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acg

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acg last won the day on February 14 2018

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About acg

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    I can't jump puddles
  • Birthday 31/07/1973

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    Toowoomba
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  1. I own two turntables... but am yet to listen to either... so I need to do this analogue.
  2. In the pursuit of trying to figure out 'what traps do what in my room', I've spent a little time experimenting this afternoon. First, I made a rough Superchunk trap from some Higgins poly thermal insulation and plonked it in a couple of corners. There is only one superchunk trap so it takes a while to move a couple of pieces at a time. I am trying to figure out what these soft and fluffy traps are capable of doing in my room and it certainly does look as though they are useful, but they may not be the end-game solution. Here is the Mega-Superchunk. It's pretty rough but you get the idea. The white Superchunk sits in the corner and then all the sheet of Greenstuff were wedged roughly in front. And the improvised Greenstuff Superchunk in the opposite corner...of course not at the same time as the Mega-Superchunk or the Under Window Bench Trap because the same sheets of Greenstuf must be used for each trap. So, what did I see? Here are the SPL traces of some of the configurations I played with: Red = Empty room, devoid of treatment Green = Superchunk behind the active speaker Cyan = Superchunk + Greenstuf Bench under windows behind seat Gold = Mega Superchunk Magenta = Superchunk + improvised Greenstuf Superchunk in other front corner Blue = Partial Superchunk behind speaker + Greenstuf Bench under windows An excellent response to treatment in the same regions as last time, but not as much effect on the 42Hz and 117Hz room modes as I had hoped. The Mega-Superchunk (Gold) shows that getting as much soft and fluffy behind the Bass Cannons as possible is going to do good things to the frequency response 60Hz-100Hz, especially if I change the filter for the Cannons from 60Hz to maybe 30Hz to try and chop off some of that 42Hz longitudinal mode. The dip at 51Hz is supposedly a tangential mode and is improved with a Superchunk in the opposite corner (Magenta) and also trapping the other corners of the room floor to ceiling, although I have not shown those measurements here. If I can get that dip filled up similarly to how 64Hz has improved then I have regular shapes that I can EQ flatter with the electronic filters in the amplifiers. That 117Hz hill does not really want to budge. I did manage to knock 6dB from it by using this monstrosity at one axial mode point... ...that is nearly two bags of Greenstuf R2.5 propped up against the wall. Obviously not a sustainable solution, but I can put a VPR both there and on the opposite wall and tune it to circa 117Hz and perhaps I will get a better result. The 55Hz axial mode was also trimmed a couple of dB with that trap, so perhaps I should tune the trap for some effect at 55Hz but maximum effect at 117Hz. To match reality to my measurements, I also tested a shorter Superchunk in the corner above the trapdoor. A full floor to ceiling trap will not fit in there if I ever want to get in and out of the room, so I shoved an old TV stand in the corner and built the Keeping-it-real-Superchunk on top to estimate the effect of losing 40% of trap height. Here are the results: Red = Empty room, devoid of treatment Cyan = Superchunk + Greenstuf Bench under windows behind seat Yellow = Short Superchunk + Greenstuf Bench under windows behind seat The shorter Superchunk in that position is still worthwhile. So, what have I learned from this: The more absorption I get in those corners behind the speakers the better. The Mega-Superchunk is not really an option, but I intend to experiment with VPR/CBA in that position. They are much more expensive, but also can be made to work at much lower frequencies, plus they should be quite effective when placed against the wall so will be able to be made bigger than the Superchunks while allowing the trapdoor to open and close. Axial traps are going to be required to tame 117Hz, and probably 55Hz axial modes as well. It also had an effect at 42Hz. There is no room so these will have to be VPR/CBA. The Bench Trap under the back windows is a good idea and works on all the biggest problem modes and several of the smaller ones. These will be built as originally planned. Going floor to ceiling in the back corners also helped the 117Hz problem, so I may have to block a window or two with an additional trap atop the under window Bench Traps. A less intrusive trap at the corner between the ceiling and the back wall (with the windows) is likely to also be helpful for this mode, perhaps more helpful plus it will not block any light. I am probably going to have to move the crossover point for the Bass Cannons from 60Hz to circa 30Hz which should knock about 3dB from the 42Hz peak. A second order filter may even be required. With room treatment that may get me flattish 16Hz to 45Hz and then rolling off from there with the next major problem at 117Hz, which is is right at the lower knee of the 110Hz Upperbass Horn which is yet to be completed, which should give me some further options to alleviate the peaks with EQ. So, now to mock up some VPR's.
  3. Nothing to do with line source behaviour Dave...I was referring to more to physical reflections due to the size and heftiness of the subs...2.4m tall x 1m deep and 400 kg...a wall of sorts...perhaps with a behaviour of reflecting more of the low frequency energy back into the wall or corner rather than letting it bounce around other parts of the room. Sound wraps around the speaker, bounces off the side wall (bricks) then the Cannons, then the side wall, Cannon et cetera, getting a little less each time as the wall and the Cannon lets a bit through, but ultimately more energy stays in the corner to be absorbed by the Greenstuf. It may only be a few or maybe 10% more energy reflected back into the trap by the mass of the Cannons. One thing that I can say is that the bass in the room improved a little just by building the Cannons in situ even without turning them on and still using the ML1's...I reckon there is something in my theory....somewhere...perhaps.
  4. @Red Spade AudioPaul, thanks for the gif...what a great idea! Please share how you did that although I could probably google it. Subjectively, I did not listen to anything but sweeps and I shifted the Greenstuff out of the room immediately because it is for something else. Because Autex have shut up shop here and gone back to NZ I have since purchased some Higgins R2.5 poly insulation to roughly make a corner trap or two for more formal testing. Just gotta find some free time to cut it up and stack it in place. One thing that I have wondered since taking these measurements is whether the physical shape of my sub has something to do with the success of the trap. It's not a normal sub...see below... 8 x 10" drivers pretty much floor to ceiling. The two bags of Greenstuf were placed behind that very speaker (horns not yet complete - but close) with the trapdoor down of course (it is up in the photo). The Cannon stack is made of steel and weighs about 400kg and I think would be a very good reflector of sound even in lower frequencies. Two things that I have been thinking about regarding the effectiveness of the improvised corner trap are: The Cannons almost make a false wall in the room and even though they do not sit in the corner they might be effective at reflecting sound pressure back into the corner trap for 'another bite'. The low excursion, floor to ceiling nature of the sound source may more evenly distribute energy into the corner trap, as opposed to all the energy coming from one spot close to the floor. Of course I am only thinking about the sound that wraps around the speaker and makes its way to that corner rather than sound that is reflected from all other parts of the room. Of course these ideas might be moot, but I am trying to think of reasons why those two bags of poly were so effective.
  5. Before (zero treatment): 2 unopened bags Greenstuf R2.5 stacked floor to ceiling in corner behind speaker:
  6. Sorta Mike, at least in theory. In practice things are sometimes different. Recently playing with two (2) unopened bags of Greenstuf R2.5 in my room I had good impact sub 100Hz, especially with cancellations rather than reinforcements. Enough of an impact for me to consider building proper Superchunk traps to see how they do. Blue = zero room treatment Gold = 2 unopened bags of Greenstuf R2.5 in one front corner floor to ceiling 15dB improvement @ 64Hz which makes the whole 60Hz-100Hz fit within +/-2.5db of the first order low pass set at 60Hz. Some effects as low as 40Hz, and some hope of taming the room modes circa 42Hz, 55Hz, 117Hz with more stuff in the right places. Cheers, Anthony
  7. I read somewhere that the Greenstuf R2.5 has a GFR of about 7,000, which seems to be just about perfect for a velocity trap like a superchunk. Cannot remember where I read that, but from memory it was from someone in contact with Autex. Of course this is the internet and everything you read here is the absolute truth...
  8. acg

    Returning LPs to Amazon US

    I had no probs with full refund including postage. Was a pain in the arse though...
  9. Glass is fine because it lets the bass through and is effectively a large bass trap which should help your frequency response at your listening chair. Symmetry, symmetry, symmetry, so a green vote for me, although from a sound perspective ther may be better options in the space. A good big room like that could sound really nice...
  10. Yeah. I've just measured how high a superchunk will need to be so that the trapdoor can still open and close: 1.35m down from the ceiling. It's not a lot of straddling trap left but perhaps I can figure out a workaround which includes normal foam or VPR/CBA in the trapdoor constriction zone. I think that I will go and get some more Greenstuf, cut it up, stack it loosely in place and then see what the measurements say.
  11. But mine are bigger, and don't have resistors in a speaker level crossover....haha. Carry on.
  12. This thread is hilarious. I can't figure out if the OP is serious, delirious, or taking the piss...he's playing the dumb card really well. It's a great read for a few giggles... @davewantsmoore, I can't remember you losing your sh&t like that before, but it's nice to know you're human!
  13. A couple of days ago I went into REW, put the mic on a very long lead and wandered around the room listening to sine waves at or near the rooms modal frequencies and the peaks/troughs that appear in the frequency response below about 150Hz. At the end of it I have a diagram of the room for each frequency that indicates the SPL at positions that I think traps can potentially be placed. Part of the reason I did this is to identify the frequencies that traps at those particular locations should target. 30Hz axial mode - this one displayed exactly the kind of behaviour you would expect from a text book...at any distance between the side walls, regardless of how close the mic was to ceiling or floor the spl was virtually unchanged. It did change of course with proximity to the side walls and was loudest there, but in general it was very well behaved. I'm not worried at all by this mode so long as I stay equidistant to the sidewalls. 41Hz longitudinal mode - this is a big one and is roughly equivalent to Fs in the Bass Cannons. The entire back wall underneath and above the widows had the highest pressures, particularly the corner nearest the Cannon. Also plenty of pressure axially from the listening position and a little less in the front corner behind the Cannon, which surprised me a little. It looks like the under window trap is going to be particularly effective for this resonance, as well as axial traps on the side wall and a corner trap behind the Cannon. Being a longitudinal mode I had assumed that I would need a trap at the midpoint of the front wall between the speakers, but that was almost the lowest pressure point of the entire room perimeter. At the ceiling directly above the listening position SPL had dropped 2dB. 51Hz tangential mode - This one has a large interaction with the ceiling directly above the listening position with SPL 4dB louder up there. Plenty of pressure also in all the room corners and even both axial and longitudinal positions. 55Hz axial mode - No ceiling interaction with this one and relatively low pressure both axially and behind the listening position but the front corners and front wall had plenty of pressure. 117Hz axial/tangential mode - 2dB quieter at the ceiling directly above the listener but has plenty of pressure in both the front and back room corners, mid-front wall and axially on the floor but not at the ceiling. 139Hz tangential mode - First measurement was above the listening position where 300mm above ear height it was 6dB louder. I did not even bother to test the remainder of the room because this is so obvious a ceiling/floor interaction. So, after all that I have concluded: a ceiling trap would be nice to handle 50Hz - 140Hz and probably higher. This needs to be either 200mm of foam or 100mm of CBA the under window traps behind the listening position should try to have effect from 40Hz and up. These are 370mm thick soft and fluffy so should work quite well at that range. the corner traps behind the Cannons should target 40Hz - 120Hz and probably higher. More about these later. Mid-wall traps set on the front wall between the speakers will be useful upwards from 40Hz to at least 120Hz, probably higher. Axial traps should be useful from 40Hz and upwards. These are really limited to VPR/CBA traps because one will have to sit above an opening and closing trapdoor and the other will sit between my desk and the wall, so space is very limited. Also, I have read some more on VPR/CBA traps and based on the work of others, have managed to form my own opinion of how they are likely to be best used. These are opinions and not based on my own experience, rather that of others. Roughly speaking, a VPR/CBA is twice as effective below 100Hz as the equivalent thickness of high quality foam/poly/melamine. Even a VPR/CBA with a total thickness of 100mm will be able to knock out room modes down to 40Hz whereas 200mm of conventional foam/poly/melamine is necessary in the same position to get similar result. Straddling corners, whether with VPR/CBA or conventional foam, is more effective < 100 Hz than having the panels hugging the wall. Straddling a corner a 10cm thick VPR/CBA has equivalent performance to a 20cm thick VPR/CBA below about 60Hz, but 60Hz - 150Hz the thicker panel is more potent. A 25cm thick VPR/CBA is no more effective than 20cm thick, so there is an upper limit on thickness when chasing 40Hz+ frequencies. With all that in mind to plan my room treatment, the under window traps seem perfect in their current design and should have an absorption coefficient of about 0.6 at 40Hz, 0.8 at 50Hz are large (6m x 0.6m x 0.4m deep) and the sit in a high pressure zone for most of the problems frequencies. Likewise, VPR/CBA 10cm thick will be made to go on the walls axially from the listening position. These are not first reflection points...I don't think I need to bother with treating those. The front corners behind the Cannons are the hardest question for me. Based on stuffing some Greenstuf bags into that corner and seeing the change in frequency response I think that a huge superchunk style trap (1200mm front face, 850mm sides) will work well perhaps even as low as 40Hz, but I am not sure. It looks like I am going to have to straddle those corners regardless of using foam or VPR/CBA, so at this stage I am considering building a big Superchunk and if I need more performance I'll add a VPR/CBA or maybe just some more foam to the front. I don't know. There is plenty of room mid front-wall between the speakers so I should be able to experiment with either foam or VPR/CBA at that location. It will end up being part of the room feature (so the horns don't stand out so much) so I have plenty of creative leeway on that trap. Which leaves the ceiling. It may be just easier to lump 20cm of decent foam above the listening chair and stuff as much as possible under the chair to try an alleviate this mode. A 10cm VPR/CBA would work well but probably not as well at 140Hz as the thicker traps. So I have a basic plan, some trepidation about the front corners behind the Cannons, but basically the room treatment decisions going forward have been made. I've just got to get some time to do it.
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