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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

    Well, if nothing else my treatment repositioning has demonstrated the limitations of the frequency response graph's ability to describe a speaker's in-room tonal balance!...

    If you look at the above FR graph in post #100 comparing untreated and treated side wall first reflections, you see very little change, and the small change in the MF is comparable to the small change in HF. However, the reality is a much larger audible shift in the speakers' tonal balance, you'd literally have to be deaf not to hear it!

    In summary, it now sounds like the mids have been turned down and the treble turned up. Transient attack and dynamics are now sharper and faster, - the presentation is now more 'edge of your seat' and less 'sit back and relax'. It's like the headphone equivalent of changing from HD600 to HD800 or DT880.

    I noticed a similar 'night and day' effect when I first installed the treatment in 2013 when I was using Tannoy Lancaster MG15 as my main speakers. However it's been many years since I've had speakers positioned in the room such that their first reflections are completely covered by the side wall absorbers. As my speakers have gradually crept nearer the front wall, the side wall absorbers should have been moved with them, but the location of the door dissuaded me from doing so.

    The change is not all positive. I've lost a fair amount of the 'envelopment' that I experienced when the walls were untreated, and the speakers are now far more revealing and unforgiving. I may need to EQ the treble down a little because it's a bit edgy now, - an observation I'd never thought I'd make about the Ditton 66! I'll experiment with toe-in too in case that alleviates it.

    PS - @darrenyeats, am I right in thinking that you use GIK Scopus membrane bass traps in your listening room? Do you mind if I ask you what your low frequency reverberation times were before and after, and how many traps were required to achieve this reduction?
     
  2. tuga

    tuga Legal Alien

    I agree. Room interaction plays a huge role in the end result and the effects are not very well represented above the transition range 500Hz
    I find that wide directivity speakers tend to do worse in lively room of normal size
     
  3. darrenyeats

    darrenyeats pfm Member

    I've got the equivalent of 2.5 Scopus traps (custom width/height). They make a measurable difference but only just! Subjectively it's a difference I don't want to go back from. You'd need a whole wall of them to make a significant measurable difference. I position them contiguously as recommended - on the rear wall. I also have helmholtz resonators, but if I started again maybe I'd just go for all Scopus? Even with the scatter plate option, you may need to do "something" else since to me they sound not much less reflective than a bare wall.

    Bear in mind mine are custom frequency 35Hz tuned (meaning thicker than usual), and the lower the frequency the harder it is to absorb. So you may do a little better.

    The most important thing you can do for bass is move the listening position, but this tends to involve a tradeoff between reinforced but more mode-y bass near the room boundary and reduced but cleaner bass away from room boundaries (obvs you want to avoid the dead center of the room which will have a suckout). The absorbers in practice will just let you move a smidgen nearer to the boundary than you would otherwise.

    No-one's "fixing the bass" with passive treatments, it's really about the positionings.

    Second most important for bass is speaker positioning. Speaker positioning is very important for mid/top though. Again trade-offs. IMO side walls and large objects are the problem for mid/top. Speakers being flat against/near the wall behind them is okay though - for bass/mid/top (mid/top depends on the design, and there being no large objects near).
     
    Last edited: Oct 11, 2021
    ToTo Man likes this.
  4. ToTo Man

    ToTo Man the band not the dog

    Thanks Darren, that's really helpful. Do you have the Scopus directly in line with your listening seat or are they spread to the sides? AIUI modal treatment does not need to be in the 'firing line' the way that reflection treatment needs to be? The reason I ask is that I'm also considering the Scopus T70 to deal with my 75Hz null, which I think is caused by SBIR but I'm not entirely sure if it's SBIR or modal or both. I'd therefore probably need to prioritise placement of the T70 to directly behind my listening seat and the T40s flanking the T70s on each side. It'll likely trigger my OCD as the T40 traps are 25cm deep while the T70 are only 10cm deep, so the wall will look very 'blocky'. I'm also not sure whether the different depths of the traps will be good or bad from a reflection POV? Maybe I'd be better just putting T40s on the wall and standing Monster traps in front of them to deal with the 75Hz SBIR?
     
  5. Old Shatterhand

    Old Shatterhand pfm Member

    Do you know active Bass traps like from Bag End or PSI Audio?
     
  6. darrenyeats

    darrenyeats pfm Member

    My Scopuses are directly behind me but my understanding is they could be anywhere on the rear wall, but I've not tried.
     
    Last edited: Oct 12, 2021
  7. ToTo Man

    ToTo Man the band not the dog

    I kissed goodbye to my hifi rack, to see if eliminating a tall, solid structure between the speakers improved the phantom centre image. I don't think it has. It's not any worse but I don't think it's any better. The imaging seems to be a bit lower in height now, like the singer is sitting instead of standing. I think this may be the result of lowering the height of my 50-inch plasma TV, which previously sat much higher on top of my hifi rack.

    [​IMG]

    I’m pretty sure I can also hear the TV having an undesirable influence on my speakers' timbre. It's not a night and day difference, but my system sounds 'glassier' now, for lack of a better word, with a slight hardness/harshness in the midrange. I also think the system now sounds a bit lightweight, I suspect this might be because my DAC is sitting on hollow cardboard IKEA boxes filled with DVDs (a temporary measure!) instead of a solid wood shelf.

    The changes in measured FR are subtle and some of these may simply be due to the small variations in mic placement in the before and after measurements. The most significant and interesting change is the increased output around 200Hz:

    [​IMG]

    Turning my attention to the ETC measurement to investigate the contribution of the TV screen, I now have a significant reflection at 21.4ms (L spk) and 19.6ms (R spk) that wasn’t there when the TV was sitting up high on the hifi rack:

    [​IMG]

    Throwing a duvet over the TV totally eliminates the reflections at 21.4ms and 19.6ms. It also has an effect on the FR, reducing the upper mids and treble:

    [​IMG]

    [​IMG]

    Putting the duvet on the back wall behind the listening seat instead of throwing it over the TV also helps with the reflections at 21.4ms and 19.6ms but it’s nowhere near as effective. It reduces the reflections by -4dB, whereas covering the TV reduces the reflections by -16dB.

    Conclusion: I need to raise my TV back up to above ear height! I think the most effective way to do this is to wall mount it, that way I can also get it as far back from the plane of the speakers as possible. However, I won’t be able to wall mount it until I actually decide which wall I’m putting my system on!...
     
    Last edited: Oct 20, 2021
    tuga likes this.
  8. jobseeker

    jobseeker pfm Member

    Must admit I’ve never been able to hear any difference in presentation when I’ve tried removing a central rack.
     
    ToTo Man likes this.
  9. darrenyeats

    darrenyeats pfm Member

    Not much mid/top energy will get thrown directly backward from box speakers, but there will be some angling slightly back. You would hear less difference with the TV duvet/no duvet if the speakers were fully toed out, and/or if the speakers were further out. Normally any large objects should be at least 1m away from speakers (ideally behind them). Duvet it is?!
     
    Last edited: Oct 21, 2021
  10. ToTo Man

    ToTo Man the band not the dog

    That might be tricky when watching concerts and movies! ;)

    The front of the TV is currently on the same plane as the tweeters. If I wall-mounted it I'd be able to get it 30cm behind the plane, plus if I raise it back up to where it was (which would in fact be welcome because I like to recline when watching TV and my eyes naturally want to spot 60cm higher than where the TV is currently!) then this should hopefully further reduce the screen's contribution. It would also free up space on the front wall to experiment with acoustic treatment. :)
     
    darrenyeats likes this.
  11. ToTo Man

    ToTo Man the band not the dog

    Digging a little deeper into the ETC, the 21.4ms and 19.6ms from the left and right speakers corresponds to an additional path distance of 7.3m and 6.8m, respectively. I used the string method to map these out and it appears the reflections are coming from the back wall and rebounding off the TV and back to the listening seat. While covering the TV with a blanket stops the rebound to the mic, it probably makes more sense to treat the back wall to block the initial reflection.
     
    darrenyeats likes this.
  12. ToTo Man

    ToTo Man the band not the dog

    I did some more experimenting with absorption on the back wall to see what effect it had on my 75Hz null. Instead of using my two GIK Monster panels, I used a couple of fold-out foam mattress cushions (similar to this: https://www.argos.co.uk/product/4177740 but around 35 years older!).

    I have no idea what the absorption coefficient of these foam cushions is but I used them because they are easier to move and cover a larger surface area (each cushion measures 186cm x 62cm x 12cm so two stacked on top of each other gives me a giant absorber measuring 186cm x 124cm x 12cm!).

    To my surprise, it turns out that, when placed with an appropriate air gap, 12cm thick upholstery foam eats 75Hz for breakfast! In fact it even takes a greedy bite out of 40Hz, which I was not expecting. The following measurement compares no back wall treatment to the two foam cushions folded out and stacked on top of each other 90cm from the back wall:

    [​IMG]

    How does it sound? Very different! I'm now very aware that all of the sound is coming from the front of the room, along and behind the plane of the speakers. The imaging is very focused and precise, but not as spacious and enveloping. The noise floor is lower and dynamics have become even faster and more impactful, perhaps too much so. Filling in the null at 75Hz is very noticeable and extremely welcome. The reduction in output between 200Hz-400Hz is noticeable and it is not welcome, but it's not as audible as I feared.

    I also tried other arrangements for the cushions, different distances from the back wall, one cushion instead of two, keeping the cushions folded as cubes, etc. Every arrangement is an improvement over doing nothing to the 75Hz null, even using just the one fold-out cushion which stops 40cm short of my listening height. However, maximum horsepower is achieved when the cushions are unfolded and stacked one on top of the other, 90cm out from the back wall. This is also of course the least practical solution because it effectively cuts off access to a quarter of my room!

    Below are the results of the other arrangements. All the measurements show that there is no free lunch. I saw this previously when I used the GIK Monster panels on the rear wall. Using broadband absorption to fix the 75Hz null also results in 200Hz-400Hz being absorbed, and from a FR point of view I don’t really want 200Hz-400Hz sounding thinner than it already is.

    Perhaps it would be better to use tuned membrane traps on the back wall to absorb 75Hz, and then put a thin 5cm broadband absorber in front of it to absorb reflections above 500Hz, thus leaving the 200Hz-400Hz region untouched? Diffusion is another option, provided I can sit far enough away from it for the diffused soundwaves to integrate correctly.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: Oct 29, 2021
    tuga likes this.
  13. RJohan

    RJohan pfm Member

    Aha! So foam mattresses do work!

    I have a heap of them plus even more pillows in a room corner as a bass trap. They are on top of one of the JBL 4331's cabs currently not in use, ca 80 * 90 * 100 cm. Just for me to keep on piling them :)
     
    ToTo Man likes this.
  14. ToTo Man

    ToTo Man the band not the dog

    I was looking back through previous measurements with the back wall untreated and noticed that moving the listening position forward another 20cm from 130cm to 150cm from the back wall makes a significant improvement to the 75Hz null. Not as good as with the absorption behind the listening position but as far as a 'free' solution goes that doesn't cost money or take up valuable space it's pretty decent! I previously dismissed this listening position because I thought it moved me too close to my speakers, but I'll give it a go and see what I think. I also tweaked the EQ for this new listening position with narrower Q filters and managed to flatten the peaks at 53Hz, 85Hz and 108Hz without making the dips at 75Hz and 97Hz any worse:


    [​IMG]
     
    Last edited: Oct 29, 2021
    tuga likes this.
  15. AndyU

    AndyU pfm Member

    I’d be more worried about the octave wide chasm centred on about 150Hz! That’s just about the whole top string of a bass guitar.
     
  16. ToTo Man

    ToTo Man the band not the dog

    That 'chasm' disappears if I pull my speakers a metre out from the front wall, but then I'm left with a -30dB null from 70Hz-80Hz which bugs me more (I listen to a lot more rock and pop music than jazz). I've tried the foam mattresses on the floor in front of the speakers but they have no effect between 100Hz-200Hz, they instead reduce output between 200Hz and 1.25kHz. The only things I've still to try are treating the ceiling (which is difficult to experiment with for obvious reasons!) and raising the height of the speakers.
     
    Last edited: Oct 29, 2021
    AndyU likes this.
  17. ToTo Man

    ToTo Man the band not the dog

    I did some more experimenting with the two spare broadband Monster bass traps I have, putting them in locations I haven't tried before, at least not recently. I tried them on the front wall, standing vertically on top of each speaker. I also tried them in the back corners in front of my GIK corner traps, first around 50cm out from the corner traps and then almost touching the corner traps.

    The following measurements were taken at my new listening distance of 150cm from the back wall which I have been using since post #114.

    No monsters vs monsters on front wall:
    [​IMG]

    No monsters vs monsters in rear corners with 50cm air gap to corner traps:
    [​IMG]

    No monsters vs monsters in rear corners with 3cm air gap to corner traps:
    [​IMG]

    I think the best arrangement is when they're almost touching the corner traps. The 75Hz null is improved and I also gain some output between 250Hz-350Hz. The only downside I see is a deepening of the 95Hz null. Overall I think this is a preferable result compared to when I placed the absorption on the wall area behind my listening seat and saw a reduction in output between 200Hz-450Hz (see the graphs in post #112).

    Putting the absorbers on the front wall on top of the speakers makes very little difference. The 75Hz gets slightly worse while the 95Hz null gets slightly better (you only notice this when you remove the 1/12 oct smoothing from the graphs).

    So I guess this tells me I need bigger corner traps?! My current GIK chunks are only 580mm wide. If I can find a way to safely stack the Monster traps on top of each other and secure them to the wall then I suppose I could just buy some more Monsters and run a column of them floor-to-ceiling in front of the GIK chunks rather than removing the chunks and DIY'ing bigger ones. I know which option my dad would prefer!!! :D If I buy more Monster panels I’ll probably go for the range-limited version and take the opportunity to reflect some MF and HF energy back into the room.

    I'm also going to order some pressure traps tuned to 42Hz as this really is going to be the only way to even out my axial length mode and make listening away from the hot-seat a more pleasurable experience.

    This is the response I’m able to achieve now at a listening distance of 150cm from the back wall with EQ applied:
    [​IMG]
     
    Last edited: Nov 8, 2021
    tuga likes this.
  18. tuga

    tuga Legal Alien

    It's looking better all the time. Why didn't you go for "moving the listening position forward another 20cm from 130cm to 150cm from the back" measured on post #114?
     
    ToTo Man likes this.
  19. ToTo Man

    ToTo Man the band not the dog

    I did. :) The measurements shown in post #117 were taken at the new 150cm distance but I forgot to note this on the graphs. If you look at 43Hz you'll see a clue (the output at 43Hz is weaker because 150cm is 20cm closer to the midpoint of my room). I've edited post #117 to make it clear that the listening distance is now 150cm.

    Even with the new 150cm distance, I still think I need more bass trapping, not only to smooth out the LF frequency response but also to tighten LF decay times. Adding the two Monsters in the rear corners has reduced ringing at 43Hz and 53Hz but only be a small amount. If I add another one and a half Monsters to each corner (which will take me to the ceiling) the ringing will improve further but I still reckon I'll need membrane traps to tame the 43Hz beast. :D

    I'm also going to experiment with some diffusion on the back wall and rear side walls to see if it really can make a small room sound bigger. I thought absorption was complicated but the mathematical theory of diffusion is on a whole new level, - studying it makes my brain hurt!... :eek:
     
    Last edited: Nov 8, 2021
    darrenyeats and tuga like this.
  20. ToTo Man

    ToTo Man the band not the dog

    I removed the stack of GIK TriTraps from the rear left corner of my room so I could gain access to the cupboard. This is the only corner I removed the treatments from, so it’s difficult to come to reliable conclusions, however my suspicion about the TriTraps eating my upper bass / lower midrange appears to have some validity, because removing the TriTraps increases output between 250Hz-400Hz. See this post on Gearspace for more details.

    I ended up removing the cupboard door and stuffing the cupboard space full of earthwool RS45 wrapped in polythene. This has improved the 75Hz null by +4dB compared to having the cupboard door shut. I've still to put the TriTraps and Monster trap back into the corner, I suspect the 75Hz null will improve even further once I do this, but this is the EQd response I now have at my listening seat:

    [​IMG]

    Before I put the 180cm tall stack of 75cm (L) X 30cm (W) RS45 slabs into the cupboard I stacked them on the floor in place of the GIK TriTraps and found they were significantly more effective at reducing ringing at my 53Hz height axial mode, which is promising.

    I've therefore decided to replace my GIK TriTraps with larger DIY ones made from the same RS45 earthwool I stuffed into the cupboard. These DIY corner traps will measure 85cm across the face instead of the GIK's 58cm and will be double the cubic volume of the GIK, so should have a good bit more horsepower down low. I'll wrap them in poly bags, make a stretched fabric grille frame to go over the front, and will most likely put some vertical wooden slats over the front to reflect some of the MF and HF back into the room. Hopefully this will also help to curtail the absorption at 250Hz-400Hz.
     
    Last edited: Nov 22, 2021 at 8:40 PM
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