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Upgrading my acoustical room treatments

Discussion in 'audio' started by ToTo Man, Sep 14, 2021.

  1. ToTo Man

    ToTo Man the band not the dog

    I've been staying up until the wee small hours recently lapping up Anthony Grimani's webinars on Room Acoustics courtesy of Audioholics' YouTube channel and feel inspired to finally upgrade / re-jig my room treatments after years of deliberation and uncertainty of the route to take. Anthony has a great way of demystifying the subject and giving real-world examples of what works and what doesn't.

    I ultimately see myself going down the 'four mono subs at wall mid-points' route as I reckon this is going to be the only way to have my cake and eat it and be able to position my main speakers where they image best without compromising on the smoothness of the bass which, at the speakers' current position, is the best I've ever had in this room. However, I'd still like to explore what improvements can be made to my passive treatments.

    I’ll write a longer post later that details what treatments I have at the moment and the potential upgrades/improvements I’m considering, but it would be helpful if I could start off with a couple of questions:

    1) My axial mode standing waves are 45Hz (front wall to back wall), 42Hz (side wall to side wall) and 53Hz (floor to ceiling). Will ameliorating an axial mode standing wave with diaphragmatic absorption tuned to that fundamental frequency (e.g. 42Hz) also ameliorate its multiples (i.e. 84Hz, etc...), or do I also need diaphragmatic absorbers that are tuned to the multiples?

    2) My experience in trying to ameliorate a null caused by suspected SBIR off the back wall revealed that the absorber worked best when placed behind and in line with my listening seat. If I moved the absorber left or right so that it was no longer aligned with my listening seat, the null reappeared. Does the same apply to peaks and nulls caused by standing waves, or can you place absorbers anywhere along the axis of the node (for pressure-based absorbers) or anti-node (for velocity-based absorbers) and still achieve the same magnitude of improvement at the listening seat? Or does it depend on how high the frequency is? For example, if I wanted to reduce the 84Hz peak caused by my 42Hz room width axial mode, would I get maximum performance from pressure-based absorbers tuned to 84Hz by mounting them on the side walls precisely either side of my listening seat, or would they be just as effective placed anywhere on the side walls?

    Thanks in advance! :)
     
    Fatmarley likes this.
  2. RJohan

    RJohan pfm Member

    Complicated questions!

    As you have 42 and 45 Hz modes (very near to each other) I would put pressure based absorbers at the room corners tuned in the 42 to 45 Hz range to do them both in 'one take'. Recently, on a Swedish forum, there was a guy who had put numerous simple card board tubes acting as organ pipes with the open ends into the corners. They would then take care of both the 42 Hz and 84 Hz (wouldn't they?). There is also active bass absorbers (a mic/amp/polarity switched/loudspeaker), even possible as DIY. And Helmholtz absorbers. What do you plan to use?

    I don't like rooms that are to damped in the mid/highs, so if I should use velocity based absorbers I would do one big (1 meter deep, or so) bass trap behind the listener.

    Hope some one else will catch on, my knowledge ends here...
     
    ToTo Man likes this.
  3. Old Shatterhand

    Old Shatterhand pfm Member

    I was told by GIK acoustics that even if you use a big amount of membrane absorbers there is no guarantee that you can eliminate such low room mode frequencies.
     
    ToTo Man likes this.
  4. ToTo Man

    ToTo Man the band not the dog

    Some more info:

    At the moment, I have the following GIK products:
    - TriTraps running floor-to-ceiling in all corners of the room
    - two 600mm x 1200mm '244' broadband absorbers on each side wall midpoint
    - one 600mm x 120mm '242' broadband absorber behind my TV.

    I also have a couple of massive GIK 600mm x 1200mm Monster bass traps but I don't currently use these (I did use one for a brief period behind my listening seat to improve a 75Hz null but got fed up having to shuffle around it all the time).

    The corner treatments made a huge improvement when I added them to my room in 2013 which was, at the time, an empty 4.2m x 3.8m x 3.25m cuboid, and reduced my reverberation times from an unlistenable and varied 500ms-1000ms to a well-damped and linear 300ms.*

    * This actually varies depending on my listening position relative to the speakers. RT60 in my current farfield listening arrangement with my speakers close to untreated walls is 325ms, but if I pull the speakers away from the wall and listen in an equilateral midfield arrangement it reduces to just 225ms.

    All of these treatments however are full-range, velocity-based absorbers, and at the time of purchase I was somewhat misled (or lets say ‘oversold’) on their effectiveness at low frequencies. I later learned that velocity-based absorbers perform best when placed 1/4 wavelength away from the wall of the frequency you’re trying to absorb, - not a very practical location for the average sized listening room, certainly not practical for me given my disability, - I don't need to maker life harder for myself by putting obstacles in my room! In hindsight I’d have been better with pressure-based absorbers, but GIK did not sell nor recommend these to me at the time.

    I’ve often wondered how I can effectively ameliorate my standing waves. The most intuitive option (in my mind at least) would be to add T=42Hz tuned membrane panels to the side walls, T=45Hz tuned membrane panels to the front and back walls, and T=53Hz tuned membrane panels to the ceiling. This would however a) be expensive; b) take up lots of space; and c) presumably only deal with the fundamental frequencies and not their harmonics? Another reason why I haven’t bothered to treat these standing waves is because I use manual parametric EQ to flatten the FR peaks at 45Hz, 53Hz and 84Hz. This is very effective but of course it does not deal with the corresponding nulls.

    I am oversimplifying Grimani’s approach, but he basically advocates the use of velocity- and pressure-based absorbers to deal with reflections and decay times down to around 90Hz or 80Hz and then use a swarm of four or more subs to smooth out the problems below 80Hz.

    In the rooms he designs, he covers every surface (apart from the floor) with 15% to 20% broadband absorption and 15% to 20% diffusion. He classes low frequency absorption as a separate treatment that does not count towards this limit. He usually uses diaphragmatic/membrane corner traps and a deep panel on the front wall behind the TV screen tuned with a low Q to capture a broad range of low frequencies.

    All of my current treatments are broadband velocity-based absorbers that aren’t range-limited, so I can’t really exclude my corner traps from my 15% allowance. So, including all treatments, I currently have 27% absorption coverage on each side wall, 19% on the front wall, 14% on the rear wall, and around 2% on the ceiling and 2% on the floor (from the corner traps). This excludes any soft furnishings I have in the room (fabric reclining chair, carpet, rug) and does not acknowledge the fact that my ceiling and floor are presumably 'lossier' than my walls.

    This is my current room layout:
    [​IMG]

    As you can see, my speakers are now VERY close to the front and side walls, If I move them away from either wall I start losing bass output between 60Hz-140Hz. The room is too small to position the speakers far enough away from the boundaries to minimise the effects of SBIR and I am also tired of body-swerving speakers when I enter and leave the room, - one of these days I’m going to trip or stumble and cause serious damage either to myself or my gear!

    These are the averaged left & right speaker measurements comparing the FR when I have the speakers near the boundaries (current position) -vs- brought out from the walls slightly to where my Tannoys used to be:
    [​IMG][

    And this is the averaged left & right speaker measurement with the speakers near the boundaries after I apply EQ:
    [​IMG]

    There is currently no treatment at the speakers’ first reflection points on the front and side walls. I suspect (or rather hope!) that this is what’s causing me to hear a centre image that varies in width from one track to the next), as I should be able to fix it with treatment. My other two theories are that the issue is being caused by my hifi rack and TV protruding out in front of the lateral plane of my speakers, or my speakers are simply too wide apart (2.85m) relative to my listening distance (2.65m), issues which are not as easy to fix.

    Grimani recommends using hemispherical diffusion on the side walls in the front half of the room (in front of the listening plane) and spherical diffusion in the back half (behind the listening plane), and prefers to intersperse the diffusion amongst the absorption and avoid clusters of just one or the other.

    Since I don’t currently have any diffusion in the room I was considering buying four GIK 600mm x 1200mm PolyFusors and putting them in the locations indicated in the above diagrams. According to GIK the Polyfusor acts as an absorber below 600Hz and diffuser above 600Hz, though if you look at the data it appears to change back to an absorber above 8kHz, - I suppose this is to be expected from a product covered in fabric?

    Would this be a good place to put this kind of diffuser, or it is too close to the speaker? Would I instead be better using a broadband absorber like my GIK 244 panel and instead place a PolyFusor where my 244 panels currently are? I need to be careful if I increase my absorber count so as not to deaden the room any more than it is. In fact, adding diffusion to the room can actually increase the effective surface area of the absorbers by directing more sound into them, so I might need to consider replacing my existing corner traps with range-limiting membrane traps so as not to over absorb the high frequencies.

    Lots to discuss! :)
     
    Last edited: Sep 15, 2021
  5. Old Shatterhand

    Old Shatterhand pfm Member

    @ diffusors:
     
  6. RJohan

    RJohan pfm Member

    Oh Dear!

    You are already far beyond where I've ever gone. But I would certainly try absorption of the first reflections behind the speakers and have diffusion for the later reflections from the side walls. But the Celestions are quite wide (?) and might not disperse high frequencies that much? No damping on the floor?

    My approach, generally, is to do damping mostly at lower frequencies and dispersion at higher so to keep the room decently live. What I want to have, in the time domain, is a direct sound, followed by silence, followed by a diffuse sound field coming of all surfaces. Not that easy to get. Just now I'm experimenting with placing the main speakers like Swedish Carlsson/Larsen designs, up against the rear wall with absorption of the first rear wall reflection, seem to work with a dose of EQ of my KEF R100's.

    I currently use a sub under ca 70 Hz, and, as it's placed for an even response in the one and only listening position, I get that. Lot's of tweaking to get it to SOUND right, though.
     
    ToTo Man likes this.
  7. ToTo Man

    ToTo Man the band not the dog

    The suspended wooden is floor is sheeted with plywood and carpeted, but no I don't have any absorbers on the floor apart from the corner traps. I think my family would have me committed to an asylum if I put absorbers on the floor in the middle of the room, though it would give me a good crash mat to fall onto! :D

    If I were building a room from scratch I'd consider digging out an absorption pit in the floor at the first reflection point (Grimani has apparently done this for some clients!), but that remains a pipe dream for me...
     
  8. Riotvan

    Riotvan Snoofer in the Woofer

    Interesting thread! As for floor absorption i decided the dog needed a bigger pillow at a strategic spot.
     
  9. geoturbo

    geoturbo pfm Member

    Hello ToTo,
    the one thing I envy most is that you have a dedicated room, the least is that you have a dedicated room with a (big) compromise which is having to Keep the speakers close to the wall or the Door remains shut (there are speakers which are meant to be up close to the wall but.. if I understood correctly I Believe you have the Biiig dip in the 150-250Hz - multiplying the fundamentals 53x3andx4, 42/45×4andx5 this appears As a very big incident of waves to me.. which with the speakers in this position eq is not even able to recoup). The limiting factor of the speakers close to the wall is imho preventing you to search for the best bass/mid bass response in this critical region which should be accounted for with careful initial positioning.

    Why not leaving the hifi rack where It is and moving the speakers on the left wall?

    Or better this reminds me of something I read Lately.. let me recall.. here It is:
    https://www.decware.com/paper14.htm
    (You could Jump at figure 2 for a quick hint, and there are a lot of other interesting lectures)

    Not sure this will work in your case (what about the TV?).. but Before spending some more Money on trickytraps maybe worth a try taking all acoustic panels out and applying What you learned so far all over one step at the time with the speakers rotated?
    Hoping here is some food for thoughts ;)
    Good luck
     
    ToTo Man likes this.
  10. guydarryl

    guydarryl pfm Member

    For stereo wouldn't you need two dogs?
     
    panditr and Riotvan like this.
  11. ToTo Man

    ToTo Man the band not the dog

    The room isn't completely dedicated to audio, I also use it as my office (I have a small table near the right side wall which has a laptop etc), but yes it's dedicated in as far as I have the room all to myself and don't have to share it, which is great! :)

    If I can go off-topic for a moment… From 2002-2010 the room I used as my dedicated audio room and office was an amazingly generous 6.85m x 4.7m x 3.25m (LxWxH), big enough to be greedy and have three different systems including multichannel! I moved into the smaller room in 2010 for a range of reasons, but mainly because it’s south-facing so stays warmer and is a nicer room to be in during the winter (very important in Scotland!). It’s also at the back of the house so receives much less road traffic noise.

    When I was in the larger room, I had almost no understanding of room acoustics and did not realise that placing speakers and seating certain distances from the wall caused peaks and dips in the frequency response (I once destroyed a pair of KEF B139 bass drivers that I made into subwoofers by clipping a 120wpc amp into them trying to get bass at my listening seat!). I spent the first few years with the speakers at the bay window end of the room and then eventually flipped everything round so that my seating was in the bay window. The latter arrangement gave better bass response but the high frequency reflections off the windows were a real mess (I fixed this at the time by putting egg crate acoustic foam everywhere behind me!).

    I wish I’d have known then what I know now about room acoustics as I’m pretty sure I could have got much better results in that room by treating it properly and locating my speakers and seating correctly. In hindsight the seating was too far away from the speakers and resulted in too much reflected sound and too little direct sound. By comparison, the nearfield setup I had on my computer desk in that room sounded much better.

    If anyone’s interested in seeing my old room I’ve uploaded photos here. And, before anyone suggests I move back into that room, that would be extremely difficult as it’s currently used as a storage room and has loads of stuff in it including huge bedroom wardrobes, so finding somewhere else to put all that shite will be a real challenge! :D

    Why did you have to mention the wide dip between 150Hz-250Hz???!!! :eek: Just kidding! :D Yes, I already noticed this dip in the graph, but interestingly, this dip isn’t as noticeable to my ears as the dip between 100Hz-150Hz that happens when I move the speakers away from the wall. The further I move the speakers away from the walls, the lower in frequency the dip moves.

    Here’s what I don’t understand. Is this dip being caused by standing waves or by SBIR? And why aren’t my corner bass traps dealing with it? According to GIK data, their TriTrap has maximum absorption between 100Hz-250Hz, so I’d expect it to help here. Unless the only way to improve the 150Hz-250Hz dip is to absorb the fundamental standing wave frequencies at 42Hz, 45Hz and 53Hz? This goes back to my original question in post #1.

    Thanks for the suggestions, it's definitely given me something to think about. :)
     
  12. rag987

    rag987 pfm Member

    Hi, my thoughts would be to try and solve as much of the problem as you can with location of speakers and listening seat before you do any more with treatments. I view treatments as damage limitation after you have done the best you can without them, not the other way around. I have a dedicated AV room, and I have been through a bit of a journey to get to where I am. Some of my findings:
    • A structure between stereo speakers destroys imaging. I first discovered this when I left a cardboard box next to my speaker. I lost all sense of imaging, and also found that there was no consistency between tracks and the image seemed to be "wandering". My first change was to stagger the speakers so the centre speaker was further behind the mains. I now don't use a centre at all.
    • Having the stereo pair away from the side and rear walls really improves imaging. Yes you lose bass reinforcement. And if pulled away from front wall enough then SBIR becomes a non-issue
    • Listening closer to the stereo pair results in significantly improved imaging, depth and width of soundstage. I now sit just inside the equilateral triangle.
    • Having rigid stone / brick / concrete walls is not as good as a more flexible structure like plasterboard for reducing modes
    So would you be able to flip your room by 180 degrees and see if any of the above are possible? Your bay window could be the recess for TV and hi-fi rack, your door no longer constrains the position of your speakers so you can pull them away from the walls, and also listening seat could be more nearfield but still away from the rear wall. You would want to avoid 1/4 and 1/2 points for seating and speakers, I think.

    Then to overcome loss of bass perhaps one or more subwoofers? Sealed 12" perhaps?

    It it helps here is where I have ended. You see my profile for more details e.g. what kit etc.
    • Dedicated AV room 8.4 x 3.5 x 2.4. Gypsum board walls and ceiling, solid wood floating floor.
    • Stereo speakers 1.6m away from front walls and 0.8 from sides
    • Nearfield listening position inside equilateral triangle
    • Pair subs mid front and mid-rear walls, x-over at 80Hz
    • Listening seat at 44% distance from front wall
    • Tube bass traps at all 4 corners, wall to ceiling. These are hollow and sealed so behave like narrow band pressure absorbers, I believe they perform well from approx 40Hz, though have not measured
    • First reflections with floor to ceiling 8" deep absorbers, stood clear of the wall by approx 8" to improve bass
    This works really well for me. My room modes are 21Hz, 41Hz and 60Hz. I use the PEQ on the subs to cut the 41Hz peak. As I have 2 subs I could use one PEQ at 41Hz and the other at 60Hz - I have not tried it yet. The 21Hz mode I just ignore as I have adjusted the subs to cut-off at low frequencies. REW shows a relatively flat SPL and that the delay across the lower frequencies is below 200ms apart from at 60Hz where it is below 300ms.

    Regarding diffusors, it is something I would like to try but I have read that there needs to be a distance of at least 2m between the diffusor and listening seat for them to be worthwhile. The Arqen site, below, has some DIY recipes that tempt me. Probably for my rear wall to start with.

    A couple of REW captures of the measured results.

    [​IMG]
    Note the 100Hz band is caused by electrical interference, potentially the USB soundcard or microphone driver-thingy.

    [​IMG]
    Yes I know something odd above 100Hz - not figured it out yet. Though I have boosted my sub a bit for the room-curve effect so that may be something to do with it.

    Some resources you may have come across:
    Multi-layer Absorber Calculator (acousticmodelling.com). Helps model the velocity absorbers.
    Optimizing Speaker Placement with Acoustic Measurements | Arqen.com. Arqen have a nice series of articles, navigation is not so good but look at the "Acoustics 101" series on top menubar and you can navigate the articles using the info box on the right or left and right arrows below each guide. Their "recipe" is different to Grimani's - see here Bass Traps 101 - Your Guide to Corner Bass Trap Placement (arqen.com)
     
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  13. RJohan

    RJohan pfm Member

    If I understand it correctly absorbers need to be quite large to do anything meaningful, particulary at lower frequencies. It's the total surface area of the absorbers compared to the total area of the room boundaries that usually is to small.

    I recognizes the 100 - 150 Hz problem from my room, in my measurements its visible as very rapid dips and peaks. My take is that it comes from the physics of wavelenghts, room size and speaker placement interacting.

    As opposed to Rag987 I think the room acoustics comes first. And there is speakers that works very well up against a wall, if they are designed for it.
     
  14. rag987

    rag987 pfm Member

    That's not what I said. I suggested a rethink before adding more treatments. In a small room treatments are important.
     
  15. RJohan

    RJohan pfm Member

    Ok, I'm with you.
     
  16. ToTo Man

    ToTo Man the band not the dog

    Thank you for this extremely detailed account of your own experience, it is very helpful and gives me several things to try.

    My sketch is a bit misleading, sorry. The bay window depth shown is the depth to the actual window, but the depth to the sill panelling below the window is only 26cm, and this depth is filled with a floor-standing central heating radiator so there is actually no space in the bay to locate any hifi equipment. Even if there was space I don't think I'd want to put my hifi gear here as it would been my window blind would be permanently closed to keep the sun off. It's a shame the in-wall cupboard shown in the lower left of the sketch wasn't deeper otherwise I could have perhaps located my gear in there and mount the TV in the middle of the left wall (i.e. rotate the system by 90 degrees instead of 180.

    My phantom centre imaging was amazing when my speakers were closer together and closer to my listening seat, so that their lateral plane was in front of my hifi rack. IIRC they were around 85cm from the front wall and 115cm from the side walls, and 200cm from my listening seat. I wouldn't want to sit any closer to the speakers than 200cm because driver integration becomes an issue (if I sit closer then I'd probably need to change to speakers with smaller distances between tweeter, mid and bass units). Putting my system and speakers on the left (cupboard) wall or right (door) wall would give me an extra 36cm distance to play with, which isn't much, but it might help me get the speakers a little further out of the 'SBIR zone'.

    Do you know approximately what distance from the front wall a speaker needs to be in a 4.14m x 3.78m x 3.25m room to avoid SBIR in the critical 50Hz-100Hz area? If I put my speakers, say 1.2m from the front wall and my listening seat 1.3m from the rear wall, I'm only going to be around 1.9m away from the speakers if I sit in an equilateral triangle. I laid it out with markers and having my speakers so far out from the wall basically eats up all my floor space, even when the room is rotated 90 degrees. I could of course solve this by buying smaller, modern speakers and using subs etc, but I'm a stubborn b*stard with an emotional attachment to his Ditton 66s!

    PS - I already have two 12" BK XXLS400 subwoofers that aren't currently in use, so it wouldn't be too much of an outlay to buy another two and use one at each wall midpoint for bass management below 80Hz. In hindsight, if I knew I might be using four subs I'd have probably bought the smaller XXLS200 model because the 400s eat up a lot of floor space in such a small room and are a back-breaker to move!
     
  17. tuga

    tuga Legal Alien

    PM sent :D
     
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  18. rag987

    rag987 pfm Member

    Try this site, he offers 3 options best to worst from flush with wall to against the the wall to away from the wall. I placed my speakers on this basis, Option 3.
    Speaker Placement 101: How to Fight Boundary Interference (arqen.com)

    I would say no harm in experimenting with your 2 subs beofre you commit to another 2. It may be acceptable. Welti has a lot to say about sub placement and he suggest 2 subs mid-wall front and back are almost as good as 4 at mid-wall points. Grimani seems to prefer 4 in corners, but I think his perspective is driven by home-theater considerations as he is aiming for 115dBA SPL so he places the subs to work at max efficiency in corners where bass reinforcement is maximum. I prefer to place bass traps in the corners.
     
    ToTo Man likes this.
  19. ToTo Man

    ToTo Man the band not the dog

    Yes, Grimani recommends corner placement of subs for max. SPL efficiency but acknowledges that mid-wall placement yields superior standing wave amelioration. My corners are treated and the one at the door isn't big enough to take a sub so I'd only be able to try the mid-wall placement.

    I haven't shown it in my diagram but my subs are currently located either side of my hifi rack. I was using them to fill in below 40Hz where my main speakers begin to roll-off, but after relocating the mains to the wall and regaining output between 60Hz-120Hz I no longer felt the need for the subs' contribution below 40Hz. I'm currently trying to source a couple of 40cm x 40cm dollies that I can put the subs on to help move them out of the way, they're too darn heavy to lift repeatedly...
     
  20. RJohan

    RJohan pfm Member

    The 'Argen' guy seems to know what he's writing about. It's just to experiment with his ideas, I think.

    About multi subs. The HT setups is much about even response in several seating positions, we audiophile loners doesn't necessarily benefit that much. I'm certain To To Man can get a very good response with two subs.
     
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