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Thread: Bass Trap and general room treatment advice please

  1. #1

    Question Bass Trap and general room treatment advice please

    Hi,

    I am building a new home cinema/critical listening room. The dimensions are 5.2m wide by 5.6m long by 3m high. The walls are 100year old 10"+ solid rock/lime mortar mix, so pretty solid. The floor is concrete with a small amount of insulation underneath. The ceiling will be 3/4" ply.

    The plan is to line the walls with paper backed rockwool panels with the soft side facing the room at the front sides and rear and with the paperside facing towards the room at the rear side walls and on the bass traps...

    For the bass traps, I plan on making wood frames for two foot square traps on the side and rear ceiling corners and all vertical corners. Rather than use cut up panels stacked on top of each other, my builder has suggested lining the frames with geotextile membrane and filling the resulting bags with blown-in fibreglass fibres from the loft space.

    This sounds a good solution to me, and because the fibre is blown in, the light fluffyness of the fibres is maximised and, from what I have read, light and fluffy beats firmer rockwool or fiberglass wool panels when the bass traps are quite substantial.

    The ceiling, between the basstraps will be treated with 3" acoustic wedge panels. When I am happy with the sound of the room, everything will be covered in fabric to make it all look smart.

    I would welcome any comments on my plan, before I set-to:-)

    Many thanks,

    Steve

  2. #2
    VIP/Donor [VIP/Donor] microstrip's Avatar
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    Quote Originally Posted by Bighairybloke View Post
    Hi,

    I am building a new home cinema/critical listening room. The dimensions are 5.2m wide by 5.6m long by 3m high. The walls are 100year old 10"+ solid rock/lime mortar mix, so pretty solid. The floor is concrete with a small amount of insulation underneath. The ceiling will be 3/4" ply.

    The plan is to line the walls with paper backed rockwool panels with the soft side facing the room at the front sides and rear and with the paperside facing towards the room at the rear side walls and on the bass traps...

    For the bass traps, I plan on making wood frames for two foot square traps on the side and rear ceiling corners and all vertical corners. Rather than use cut up panels stacked on top of each other, my builder has suggested lining the frames with geotextile membrane and filling the resulting bags with blown-in fibreglass fibres from the loft space.

    This sounds a good solution to me, and because the fibre is blown in, the light fluffyness of the fibres is maximised and, from what I have read, light and fluffy beats firmer rockwool or fiberglass wool panels when the bass traps are quite substantial.

    The ceiling, between the basstraps will be treated with 3" acoustic wedge panels. When I am happy with the sound of the room, everything will be covered in fabric to make it all look smart.

    I would welcome any comments on my plan, before I set-to:-)

    Many thanks,

    Steve
    IMHO you will have a difficult room to deal with. It is almost a square, and very solid walls. Solid walls are a no in such cases - they do not absorb bass and reflect the bass energy, creating peaks and nulls in the frequency response. IMHO you will need a good experienced acoustician to help designing the bass traps of your room, as just using rockwood will not be enough. My room has two feet thick massive stone walls and the only way to manage bass quality was adding very large membrane bass traps.
    Under construction around the DCS Vivaldi 2.0 stack and a pair of Wilson XLF's : Forsell Air Force One, ARC Phono 2SE, cj GAT + Lamm M1.2 ref, TA OPUS MM2 +TA XL digital, TA XL gen V power cables and CenterStage footers ...

  3. #3
    Thanks microstrip, I recognise its not ideal, hence going so heavy with basstraps. Also, the infinite baffle subs in the loft space placed at the 1/4-1/3 wavelength area should ensure a decent bass response. Time will tell though:-) Heres a sketchup model I made - https://youtu.be/NA2yIPEaltI

    Cheers,

    Steve

  4. #4
    I have had some thoughts on the ceiling, I was going to have a plywood ceiling with foam wedges glued on, but now I am thinking of doing away with the ceiling and wedges and stapling some acoustic transparent fabric across in its place and then filling the super chunks, soffits and space between the joists from above in one go with blown-in light and fluffy rockwool/fibreglass and then boarding over the top of the joists. Here's a video I have done using sketchup.

    https://youtu.be/9qaqpgumWzU

    I have checked that there are no first reflection points on proposed corner traps and soffits.

    https://youtu.be/20Y_ZuAns58

    Cheers,

    Steve

  5. #5
    VIP/Donor [VIP/Donor] flyer's Avatar
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    The fact that you are paying attention to the bass from the start is excellent. The fact you do that in the corners and rear ceiling corners is very good. But the fact you intend to do this with real bass traps, i.e. filled with insulation , well... not so good in my opinion.

    You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy. By putting the volume harder, you will be faced with a load of other problems in the mids and highs which, to solve them as well, you will be adding in even more absorbers. This may turn into a never ending story because each time you absorb excessive energy (or rather energy that is not good enough diffused), you will want to compensate that by addding power in your system and each time other phenomenons pop up, until all your walls will be covered with absorbers and you are frustrated you do not get hall ambiance you look for in the recording, nor the stage width, etc.

    I know I paint a pretty grim image here, sorry for that. I am just not a fan of bass trapping ... bass.

    Microstrip refers to membrane traps of which are already much better suited. But given your (unfinished) room condition, the fact you have space for it, the fact you intend to fill corners up with traps anyway, I would firmly recommend to go with adjustable helmholtz resonators: no loss of energy, tunable till the lowest frequencies (and automatically their harmonics) and flattening out the bass response.
    Aries Cerat, SMT, Entreq , Stacore and Stenheim distributor for Belgium, The Netherlands and Luxembourg - Tenor Audio and 432EVO Audition Center
    www.ultisone.be

  6. #6
    Steve,

    Quote Originally Posted by Bighairybloke
    I am building a new home cinema/critical listening room. The dimensions are 5.2m wide by 5.6m long by 3m high. The walls are 100year old 10"+ solid rock/lime mortar mix, so pretty solid. The floor is concrete with a small amount of insulation underneath. The ceiling will be 3/4" ply.
    The only issue I would address at this stage is reverberation time. With the walls and floor you describe it must be rather high(ish), just a guess. Since you have ceiling height an acoustic ceiling could be an option. In our living/listening room (8x5x3 m, walls of brick 'n mortar, concrete floor with tiles) I have a stretched synthetic fabric at 2.5 m with the space above loosely filled with rock wool, WAF = 1. Acoustics is dry but not too dry. That ceiling also treats the room modes, both vertical and horizontal.

    Should room modes be an issue active treatment such as source-to-sink subwoofers could be a option. Some of these subwoofer based approaches have been tested in an AES paper, which I could send via email if you are interested:

    http://www.aes.org/e-lib/browse.cfm?elib=16324

    As far as early reflections are concerned, there is no evidence that these are detrimental as a matter of principle. I had a good look at the available scientific literature and have prepared a write-up.

    Klaus
    Speakers: Klein + Hummel O500C, Electronics: Funk MTX preamp, Rane PS1 phonostage, Analog: Michell Gyrodec, SME 309, Shure V15VxMR, Digital: Tascam CD-RW4U, Tascam MD-801R

  7. #7
    Quote Originally Posted by flyer
    You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy.
    The only substantial contributors to sound pressure level at listening position I know of are direct sound and first reflections. Corner traps don't affect any of these so why would the level be lowered?

    Klaus
    Speakers: Klein + Hummel O500C, Electronics: Funk MTX preamp, Rane PS1 phonostage, Analog: Michell Gyrodec, SME 309, Shure V15VxMR, Digital: Tascam CD-RW4U, Tascam MD-801R

  8. #8
    Quote Originally Posted by KlausR. View Post
    Steve,



    The only issue I would address at this stage is reverberation time. With the walls and floor you describe it must be rather high(ish), just a guess. Since you have ceiling height an acoustic ceiling could be an option. In our living/listening room (8x5x3 m, walls of brick 'n mortar, concrete floor with tiles) I have a stretched synthetic fabric at 2.5 m with the space above loosely filled with rock wool, WAF = 1. Acoustics is dry but not too dry. That ceiling also treats the room modes, both vertical and horizontal.

    Should room modes be an issue active treatment such as source-to-sink subwoofers could be a option. Some of these subwoofer based approaches have been tested in an AES paper, which I could send via email if you are interested:

    http://www.aes.org/e-lib/browse.cfm?elib=16324

    As far as early reflections are concerned, there is no evidence that these are detrimental as a matter of principle. I had a good look at the available scientific literature and have prepared a write-up.

    Klaus
    Thanks Klaus, I'd appreciate any info you have, can you pm me? I have not heard of source-to-sync subwoofers. That will be an interesting read. Due to the positioning of the IB subwoofers at the anticipated room mode nulls, I am hopeful room modes will be kept in check. The copious bass traps are there to control bass decay times as much as possible.

    Do you have any pictures of your cieling? I am envisioning using a fabric for holding the loose fill in place and another layer (black Devore velvet) for the WAF layer.

    I'd appreciate your thoughts on the proposed treatment of the wall, i.e. Having the insulation bats with alu foil side facing towards the wall at the first reflection points and rear wall and facing the room elsewhere to add a bit of life back in to the room. I.e. The back of the room at the sides and also on all sofit and corner bass traps too.

    Regards,

    Steve

  9. #9
    Quote Originally Posted by flyer View Post
    The fact that you are paying attention to the bass from the start is excellent. The fact you do that in the corners and rear ceiling corners is very good. But the fact you intend to do this with real bass traps, i.e. filled with insulation , well... not so good in my opinion.

    You intend to kill/attenuate the bass energy. By doing so, you will have to put the volume harder to compensate for that loss of energy. By putting the volume harder, you will be faced with a load of other problems in the mids and highs which, to solve them as well, you will be adding in even more absorbers. This may turn into a never ending story because each time you absorb excessive energy (or rather energy that is not good enough diffused), you will want to compensate that by addding power in your system and each time other phenomenons pop up, until all your walls will be covered with absorbers and you are frustrated you do not get hall ambiance you look for in the recording, nor the stage width, etc.

    I know I paint a pretty grim image here, sorry for that. I am just not a fan of bass trapping ... bass.

    Microstrip refers to membrane traps of which are already much better suited. But given your (unfinished) room condition, the fact you have space for it, the fact you intend to fill corners up with traps anyway, I would firmly recommend to go with adjustable helmholtz resonators: no loss of energy, tunable till the lowest frequencies (and automatically their harmonics) and flattening out the bass response.
    Thank you flyer, I was under the impression that by trapping the bass, or rather reducing its unwelcome hanging-around is a good thing. The loss of energy is not so much of a concern as the IB subwoofers in the cieling will be pretty powerful and have meaty amp.

    Ps:love some of that high end gear you sell:-)

    Regards,

    Steve

  10. #10
    VIP/Donor [VIP/Donor] flyer's Avatar
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    Quote Originally Posted by KlausR. View Post
    The only substantial contributors to sound pressure level at listening position I know of are direct sound and first reflections. Corner traps don't affect any of these so why would the level be lowered?

    Klaus
    Hi Klaus, I agreed with your previous post regarding the ceiling solution which, if well worked out, effectively will address both the bass issue as the reverb time. To do so, ideally the ceiling would have to be a mix of diffusing and absorbant effect and along the wall/ceiling corners air tight resonators tuned to the problematic frequencies. Again, that is ideally... what you describe is certainly better than nothing, albeit there is a danger to deaden the sound too much as well. Proper execution...

    Regarding your question above. SPL is a result of all the energy arriving at one's ears, direct, 1st, 2nd, 3rd, ..., 100th etc reflections... You are right that the direct sound and first reflection will have highest amplitude.

    Corner traps will have effect on bass (depending of course how they are conceived and executed), but let's assume just 'bass' as a generic. Speakers are designed to be as linear as possible, not taking room modes into account.
    If you, through bass treatment, want to handle those room modes then there are two ways:

    either you absorb the bass energy (converted to heat (well, think of millidegrees) by the absorbing material) => bass trapping
    or you store the excess energy of the bass (think milliseconds as well) and release it again when the null follows after that excess => (helmholtz) resonator

    In the case of bass trapping, you are not really dealing with the room modes, you only combat the effects of it. With a resonator, you are root solving the modes since you are simulating then a larger room than it really is.

    Beware, that is all quite theoretical and none of the two above statements is ever going to be 100% accurate, far from it.

    What is not theoretical is the following: Room modes will reinforce or null bass frequencies (and their harmonics till well in the medium range). If you absorb (without releasing again like a HH resonator does) the reinforced frequencies, then only the nulls and the lesser excesses are remaining to hear... so, you will want to regain a certain pressure in the bass, resulting in increasing the volume to compensate for that. But now you start to have too much energy in the mids and/or highs (depending on room etc), so you want to attenuate those as well...


    Anyway, my theory is the following:
    First make sure the bass room modes are taken care of (and that is not so easy, reason why a tunable resonators is the best available).
    Then make sure there is enough diffusion at the first reflection points (if you put absorbers there, you will not get a reflection from that point and hence you kill again sound and above all reduce sound stage and density)
    And once the two above are done, only then evaluate the room reverb time + its value in the frequency range. You must then foresee absorbing material to get the reverb time down to an acceptable level. You should also take possible flutter echo into account whilst treating the reverb, if properly placed you might kill two phenomenons at once.


    I struggled for 10 years with absorbers etc before I started to take classes in acoustics for 6 months.

    PS: sorry but I am not into subs or DSP solutions... I think these should be added at the end to finetune, not at the beginning to counter root causes, but that's me :-)
    Last edited by flyer; 04-06-2017 at 01:16 AM. Reason: PS added
    Aries Cerat, SMT, Entreq , Stacore and Stenheim distributor for Belgium, The Netherlands and Luxembourg - Tenor Audio and 432EVO Audition Center
    www.ultisone.be

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