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  • Diaphragmatic Bass Traps

    I was planning on holding off and including this in a piece I am writing on fixing my room acoustics and the DIY diffusers and bass traps I have built, but I was so surprised by the results, I figured I would share a preview. Even after I have installed numerous RealTraps (and home made Owens Corning) absorptive bass traps and two huge (8'x4'x4') corner absorptive bass traps, I still had some bass overhang in the region below 100hz and particularly in the 30hz -50hz region. While the absorptive bass traps did a nice job smoothing the bumps out above 125hz or so, they simply could not significantly impact the lower bass. My hearing and the charts from Room EQ wizard told me that in the lower bass region I had a decent sized bump and moreover, the waterfall graph showed the lower bass sticking around too long. I have always found bass overhang to be particularly pernicious, as it not only muddies the bass, but also smears the rest of the bass and mid range - masking detail and closing the soundstage down. So, I did alot of research and determined that my best bet was to go for a diaphragmatic bass trap designed to address the lowest frequencies (unlike the absorptive traps, the diaphragmatic traps, like the Helmholz resonators, tend to have a relatively narrow operating range). Acoustic Fields has plans you can purchase for the bass traps. The critical dimensions are those relating to the depth and placement of the resonators, absorbers and the back. I designed mine to fit under my diffusers in the front of the room (where the bass energy would be the greatest). As you can see from the pics, the resonators are 1/4" and 3/8" mdf boards. In front of the resonator goes a 2" thick Owens Corning fiberglass board and 3" behind the resonators is pegboard and area for 2" of absorptive material. This could be more fiberglass board, but I decided to give a try to a suggestion by Acoustic Fields and instead use 2" of activated carbon. Dennis Foley of Acoustic Fields says on his material and video that the activated carbon is numerous times more absorptive of bass waves than the fiberglass boards. One critical aspect of these bass traps is that they must be absolutely rigid and dense, so the unit is built of 2" of Ranger board (which is mdf that uses finer particles - it isquite a bit denser and heavier than normal mdf). The 1" Ranger board sheets were laminated together using Green Glue. For the back of the unit (which is really critical), I used 3" of the Ranger board. As you would imagine, the units are very rigid and very heavy (about 350lbs.).


    Prior to installing the units in my basement listening room (thanks so much for your help Steve Weiss!), I played a bunch of bass-heavy music - e.g. Morph the Cat, Plant and Krauss, Polly Come Home, some Ray Brown pieces and some deep, deep organ pieces, etc. This all confirmed what I had been hearing re the lower bass overhang. I then installed the units, got some ice for my back, and listened. Well, I was blown away - the bass overhang was almost entirely gone. The bass sounded faster, more detailed and articulate. The mids and highs really opened up, as did the soundstage. Indeed, it became clear to me that I have to readjust the settings on my subwoofers, as the music sounds a bit tilted towards the highs now. I have not checked the readings with Room EQ Wizard, but I know what I am hearing. It's quite remarkable.

    As I mentioned, I am planning on posting at some point a more thorough discussion of my work on dealing with the acoustic issues in my listening room.

  • #2
    Hey Richard Glad that your time spent has resulted in what appears by your words to be some excellent results I guess what Dennis professes is on the mark :-))

    Comment


    • Cohnaudio
      Cohnaudio commented
      Editing a comment
      I have to admit, I was doubtful (I'm a skeptical person generally). The thing that kept gnawing at me was whether I was spending so much time, energy and money for nothing. From my research (particularly from the white papers published by the BBC), I knew the diaphragmatic bass traps were the way to go for the lower bass. The question was, how many traps did I need to hear a difference. I can't say whether the use of the activated carbon, instead of the fiberglass board, made the difference, however, I intend to check the effectiveness of the carbon on its own as an absorber, but creating some pegboard sandwiches with the carbon and placing them in the corners and listening and measuring whether there is a difference over the fiberglass. I am a firm believer of both listening and measuring.

  • #3
    Very ambitious and impressive Rich. I look forward to learning a lot more.
    Tom
    TIDAL Audio Agoria Loudspeakers; VAC, Merrill Audio & Bricasti electronics; Spiral Groove SG-2 TT with Centroid Arm & Transfiguration Proteus D Cartridge; Ampex ATR-102 with Merrill Audio Tape Preamp & Ampex electronics & Tape Project/Technics RS1500 reel-reels with Bottlehead Tube Repro Pre; Sonore Signature RenduSE Network Player; Bricasti M12 DAC/Source Controller; Ansuz Cables & Power Distribution

    INDUSTRY AFFILIATION: Dealer- XtremeFidelity.net (VAC, Bricasti Design, Merrill Audio, Sonore, Ansuz, Aavik, Synergistic Research & others)

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    • #4
      Great undertaking! Room acoustics are the last frontier for me, i.e., dedicated listening space.
      Vbr,

      Sam

      Comment


      • #5
        Thanks .It has actually been about 5 years of experimenting and I do more feel I'm pretty close to where I want things .I have some real challenges in my listening area

        Comment


        • #6
          Serious stuff here, very impressive!
          Speakers: Horning Eufrodite Ellipse III on Stillpoints Ultra 5S feet
          Amplifier: Silver Circle Audio 300B Monoblocks with Takatsuki 300B tubes
          Preamp: TW Acustic RPA-100 Line Stage
          Phono Stage: TW Acustic RPS-100
          Turntable - TW Acustic Raven AC w/ Black Knight Anniversary Upgrades
          Tonearm: Kuzma 4-Point 14" with Air Tight Opus-1 Cartridge
          Tonearm: TW 10.5" with Miyajima Zero Mono
          Vinyl Care: Audio Desk Pro Ultrasonic & VPI 16.5 RCM
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          Wire: Custom Furutech PC's & IC's + Inakustik LS-4004 Air Speaker Cables
          Tweaks: Stillpoints Aperture II Panels & Stillpoints Ultra 5S's & SS's sprinkled about - SRA Ohio Class and Symposium Super Plus Platforms

          Comment


          • #7
            Excellent post. I have wanted to move beyond the typical footers, stands, speaker placement, power to optimize playback. The room is spoken of as so critical, yet no one seems to know how to address it, so few talk about it. Thanks for venturing into the space and sharing your results.
            PAP Trio 10/Voxativ & PAP Trio 15 Horn speakers, Ampsandsound Casablanca monoblocks, First Sound Audio Mark 3SI preamp,
            Mojo Audio Deja Vu server, Mojo Audio Mystique V3 DAC, The Linear Solution Ethernet Switch, Blue Jeans Ethernet cablling,
            Akiko Corelli, Custom power strip direct wired to panel with OFC copper wire. Inakustik Ref Air 2404 Speaker cable. Genesis and Inakustik NF2404 Air Interconnects.

            Comment


            • #8
              I really cannot overstate the impact the proper acoustic treatment has had on the sound in my listening area. The problem is that there is so much misinformation out there. The best sources I found were the Handbook of Acoustics and the BBC white papers on diffusers and bass traps. As I am not an engineering kind of guy, much of the mathematics were over my head, but through trial and error, I was able to fix many of the problems of my room. I am fortunate in that I have been building fine furniture for over 20 years and have a full workshop, so I did not have to buy the difffusers and bass traps (the larger diffusers - e.g. QRD 23s, which are 18" deep and 2'x4' and the diaphragmatic bass traps are quite expensive). Having said that, anyone with basic skills and some area to work with plywood can build these things - nothing complicated about the design. Indeed, I came up with a way of building the diffusers more efficiently/quicker and which results in a much more rigid structure (which is critical for diffusers).

              Comment


              • #9
                I am about the start my own versions of these Diaphragmatic absorbers and am interested in some of the details of your units. As an aside, I'm waiting for activated carbon to be delivered in the next few weeks.

                1) The pictures show construction details different from the "plans" you can buy from Acoustic Fields. Any reason why you chose to go a different route?
                2) How many and what size are the absorber units you have in your room?


                I'm also curious about the details of your diffusers. Specifically, what material are you using for the vertical slats?

                Acoustic Fields claim these need to be solid hardwood (not plywood) because of its preferred sonic signature but I have a hard time believing that they are actually using such thin and large panels of solid hardwood due to expansion/contraction and the difficulty in machining it.
                Speakers: Rockport Avior
                Amp: Ypsilon Aelius Mk II Silver Edition
                Preamp: Pass Labs XS
                Phono preamp: Ypsilon VPS-100, Slagle Silver SUTs
                Phono: Continuum Criterion, Kuzma 4-Point, Lyra Etna SL (and others)
                Digital: LauferTeknik Memory Player, dCS Scarlatti DAC
                Tape: Studer A80 RC, King/Cello preamp

                Comment


                • #10
                  I spoke with Dennis Foley today. Can't believe he made time to talk with me. His is a real company. As in hundreds of employees and 10s of millions in revenue. Very soon he will do major projects only. Having said that, he said his production line is very efficient. If you want to build a panel yourself, you need $650 in materials or you pay him $900 and its complete and done right. The channel that holds the carbon has around 50 of so internal braces. Time spent cutting and assembling has to be around 12 plus hours. My estimate. Getting materials and performing the build. I was thinking the same thing, build my own. I just don't see it now unless there was some non standard shape they did not produce.

                  I might get a couple panels for behind my speakers and to cover my windows and door pass through. He talked of room consistency. Your screwed if you have holes and panels of glass. His window panels look nice as they glide on rails to open and close. I can do the same with my door pass through.

                  I still have a hard time getting my head around broad band absorption of 30 to 6500 hertz. I guess if done right I guess it could help. There appear to be a lot of studios contracting him for their control room work, as well as high end consumers with 2 channel audio. Cohnaudio is saying it worked.
                  PAP Trio 10/Voxativ & PAP Trio 15 Horn speakers, Ampsandsound Casablanca monoblocks, First Sound Audio Mark 3SI preamp,
                  Mojo Audio Deja Vu server, Mojo Audio Mystique V3 DAC, The Linear Solution Ethernet Switch, Blue Jeans Ethernet cablling,
                  Akiko Corelli, Custom power strip direct wired to panel with OFC copper wire. Inakustik Ref Air 2404 Speaker cable. Genesis and Inakustik NF2404 Air Interconnects.

                  Comment


                  • #11
                    Re the diaphragmatic bass traps, i used the front to back dimensions on the Acoustic Fields plans. I have not yet found a good explanation of how to determine the appropriate dimensions, so I had to rely on their calculations, which they say (and show some testing results) are effective down to 20hz (though significantly more effective from 30hz to 125hz). There is a book for bass traps and diffusers that may have explanations on figuring the dimensions, but I have not yet purchased it. I could not find any other useful information on design on the internet. I confirmed with Dennis Foley that the critical dimensions were the front to back dimensions, so I adjusted the width and height, so that I could place the traps under my diffusers. I built 5 of them and they are about 14" deep by 14" high by 52" wide. I also made the traps with 2" of Ranger Board and used 3" for the back. It's critical that the units be absolutely rigid (other than the membranes). I might build some more units, but I have some furniture projects to finish first. Moreover, I have gotten some anomalous readings with REW (which could be the result of a number of things in the room), so I want to track that down first.\


                    Re the diffusers (I built both QRD-17s and QRD-23s), the only wood that is practical for the fins is 1/4" plywood (though it must be high-quality, multi=ply plywood, not the crap you get from Home Depot - to avoid vibration, which significantly degrades the performance of the diffusers). Hardwood fins are not practical for a number of reasons: good luck finding solid wood that wide (18:" for the QRD 23s), at that thickness and width they will almost certainly warp, you would need to construct the cabinet to allow for wood movement, which I don't think works for the design. For the QRD-17s, Acoustic Fields suggests using wood blocks to set the proper well depths. That works, but it adds alot of work, doesn't add much to rigidity and adds lots of weight. When I did the QRD 23s, I was determined to find a better way. It just so happens that the polystyrene foam boards used in construction (around the outside of the house) and found in Home Depot are exactly 2" wide (the width of the wells), are incredibly strong and rigid and come in sheets wide and long enough for any diffuser. You simply cut the foam to the appropriate length and width (based on the depth of the various wells), minus 1/4" for the plywood cap, glue the fins to the foam and glue the whole thing up in the right order et. voila! For the well that is the full depth of the unit, you simply place a piece of the foam covered in plastic tape, so it doesn't get glued in. Here's another trick, to glue the wood fins to the foam, use Glidden Gripper, which is kind of a paint and kind of a glue. I did extensive testing vs. the polyurethene glues etc (you can't use anything with solvents). The Gripper holds better than any other glue and can be pained on with a roller, which speeds things up. When the whole thing is put together and clamped up with the sides and top/bottom, it is incredibly rigid.

                    I hope that is helpful

                    Comment


                    • #12
                      Originally posted by Cohnaudio View Post
                      I hope that is helpful
                      This is extremely helpful, thanks.

                      Originally posted by Cohnaudio View Post
                      There is a book for bass traps and diffusers that may have explanations on figuring the dimensions, but I have not yet purchased it.

                      The Master Handbook of Acoustics gives the following formula for the frequency of resonance for a flat, unperforated panel:

                      f0 = 170 / sqrt(m * d)

                      where f0 = frequency of resonance in Hz
                      m = surface density of panel
                      d = depth of airspace

                      This may be the book and formula you are referring to.. If so, it's obvious that you lower the frequency by increasing the mass of the panel or by making the depth of the absorber larger.

                      Great idea in using polystyrene foam boards for the diffuser construction!
                      Speakers: Rockport Avior
                      Amp: Ypsilon Aelius Mk II Silver Edition
                      Preamp: Pass Labs XS
                      Phono preamp: Ypsilon VPS-100, Slagle Silver SUTs
                      Phono: Continuum Criterion, Kuzma 4-Point, Lyra Etna SL (and others)
                      Digital: LauferTeknik Memory Player, dCS Scarlatti DAC
                      Tape: Studer A80 RC, King/Cello preamp

                      Comment


                      • #13
                        Rich has convinced me you can make these bass traps and diffusers for a very affordable price. Just disappointing they are so deep.
                        PAP Trio 10/Voxativ & PAP Trio 15 Horn speakers, Ampsandsound Casablanca monoblocks, First Sound Audio Mark 3SI preamp,
                        Mojo Audio Deja Vu server, Mojo Audio Mystique V3 DAC, The Linear Solution Ethernet Switch, Blue Jeans Ethernet cablling,
                        Akiko Corelli, Custom power strip direct wired to panel with OFC copper wire. Inakustik Ref Air 2404 Speaker cable. Genesis and Inakustik NF2404 Air Interconnects.

                        Comment


                        • #14
                          Originally posted by Cohnaudio View Post
                          Re the diffusers (I built both QRD-17s and QRD-23s), the only wood that is practical for the fins is 1/4" plywood (though it must be high-quality, multi=ply plywood, not the crap you get from Home Depot - to avoid vibration, which significantly degrades the performance of the diffusers). Hardwood fins are not practical for a number of reasons: good luck finding solid wood that wide (18:" for the QRD 23s), at that thickness and width they will almost certainly warp, you would need to construct the cabinet to allow for wood movement, which I don't think works for the design.
                          I completely agree and is the reason why I question Acoustic Field's assertion that they only use solid wood in the construction of their diffusers.

                          I am planning on using 6mm Baltic Birch for the slats when I get around to it....
                          Speakers: Rockport Avior
                          Amp: Ypsilon Aelius Mk II Silver Edition
                          Preamp: Pass Labs XS
                          Phono preamp: Ypsilon VPS-100, Slagle Silver SUTs
                          Phono: Continuum Criterion, Kuzma 4-Point, Lyra Etna SL (and others)
                          Digital: LauferTeknik Memory Player, dCS Scarlatti DAC
                          Tape: Studer A80 RC, King/Cello preamp

                          Comment


                          • #15
                            Rich, had a brain storrm. What if I built the front portion of the diaphragmatic bass trap up to the perforated carbon portion. At that point I seal the diaphragmatic baffle portion to the wall. I let my drywall and insulation act as the carbon trap section. Per the plan I may have to make a couple holes in the drywall to allow air movement between the chambers. The only thing I would be lacking would be another sealed chamber on the outside of the house. Technically I could make that, but would much prefer to let it vent to the outside world and go away. I'm not sure why reflected trapped noise would be a requirement.
                            Hmmmmm.

                            I was also thinking of the scattering device. What if I were to take the Deep chamber and angle it so that it would not require so much depth. Or shall I say extension into my Listening Room. The small Chambers would then be increasing in debts as they went down that angled trap. Make sense what I'm thinking.
                            Hmmmmm.

                            I love what you have done. Your putting real technology into your listening environment.

                            PAP Trio 10/Voxativ & PAP Trio 15 Horn speakers, Ampsandsound Casablanca monoblocks, First Sound Audio Mark 3SI preamp,
                            Mojo Audio Deja Vu server, Mojo Audio Mystique V3 DAC, The Linear Solution Ethernet Switch, Blue Jeans Ethernet cablling,
                            Akiko Corelli, Custom power strip direct wired to panel with OFC copper wire. Inakustik Ref Air 2404 Speaker cable. Genesis and Inakustik NF2404 Air Interconnects.

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