is out... Again, something for the likes of horns and bass bins... And you have a pair of nice Focals and a nice REL subwoofer... Easy peasy...

Forum Etiquette Question
Btw, quick question.. as I'm haven't been active in forums yet. Should I keep all my posts in this thread ("Absorbers as bass traps..") or split it up into different topics as I go along?

That is really up to you and the Mods... Makes it easy that it is all in one thread...

Finally... REW Measurements - Main Listening Position... For some reason the quote feature did not work, so I will be brief... I agree that file 277.5.mdat looks the best... With 1/6th smoothing it seems pretty flat and a little Roon DSP with a room curve should clean things up... Something else you might try is moving the Focals forward and backward a few inches... Just to see how things change... Having your MLP in the exact center of the room does not seem intuitive to me... What is the distance of them from the front and side walls? Sorry, I know... With those footers it will be a pain and worth it... Have you tried toeing them in at all yet?