2 channel room bass boom

Discussion in 'Home Cinema Buying & Building' started by Bosh964, Sep 19, 2007.

  1. Bosh964

    Bosh964
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    Hi

    Im using a high end 2 channel Naim system using boundary re-inforcement speakers in a modern carpeted domestic living room meticulously set-up by 2 well respected Naim dealers. It is 18x12x8 and very rectangular with dot/dabbed drylined walls

    I am experiencing a 45Hz bass boom which is clearly a room width omode problem which manifests itself as a pronounced boom on any recording at the point which produces this single frequency. I can remove this by parametric EQ with a narrow Q and -9db gain but this destroys the fidelity. All other bass is fast and lean a'la Naim's house style, the problem is how to cure this boom.

    I have tried RATS tube traps which only go down to 65Hz and didnt help and 3 of CADs 4'x2' RPG 50Hz modex traps which eat in to it slightly. However, without filling the room (the family have to use it too) with more and potentially "wasting" wads of money how do I know whether I need to add 40Hz ones or more 50Hz ones?

    It has been suggested that I render the walls properly or add a second layer of plasterboard as it is the cavity that is resonating but an acoustics guy I spoke to said this would merely trap the boom in an make it worse

    I am willing to try anything that will work but as most bass trapping seems to be broad band, very large, very expensive and very much (to me anyway) a stab in the dark, I thought I would throw it open to you guys for advice and suggestions

    Thanks in advance
     
  2. mattym

    mattym
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    the rpg modex from CAD will have been made by my company at some point, where do you have them placed, and when did you get them? i havent made any 4'x2' for a while!

    i would think you need more trapping, 3 isnt really enough for that surface area, though placement of these may help, with that in mind, where are they placed? When the speakers were set up, where they positioned by naim also?

    if possible, can you post a piccy of the installation so i can see where youve got the traps in the room etc?
     
  3. Bosh964

    Bosh964
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    Hi MattyM

    I acquired the bass traps from a guy I bought some used Naim kit from who I had "spoken" to for a few years on the Naim forum so I have no idesa of their age. Can they lose effectiveness over time?

    The speakers fire across the room placed 4" from the 18' fire place wall. This is the only place they can be placed as the Left hand wall has an 8' patio window on it and the right wall transmits the sound into the kids bedroom.

    The listening position is against the wall opposite the fire placel. I had the traps stood on the floor against the right wall running from the rear wall/right wall corner but am not currently using them

    If you are at RPG is it best to speak to you directly?

    Thanks

    David
     
  4. dj-dulux

    dj-dulux
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    I agree with Mattym, having only 3 is not going to have much effect, I would think at least double that would be the minimum. Also position will be important and Mattym is probably best to advise on this.

    You say you listen close to the wall, how close? Listening near the boundary is not good as you get an effect called 'pressure doubling' occurring, try sitting further away from the wall, does it have an effect?

    Dupe...
     
  5. Bosh964

    Bosh964
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    HI

    I decided to go down the re-lining but not getting much joy from the 6 tradesmen I have had in to quote. Only 2 were interested in the job, the first quoted £1700 for trad re-plaster of 3 blockwork walls and second layer of Soundblock on ceiling and 4th stud wall but couldnt start until June. The other doesnt seem keen to trad plaster and is quoting £2500 for a second layer of soundbloc to all walls but can start late March

    Should I haggle with the latter, wait for the former, or get some more quotes?

    Thanks

    David
     
  6. Isco 3

    Isco 3
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    Did you manage to solve the 45Hz issue?
    There are a 2-3 ways of solving your problem. For a start, are you a cd-listener?

    From the desription of your room, I guess (just guessing) that you have other
    problems in there. The fact that you will end up with a good frequency response, does not mean that you will have a good sound. However, a good sound should have a nice response.
    One simple example:you have your speakers 4" from the back wall;that generates a reflection of high intensity that has a small delay. According to Haa's theory, all reflections coming within a short delay from the diredt sound, do not get perceived as disturbance.
    But, early reflections can add colouration, echo or support to the direct sound. That back wall reflection is a colouration , and changes the characteristics of the original signal. You might know it as comb-filtering.

    Back to your 45Hz. :rolleyes:
    If you are a cd listener, you could add a DSP device between the transport-DAC of your system. All done in the digital domain, fidelity maintained. Be aware, that doesn't solve all your problems, just response.
    You can alternativelly use a subwoofer to make calibration easier.
    You can build a resonator, with a center frequency of 45Hz.

    ps: How did you determine that 45Hz was the issue? Do you own any measurement equipment?
     
  7. Bosh964

    Bosh964
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    No the 45Hz boom is the (main) issue, the re-plaster is hopefully to stop the walls resonating at that frequency by mass/stiffness damping. They are typical new build 9mm boards dot and dabbed with gaps in the dabs along the joins and top and bottom!!. My previous house had the same size room with solid walls and no boom and friends with more solid walls have much tighter bass

    The speakers are 10cm (4") from the wall and are designed for boundary re-inforcement (Naim SL2)

    I have a Behringer DEQ2496 room correction parametric EQ and and Alesis Parametric EQ, both of these I have tried in the tape loop as my CD is a Naim "integrated". These do kill the boom using 45Hz as the centre point with -9db of gsain and a narrow Q, but they destroy the sound quality

    I have got some of Mattys 50Hz which partly cure the problem but I need a few more and they are large and intrusive. The waterfall room response response was measured by Absolute Audio
     
  8. Isco 3

    Isco 3
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    I hope it works out at the end.
    I do not believe replastering will do anything, to be honest. How can you asuume that replastering will have an effect at all? What equation diid you use to come to that conclusion?
    Also, the fact that your previous room had the same dimensions does not mean much. Two identical rooms with the only difference the location of a door, would behave differently. I have a feeling you expect the replastering to make your room behave like a solid-wall room, but it won't.
    The boundary compensation that your speakers have is a good thing, and it works like a shelving filter of an EQ. It still doesn't address any early reflection issues coming from that wall. It just gives you a different response.

    If you only have a 45Hz problem, you are doing really well. Most people do not even know that they have a problem. Using your Behringer as a DSP will do the job. The money you are spending on plastering alone could buy you a DAC. You can use you player as a transport.
    You could actually borrow a DAC to try out, in case you are not happy with the plastering approach.
    Anyway, when the work is done, it would be nice to write back and let us know how it all went.:thumbsup:
     
  9. dj-dulux

    dj-dulux
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    Agree with VIP here until proven, I struggle to believe that the wall re-plastering will solve your problem. It's a possibility, but I would want evidence before spending money to sort it out!

    Walls with flexibility are not great and can cause acoustic problems due to re-radiation and weak boundaries with strange impedances which can affect room and loudspeaker loading. All loudspeakers use boundary effects in some way to boost low frequency output, so I'd start with changing the loudspeaker position to see if you can affect things. We can measure this wall impedance and FRF (frequency response Function) to establish if this is the case as a natural mode of the wall (animation of the wall deforming). It is also easy for us to perform the measurements to establish how the loudspeaker forces the wall to deform and radiate (more important), so stiffness can be added in the correct place etc.

    Both the Behringer and Alesis products are awful; please don't use these as a solution, OK for diagnosing! Can you post the room response data from the measurements; it may help us look at this more objectively.

    If the solution can be achieved passively, this is the best solution and we can derive this with measurement. You have a few options yet!

    Dupe…
     
  10. Bosh964

    Bosh964
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    Thanks for the reply

    Have experimented extensively with speaker placement, and got the best compromise but still not solved by any means

    Have got the waterfalls at home, must remember to post them later. They do show other problems but the 45Hz is the real problem

    How do you measure the wall impedance, modes, deformation radiation and FRF?

    I agree the Behringer and Alesis are terrrible, the room EQ on the Behringer doesnt even ID a 45Hz problem
     
  11. dj-dulux

    dj-dulux
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    I use a couple of tools, WinMLS and LMS systems. Combine these analysis systems with accelerometers, microphones and instrumented hammers and you can work most things out.

    www.lmsintl.com
    www.winmls.com

    Due to the cost, its not used too much in a domestic enviroment, but it could... If you could get a detailed plan of the room, with listening and speaker positions it may also help us to investigate.

    Dupe...
     
  12. Bosh964

    Bosh964
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    HOw do you post JPeg piccies here?:confused:
     
  13. dj-dulux

    dj-dulux
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    Just another thought, on the Behringer try and move the microphone a meter or so in any horizontal direction, its not inconcievable that you have the mic on a pressure minimum and hence its not seeing it. Dupe...
     
  14. dj-dulux

    dj-dulux
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    If you go advance you can add small JPG files (click on the paper clip), but its best is to upload them to Flickr or similar and link using insert image icon.
     
  15. Isco 3

    Isco 3
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    The last accelerometer I came across (using light and not mass) was £150k :eek:
     
  16. dj-dulux

    dj-dulux
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    VIP,

    I have a lovely scanning laser vibrometer which is of little use these days as none of the current software packages I use support it. Still works as a vibrometer, but don't scan no more so doing wall modals etc takes ages now! Think it was only about £50k, the analysis stuff is only viable cost wise due to my other work interests..... An LMS system costs best part of £60k depending on licences and number of channels....

    But the WinMLS stuff with mic and sound card is great and costs around £500 depending on the version for pro use. Well worth the investment, I see you are a Salford guy, join the club! I guess you will be using WinMLS or MLSSA there?

    Dupe...

    Bosh, any progress on posting the waterfall plots?
     
  17. Bosh964

    Bosh964
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    Heres the waterfall, quite simple when you know how

    The room is rectangular 5.3m x 3.75m. The 5.3m speaker wall has a 450 window 300 in from the RH wall and a fire place with external chimney in the middle. The rear wall has the door 150 in from the RH wall. The LH 3.75m wall has an 1800 patio door in the middle. The LH wall is a very flimsy stud wall the others are dot and dabbed over block work
     

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  18. dj-dulux

    dj-dulux
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    Bosh, thanks for the graph. At first sight I have to say it’s typical to what I would expect in a residential property. As you have said earlier the main problem is at around 45Hz and we have a long decay at this frequency, this ties in with ½ wavelength across the room (3.75m). I am assuming that the measurement is accurate and repeatable etc.

    OK, what to do to improve? We need to understand what is happening in the room, the room is clearly affecting the audio produced by the speakers and it’s not desirable. There are a few things we need to look at:

    1. Are the room boundaries (walls, windows) adding to the problem, especially the decay?
    2. Can we do anything to break up the strong 45Hz mode? Big bits of furniture etc.
    3. Possibility to add acoustic treatments to improve the 45Hz problem?

    The first point is possible to analyse; first we have to establish what the room boundaries are doing. Are they is resonance at 45Hz, natural mode? Or are they being forced to deform by the speaker, forced mode? This is a similar measurement to what has already being done, but instead of just a microphone we use an accelerometer and a microphone. We can then start to correlate the two signals to establish if the two are interlinked. This method will help establish if we have a forced mode, to establish if it’s a natural mode we simply hit the boundary with a hammer and look at the accelerometer response. Very advanced, you would normally use a hammer with a force cell, but not necessary for this type of work really. By looking at the two results we can start to establish whats going on, forced or natural modes.

    Why do we need to establish the above? The solutions to either problem are potentially different. If it’s a natural mode of the boundary combining with the room dimension mode then adding extra mass to the boundary may be enough to solve the problem, if it’s a forced mode then we have to go down the route of adding stiffness.

    Using the above and some math we can calculate the boundary stiffness and then calculate what we need to do to solve the problem. Add mass or stiffness?

    The second point is not usually possible due to the long wavelengths.

    The third point is possible and this is one of the avenues you have looked at. The problem is at 45Hz you need some serious acoustic treatments or solutions.

    In reality the solution will come in the form of all 3.

    Where are you based?

    Dupe….
     
  19. Bosh964

    Bosh964
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    DJ-Dulux - Thanks for the comprehensive explanations. The forced/natural mode theory certainly helps me get my head round and rationalise what could be going on and also gives an empirical diagnosis to the potential answer

    So what yI think you're saying is that if the problem is caused by a natural resonance of the wallboard, then removing it and plastering should remove the problem. Whereas if it is a forced resonance adding stiffness by filling the cavity with expanding foam or adding a second layer of board should do the job. Basically it’s a 50:50 chance you’ll choose the wrong one without some expert input!!

    We are just off M1 junction 30, about 75 miles from Kenilworth, could you help with this?

    Bosh
     
  20. dj-dulux

    dj-dulux
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    Bosh, it sounds like you pretty much got it. I have attached a diagram (simplified) which helps explain a little further.

    We have 3 regions which effect how a boundary behaves, stiffness, mass and damping control. We often here people say 'add damping' but this only affects the resonance and nothing else, so is often not really what they mean.

    The 3 are interlink very strongly, for example lets add another layer of plaster board, we add mass, a small amount of stiffness and some damping. So by adding the plaster board we potentially reduce the resonance frequency of the boundary, lower the amplitude of the resonance peak due to the damping and flatten out the curve a little overall. This may be enough, but may also move the resonance into a worse place!

    Its swings and roundabouts and more to do with experience than the math much of the time.

    The other thing that I should touch on is resonant absorbers, these are a panel with absorbing material behind (similar to a stud) wall on a solid wall. By tuning the panel mass to the problem frequency we can improve the acoustics. The absorption behind the panel eats up the vibrations and prevents re-radiation which is what you probably have now. I’m not sure how effective these would be at 45Hz as any practical absorption material would be fairly useless at 45Hz, I’ll have a look at some numbers later.

    Dupe…
     

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