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Designing a listening room - wall construction and damping

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  • Designing a listening room - wall construction and damping

    • The essence of a well-damped anything is decoupling.
    • Decoupling means introducing losses into the transfer from energy from place A to B.
    • Losses reduce efficiency
    • Lower efficiency means less sonic contribution

    In short we can say as a universal truth that the more damping we introduce in the listening environment, the less that environment contributes to the final sound we hear.

    Example: listening to your hi-fi in the bathroom is listening without damping: the walls are hard and efficiently reflect sound; by implication those walls have no damping. Conversely, listening in an anechoic chamber is listening with total damping; the surfaces do not reflect sound at all.

    The perfect listening environment must not be acoustically dead like an anechoic chamber, and must not be as acoustically live as a bathroom. The ideal listening room falls somewhere between totally damped and not damped at all.

    If you have the chance to design a room from scratch, then one thing to consider is decoupling the wall panels from the structure of the building. That's going to reduce the efficiency of the panels to radiate sound and is going to give you far less contribution when they do flex and bend at low frequencies. The cost is negligible, if you have the opportunity to implement the wall-panel isolators in the room design. It will give you a simple mechanical solution which is cheaper, easier to implement and easier to comprehend than DSP or tube traps or the equivalent. Wall panel decoupling ameliorates the wall-wobble problem at source.

    Excellent video here. The secret is the rubber component which is a good damper plus the small panel area between the dampers which pushed any residual resonances upwards in frequency and to a lower amplitude.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  • #2
    Decoupling

    This thread is very relevant to my position and I recently stated that I now have to deal with the room as the next hurdle; you may remember my saying that reflections from the rear of my listening chair now produce a pressure zone, and this has never been significant before. So I am now looking at the realm of acoustics.

    Surely with regard to your bullet point one, if something is 'ringing', changing its material to one with a lower Q is the way to reduce that, and then also decoupling the source from it.

    My purchase of 17kg granite under speaker plinths, which I was vary careful to get a 'No quibbles' money back guarantee on a few months ago, made an enormous difference to the sound, much I think due to elimination of energy travelling into the floorboards. (I had regarded this as a real candidate for the snake oil category - hence the guarantee.

    I had thought some years ago of removing the plasterboard from the ceiling, leaving the floor joists exposed, and filling in between them with BAF wadding resting on sheets hanging down in half cylinder loops between them full of the wadding.
    (I think that the ceiling height is unfortunate with regard to vocals).

    Placing boards on the wall is more than slightly inconvenient with regard to perhaps moving, and also reducing the room volume.

    Comment


    • #3
      Real-world building materials

      Originally posted by Pharos View Post
      Surely with regard to your bullet point one, if something is 'ringing', changing its material to one with a lower Q is the way to reduce that, and then also decoupling the source from it....
      Maybe so, but in the real world, building materials used in walls, floors and ceilings are self supporting, hard, rigid, flat materials. A by-product of that rigidity is that when they are hit with a hand or hammer, they audibly ring, which, as you say, is an indication of 'Q', their resonant characteristics.

      If walls, floor and ceiling were made of a low Q material - say, heavy rubber sheets - that would increase their damping and lower Q in the audio band, but those heavy materials would have no rigidity, and would be impossible to position (even dangerous) as floor/ceiling panels where they would always be under the influence of gravity. There are also fire retardation issues because damped materials tend to include rubber and are consequently combustible and are unlikely to meet building regulations and insurance requirements. They are also likely to smell terrible.

      A pragmatic solution is to use existing hard, safe, approved materials (plasterboard, chipboard etc.) but to decouple them as best as possible from the structure of the building, as shown in the video. Not ideal, but a far better solution than rigidly nailing those panels to the underlying building structure where, at critical frequencies, they will sing like crazy.

      Whatever material solution you go with, you must always be aware of the consequences of collapse and/or the spread of fire. If there is a fire and your insurers or local government authority believe that you have put yourself or your neighbours at risk due to your selection of non-standard materials and/or non-standard room construction, you will be in serious trouble. This greatly restricts your acoustic options.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #4
        Coupled listening rooms

        I suspect that the decoupling mainly helps at lower frequencies, is that right?

        I sort of deal with the acoustics-problem. Funny enough I DO have an acoustically treated room (including loads of mineral wool in corners and Vicoustic-products), but I plan an additional room beside the listening-room that will be open to that one. Resulting in two rather square rooms that are open one beside the another.

        So I do think about making the second room somewhat acoustically neutral, hoping to do something that might not change the existing acoustic in the first one although they are open.

        I suspect that I would need to make the second/additional as acoustically dead as possible. Sadly I do like modern living-spaces with concrete, natural stone and such. One thought was to make that modern architecture stuff without thinking about the problem too much and just add an acoustic ceiling (maybe even wooden like this one: http://www.lignotrend.de/holzbau-pro...erkleidung.pdf ) and additionally separate the two rooms with a rather heavy curtain.

        Comment


        • #5
          Just considering options

          Yes Alan of course those safety considerations are important; I am at a 'toe in the water' stage in considering the room, and of course the building materials have to be structurally adequate and 9" rubber walls would not be appropriate.

          Isn't it the case that the real problem, rather then self resonance as I suggested or inferred - is not that of the walls themselves ringing, but the room dimensions causing eigentones?

          I suppose in your position you are always considering your relationship with authorities and regulations, and from many angles.

          Having recently placed 12" of glass insulation in the roof space of my extension, I was amazed at the reduction in acoustic energy in there - barely a trace of one's own voice.
          Last edited by Pharos; 17-01-2013, 06:46 PM. Reason: errors

          Comment


          • #6
            Acoustically inert rooms

            I have no expertise in building construction, merely surface and DSP treatment. The experts in the UK are the Building Research Establishment. Not much on their site about acoustic construction matters but interesting paper here.

            The last word in 'removing' the room from the listening experience at home is to completely eliminate reflections that inevitably occur as the sound leaves the speaker and touches every surface in the room without exception - even the inside of closed drawers - and then most are reflected back to us at the sweet spot. We take that for granted (luckily) but if we don't want to have those reflections, we need highly absorptive surfaces that will soak-up the sounds leaving them so weak that they simply cannot continue their journey on to our ears at the sweet spot. Whether we like listening in such an acoustically inert space is quite another matter.

            Here is a look around a huge anechoic chamber - far bigger than the BBC's now defunct Kingswood Warren chamber, but built for a much different purpose.
            Alan A. Shaw
            Designer, owner
            Harbeth Audio UK

            Comment


            • #7
              An alternative strategy - diffuse sound don't absorb it!

              We've looked at the anechoic chamber which has the intention of absorbing sound from the lowest to the highest frequencies in a predictable way. But there is an alternative strategy: spread the reflected sound around = diffuse the sound.

              Here is a video of making your own diffuser. It shows how one small diffuser panel can be made. What it doesn't say is that to have any noticeable effect you would need to cover most or all wall/ceiling surfaces with these panels in your listening room. One panel alone is unlikely to have any detectable effect if it represents only a percent or so of the total reflective surface area of the room. More on the diffuser design here.

              Very important point that's easy to miss: at the start of the video it says that this design is only suitable for diffusing frequencies from about 1.5kHz (not 1.5khz). That is the presence region of audio. So in the bass and midrange, right up to an octave below the tweeter, this solution is acoustically invisible: it has no effect whatever no matter what percentage of the room is covered with these panels. If the room has problems in the bass and mid frequencies (most do) this is not the solution for you.
              Alan A. Shaw
              Designer, owner
              Harbeth Audio UK

              Comment


              • #8
                No! Absorb don't diffuse!

                And here is the counter solution: broad-band absorption, of the type used by the BBC (in the golden days). This acts the same way as the anechoic chamber - it converts sound to heat and kills its energy, and once sound is heat, its potential to mess-up our listening experience has gone.

                There is a direct correlation between the size of the absorber panel as a % of the room surface, the acoustical attenuation of the materials in and on the surface of the absorber and how deep it is. We've covered that before looking at the BBC's 1951 room design approach. A commercial manufacturer of such panels is here. Note the emphasis on fire retardation.

                The downside of these absorber panels is that they do soak-up light and make the room darker. Also, they are difficult to clean because, by definition, their surface covering is usually porous cloth which attracts dust. So these panel diffusers have tended to be less popular in contemporary minimalist environments of glass and chrome (i.e. modern studios) despite their undoubted excellent acoustics.
                Alan A. Shaw
                Designer, owner
                Harbeth Audio UK

                Comment


                • #9
                  Philosophy of treatment options

                  My proposed removal of the ceiling plasterboard and filling in with BAF, as well as into hemi-cylindrical 'droops', was following from my memories of the (very expensive) acoustic traetment of the BBC's studios. I seem to remember wooden slatted sheets with a gap behind of an inch or so, which were also artistically pleasant.

                  I saw this elsewhere, but not recently; there was more recently, a wide usage of mineral tiles which have slits or crevices on their surface, and they were heavy.

                  The whole issue of the listening room seems to be a can of worms, with there being several philosophical approaches to the idealisation of a stereo listening room. LiveEndDeadEnd is one, and there are also several 'proprietry' layouts, one of which places the speakers along the long wall, firing along the shorter one.

                  From what I have just finished reading on the matter, the recording studios use very flat speakers, but with a slight extreme-HF roll off, and spend a great deal of money, really a lot, removing reflections towards the mixing desk position by treating the room, this giving a wide 'sweet spot' so that engineers, producers, and the musicians can all listen to the results. Much of the room volume is lost in this process.

                  Our domestic venue places other constraints on us, especially perhaps those with a wife

                  Near to my left speaker on the wall to the left of the listening area, is the protrusion of a chimney breast of about 15", and after putting a curtain extending around it there was a significant improvement.
                  Last edited by Pharos; 18-01-2013, 12:09 AM. Reason: Errors - as usual

                  Comment


                  • #10
                    Room placement

                    interesting site about the room and placement of speakers,
                    could be a useful help I think.

                    http://www.cardas.com/room_setup_main.php

                    Comment


                    • #11
                      My new room

                      Thanks Alan!

                      Very thankful for you for starting this thread most especially that I have just acquired a new house...

                      Among the 4 rooms, I have identified 1 room to be my dedicated listening/audio room...

                      I will monitor this thread. Thanks! :D

                      Comment


                      • #12
                        Sonic demo of before/after room treatment

                        On another thread here we are looking at audio waveforms, and trying to interpret the fast-rising vertical streaks as evidence of percussive sounds. Or hand claps, or gun shots or similar. The point I made was that our ears are optimised to use the energy in high frequencies to gauge the size of an unknown space, even blindfolded. That's why the best way to excite an unknown room is with a good, energetic, loud, sharp hand clap; the type of which leaves your hands stinging afterwards. That generates lots of HF energy and it's that energy we want to reflect off the room surfaces and eventually reach our ears.

                        Here is a video about DIY room treatment. Notice how the before and after room treatment sonic comparison is most vivid with the high hat: by absorbing the bright, toppy energy in the instruments reverberation tail, what frequencies do travel on from the walls to our ear does not contain enough HF to determine the size of the listening space.
                        Alan A. Shaw
                        Designer, owner
                        Harbeth Audio UK

                        Comment


                        • #13
                          Ideal dimensions for the listening room? The legendary Watford Town Hall

                          Intrigued by recent listening to new recording of Britten's War Requiem I dug more deeply into the net to find out what special is in Watford Town (Assembly) Hall, later renamed to Watford Colosseum. I checked more than a dozen of cds with music recorded in this hall and I must confess IT HAS ENORMOUSLY GOOD ACOUSTICS. No wonder - it was acoustically designed in late 30s by legendary pioneer in this discipline - (Philip) Hope Bagenal. Although its "shoe-box" shape, the hall has very good acoustical parameters - look at link http://www.classicconcerts.org.uk/wc...l%20Report.pdf

                          More about its restorarion I found at http://news.bbc.co.uk/local/threecou...00/8878377.stm
                          The conclusion - no one of today professionals did not even dare to think of any improvements to the original design of Charles Cowles-Voysey and Hope Bagenal team!

                          But going back to my question - Alan, what should be, according to classical acoustic theories or researches done (if there are), the optimal dimensions ratio (e.i. W/L/H correlation) of the usual room destined for listening to classical lodspeakers?

                          All the best.

                          P.S. I had very bad experience some time ago - my friend asked me if something can be done in a room with its height = width (almost square section tube!) to adopt it for his home "listening studio". The simple clap test you frequently propose in your posts alienated him from this idea immediately.

                          Comment


                          • #14
                            Breaking-up room symmetry

                            I have read that the optimum dimensions for a listeing room are 1:1.6: 2.3, and in other books, 1:1.6:2.6.

                            The idea being to spread maximully the modes.

                            Comment


                            • #15
                              Anechoic Chambers and Music Listening

                              Whilst it has always been said by those that seem to know that an anechoic chamber is not the best listening room, I would like to discuss why that is. I should point out that I have never listened to music or anything else in an anechoic chamber.

                              Placement of the speakers and listening chair

                              I assume firstly that there are no restrictions on placement regarding room modes and reflections. In other words, proximity to room boundaries has no consequences. If that’s the case one can place these items anywhere in the room for stereo image as long as each speaker is equidistant from the listener’s ears, suitable toe-in is applied, and there is enough amp power to drive the speakers.

                              What the listener hears

                              He will hear only the direct sound from the speakers to his ears (with head related transfer too) and his brain will concoct a stereo image. What he hears then is dependent entirely on what is on the recording.

                              What was the intention of the mixing engineer

                              This to me is the crux of the matter. Firstly the engineer’s mixing room will not be perfect but presumably a good engineer will make allowances for that. Secondly, does the engineer make some assumptions as to the circumstances of the listener - car, average domestic setting, headphones….?

                              Is it for this last reason that an anechoic chamber does not make a good music playback arena?

                              Has anyone listened to music in an anechoic chamber and if so, I would be interested in your comments on the sound?

                              {moderator's comment: Alan has previously exampled music recordings made in a free-field room (aka anechoic chamber}

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