Lawsy Posted September 13, 2012 Posted September 13, 2012 I'm hoping to gain some advice on a few idea's I have regarding my room and its dismal acoustics. Pretty bad hey... Essentially, I have no choice but to have my TV area short ways across the bottom floor of our concrete block we like to call our house. The distance is 3.5m across, meaning a nice fat mode at 50hz, and when I say fat, I mean in the order of 15db boost levels of fat. But instead of describing it, here's a sketch I've been working on to demonstrate. As you can see, this room is a nightmare acoustically. It is a giant concrete acoustic killer... Firstly, the bass in the graph above, although terrible, is worse than the graph portrays, so we need to attack bass response first; my first point of call is this corner. I plan on creating a bass trap for this corner, in place of the lamp, similar to this. I plan on using 10mm thin pine, with the top of the enclosure being more skeleton than solid so that sound can drop straight down into the trap as well. The depth is 200mm, the rear wall side is 800mm, the side wall side is 700mm and the height will simply be to the same level as the couch, covered in the same material as the couch. Very wife friendly, even if I do say so myself! If this is a bad idea, and I should stop now, please let me know. Otherwise, I'd like to gather a bit of help regarding the best way to go about this. What depth of fibreglass, or conversely, how much air gap would you guys recommend? Also, lets talk about fibreglass for a minute. I've been researching this (acoustics, specifically bass trapping), on and off for years... Yet in that time I can't recall seeing measured proof that lighter materials, with lower flow resistivity, is practically better. If someone could link me to some sort of comparison of two bass traps taking up a similar amount of real estate, one with roughly 48kg/m3 and one with much lighter non-compressed fibreglass, with hard measured evidence of the comparison, that would be most helpful since I'm not really convinced that the lower flow resistivity is actually any better than rigid compressed fibreglass for corner filling... Now, I realise this is behind a couch, but the bass is punching pretty much straight through it and is still loading off each surface- remember, no rug here, its 2 concrete walls + a tiled floor and its punching straight back through the couch and causing grief. The reason this is my first stage as its the most wife friendly, but hey, if the collective belief is that I'm wasting my time, then idea's are most welcome as I've been reading this forum the last couple of days and some of you guys have some great idea's with unique engineered solutions. Right up my alley. Thanks guys, look forward to hearing what you guys think/could improve/should or shouldn't do. Cheers.
Wembley Posted September 14, 2012 Posted September 14, 2012 I've recently played around with acoustic treatment for a room myself, although I went for full height corner absorbers. I'll get around to writing up the experience and measurements when I finish playing and my trials with conversion of some speakers to active (both multi-year projects as I'm easily distracted). I suggest the following link for modelling different absorption options: http://www.stanleyhallstudios.co.uk/pacalc/ You will probably need a reference like Bob Gold's absorption coefficient information: www.bobgolds.com/AbsorptionCoefficients.htm For myself, I found some 22kg/m3 "fluffy" style batts (R2.5HD Wall Batts for the record) that I used to fill each of my 4 corners in a 430x580 triangle shape. The different sizes for the edges were to accomodate the windows and doors in the room and as it happened made for good use of a standard size batt, rather than any acoustic magic. The general rule is the more depth the better for absorbing bass frequencies. With only 3 of 4 corners filled to date I have taken T60 from over 1000ms to under 250ms which was a major part of what I wanted to address. From my general reading, on the question of high density vs low density, it's a matter of what depth of absorption you intend to have. High density is better for lesser depths while if you can get over 600mm then less density becomes preferred (all depending on which source you read, but that was my ultimate determination). I went for the middle ground option, in a "superchunk" style arrangement. You're probably aware already but there is limited measured data out there. A search on "superchunk" bass traps will get you some measurements, but not a lot, and a significant amount of debate over the best way to make one and the best materials and whether commercial options are a better use of your time and money. I don't have them to hand, but if you want any measurements I've done with corner traps let me know via a message and I'll see what I've got. I'm not entirely sure how much use it'll be as i didn't do any comparitive measures between different styles of fibreglass, but treated vs untreated room. And none of the calculators I have found do modelling for corner traps, just absorbers mounted on walls from which you have to extrapolate results. I plan to mount a 100mm deep panel on my rear wall for more trapping, and measure for a panel at first reflection points in the future, but it may be a while. I believe that some others have been purchasing and experimenting with polyester based panels so you might get some better information from them on higher density panels. Regards, Damian.
Luc Posted September 14, 2012 Posted September 14, 2012 I'm following this topic as I have similar decisions to make in the near future. Good luck with all, both of you.
Lawsy Posted September 14, 2012 Author Posted September 14, 2012 I suggest the following link for modelling different absorption options: http://www.stanleyha...s.co.uk/pacalc/ You will probably need a reference like Bob Gold's absorption coefficient information: www.bobgolds.com/AbsorptionCoefficients.htm For myself, I found some 22kg/m3 "fluffy" style batts (R2.5HD Wall Batts for the record) that I used to fill each of my 4 corners in a 430x580 triangle shape. The different sizes for the edges were to accomodate the windows and doors in the room and as it happened made for good use of a standard size batt, rather than any acoustic magic. The general rule is the more depth the better for absorbing bass frequencies. With only 3 of 4 corners filled to date I have taken T60 from over 1000ms to under 250ms which was a major part of what I wanted to address. From my general reading, on the question of high density vs low density, it's a matter of what depth of absorption you intend to have. High density is better for lesser depths while if you can get over 600mm then less density becomes preferred (all depending on which source you read, but that was my ultimate determination). I went for the middle ground option, in a "superchunk" style arrangement. Regards, Damian. Thanks for your input Damian, appreciate that - sounds like you've really nailed some of those decay issues (750ms improvement in RT60 is really significant, that would most definitely be audible, nice job). As you can see from my pictures, I'll be able to fill only one corner as the room simply extends left of stage for a total width of 11 meters, and bass has lost enough energy by the time it reflects back from the far left that it is of no concern to me. As for flow absorption, yeah I've got the spreadsheet and have spent hours going over the data... And yet, I'm still not convinced. I'd really love it if someone with serious cash could do some actual testing of various absorbers so we can have some empirical evidence of this. There is just far too much conjecture and hearsay from keyboard experts. As an engineer, I understand the 'theory' behind flow resistivity, but I don't see how going to the far extreme of 'fluffy' is ever going to perform better than a porous absorber that bass is clearly able to penetrate (and therefore continue to be absorbed). Anyway - I did something interesting this afternoon, under the stairs is a corner with nothing in it. See below... So I placed my SPL meter under there, about 500mm out from the corner, and the meter started clipping its ass off from about 50hz up to 110hz at the same output level... Seeing as this corner has so much energy, then it's probably a good spot to absorb some of it. We were going to put shelves under there but before we do, I'm thinking of stacking that corner with as much absorber the wife will allow... After this, I'm going to make some various thickness panels, anywhere from 25mm to 75mm of various square sizes to place behind the TV, speakers, and rear wall. My wife friendly excuse/reasoning is that it will be decorative having slightly different thickness's; it will be more work for me as I'll have a lot more framing to do, but it should work well and look good doing it... For this job, I found a fletcher product that has absorption properties better than anything I've ever seen before. One of their products, mineral wool slap, at a thickness of 100mm's and a density of 96kg/m3, has an absorption coefficient of 1.15 at 125hz... http://www.insulation.com.au/products-1/mineral-wool-slab - download the datasheet, seems better than anything I've ever seen. It probably costs its weight in gold though... Any thoughts?
Nigel Posted September 14, 2012 Posted September 14, 2012 I reckon your fr plot is a good candidate for a Helmholz resonator, which can attack a narrow freq, in your case 50Hz. I presume this room has hard surfaces. Are there any rugs, curtains, furniture ? A narrow room with the speakers pointing at you needs to have some good absorbers/diffusers on the wall behind you.
Lawsy Posted September 14, 2012 Author Posted September 14, 2012 (edited) I reckon your fr plot is a good candidate for a Helmholz resonator, which can attack a narrow freq, in your case 50Hz. I presume this room has hard surfaces. Are there any rugs, curtains, furniture ? A narrow room with the speakers pointing at you needs to have some good absorbers/diffusers on the wall behind you. The rear wall is going to be stage 2 - this is where Fletcher's 96kg/m3 mineral wool I believe would be suitable. If you compare the data to 703 and 705 of almost any thickness, it blows it out of the water - I don't know what or how it does what it does, but if those numbers are to be believed then I could get the same results with less volume of insulator, or better results if I keep the volume the same. Even 100hz and below should surely be slightly attenuated with a mere 50mm air/50mm absorber panel on the back walls. I can't help but feel that it is going to be expensive though... Regarding Helmholz Resonator's - thankfully, the one good thing about firing across the short distance in my particular case, is that each seat gets the same 50hz boost, even the seat to the left that isn't shown in the graph - it is at least even. Although I won't be able to remove that boost, I can subdue its decay a little and that will be fine with me. Its the 80hz + area that is uneven, lingering with massive holes that ruin a good bass mix. As for hard surfaces, carpet, furniture etc - other than the obvious from my first post, the curtains are nothing substantial enough to warrant mention. The rug is 3500 x 2500 long pile shag which does a great job removing reflections from the floor (however, I am contemplating an underlay to improve things further), and the couch is substantial yes, but clearly it is not bass trap with the above graph - I've even considered flipping the couch upside down and filling what I know to be a cavity with insulation, then re-staple the material. But I don't want to risk ruining a new couch... Edited September 14, 2012 by Lawsy
Nigel Posted September 14, 2012 Posted September 14, 2012 Fair nenough, you are putting lots of thought into it and I'm sure you'll get a good result. Incidentally 50mm absorbers tend todo not much for bass, as i undderstand it..
aechmea Posted September 14, 2012 Posted September 14, 2012 Welcome Lawsy, Glad to see that you are working on the acoustics of your room - an oft forgotten area which when attended to will give better results than spending heaps on new equipment. However, you have a bit of a problem with your concrete coffin! Treatments will only work to a certain extent ie a couple of dB amplitude here and there, probably reduced resonance/boom by reducing the reflections. My room looks like a warehouse for a fibreglass factory but even so the (absorption) effects are small in comparison with the peaks. With a huge 15dB lump like the one you have, I reckon that only speaker placement and EQ will solve it to your satisfaction. With careful speaker placement you can avoid exciting the mode by having your speakers in an area of high particle velocity (1/4 wavelength) rather than high pressure areas (near the boundaries). Unfortunately 1/4 wavelength areas at those frequencies are out in the middle of the room which is in general completely impractical! Therefore I think that your best bet given the size and orientation of the room and the size of the lump, is to use EQ to slice the top off that mountain but still use treatment to soften the room (which I guess is pretty bright) at other frequencies. You would only need 2 bands - one to bring the pointy peak at 48-54 down to that flatish level 5 dB lower and then another to bring all of the 45-80 range down by another 10. {I use full sheets of 32Kg/m3 semi rigid fg 100mm thick, a bit out from the wall 64 Kg/m3 preformed tubular rigid fg 400mm diameter [home made tube traps] in corners R4? woolen batts 200mm thick, here and there carpet, curtains and substantial furniture Speakers, particularly subs are well out from the walls into the room digital EQ with DEQX to lop off remaining peaks}
Paul Spencer Posted September 15, 2012 Posted September 15, 2012 Your room isn't dismal, it's normal for a small room. This is typical! A couple of points. Firstly, there are two basic types of traps - resistive broadband and tuned narrow band. Broadband traps require spacing out from the wall, this is where you take a trap the width of a door and straddle the corner. Once the thickness of the trap is around 100mm with suitably dense material (48kg stuff), it is the distance from the front to the wall that determines how low it will go. Thickness and density are related. 48kg stuff is the economical option - costs about $80 for a 1.2 x 2.4m 50mm thick sheet of Fletcher rigid fibreglass. It's easy to cut but nasty on lungs and skin. You can go a step up in density but it's not really worth it. As the trap gets thicker, the material density seems to matter less. Foam is not really the right stuff, for example, but once you get to about a foot thick you can get good results out of it. I've used foam that thick in big panels and measured effect down to 25 Hz. 1 ft foam traps make a just visible but not remotely audible difference. Your behind the couch location does not allow for a useful broadband trap below 100 Hz. It will change the midrange only. If you want to make a trap in that location of that form, it will need to be a tuned pressure trap. They are narrow band and require trial and error. They work best right on the boundary - it's velocity vs pressure thing. A broadband trap is useless right on the boundary because they work on velocity rather than pressure. As a result you only get effect at higher frequencies. The simple solution in a small room like this is broadband traps as big as you can manage. Consider bulkhead traps and as many corners as you can get. Then add EQ. Combined together you can get a near ideal result, if you use enough traps. My room has a similar arrangement, although the mode is at 44 Hz due to a slightly bigger room depth. If you are doing a tuned pressure trap, then behind your couch is a good location, since it is a corner, hidden away and in the path of your depth first order mode that is dominating. Your strategy could be to first tackle that mode with a virtually invisible pressure trap, and then deal with a broadband solution for 50 - 200 Hz with bulkhead and corner resistive traps. The requirement of making them big is thereby lessened. Now keep in mind a couple of things. First, if your speakers are rolling off at 50 Hz then you won't see any issues lower down. A sub may illuminate a few more. Second, clean low distortion bass in a room that is well damped can sound lean. If you get it perfectly flat under those conditions, the bass will be both under control and underwhelming! That is where I then experiment with a room curve. The end result typically sees bass running at a higher level and you get a more visceral impact without sounding overly bass heavy. I had one comment that my system (at that time) sounded like a similar balance to his system which was calibrated flat. At the time I had the bass running +15 dB! Looking at a frequency response plot, you can't tell if it sounds balanced or not.
Lawsy Posted September 15, 2012 Author Posted September 15, 2012 Welcome Lawsy, Glad to see that you are working on the acoustics of your room - an oft forgotten area which when attended to will give better results than spending heaps on new equipment. However, you have a bit of a problem with your concrete coffin! Treatments will only work to a certain extent ie a couple of dB amplitude here and there, probably reduced resonance/boom by reducing the reflections. My room looks like a warehouse for a fibreglass factory but even so the (absorption) effects are small in comparison with the peaks. With a huge 15dB lump like the one you have, I reckon that only speaker placement and EQ will solve it to your satisfaction. With careful speaker placement you can avoid exciting the mode by having your speakers in an area of high particle velocity (1/4 wavelength) rather than high pressure areas (near the boundaries). Unfortunately 1/4 wavelength areas at those frequencies are out in the middle of the room which is in general completely impractical! Therefore I think that your best bet given the size and orientation of the room and the size of the lump, is to use EQ to slice the top off that mountain but still use treatment to soften the room (which I guess is pretty bright) at other frequencies. You would only need 2 bands - one to bring the pointy peak at 48-54 down to that flatish level 5 dB lower and then another to bring all of the 45-80 range down by another 10. {I use full sheets of 32Kg/m3 semi rigid fg 100mm thick, a bit out from the wall 64 Kg/m3 preformed tubular rigid fg 400mm diameter [home made tube traps] in corners R4? woolen batts 200mm thick, here and there carpet, curtains and substantial furniture Speakers, particularly subs are well out from the walls into the room digital EQ with DEQX to lop off remaining peaks} Thanks for your input mate, you're confirming what I thought was the case here. Concrete walls are a real pain in the ass... As for acoustics, I've researched the topic on and off for many a year now but only recently did we buy our own house - now we can do what we want with it - brilliant! Even through my researching, I've still got much to learn and there is so much conflicting information out there that it can be difficult tell which information is trustworthy. The thing that IS working to my benefit is that, though not idea for stereo imaging, essentially having no left wall is removing the 6th reflection surface for bass to get excited off. If it wasn't for this, I'd have what I suspect would be a suck out in the right seats at around 75. That would be awful to deal with, removing what is a symmetrical boost along the seating positions we care about, for an asymmetrical flip from the main set to the corner seat (the two spots we use the most, the wife loves the corner and I lean over towards her from the middle). This just wouldn't do - every time I would lean over to the Mrs. I'd be shifting my head through a transition and that would drive me utterly nuts... As for EQ, if I drop the bass response then I lose pretty much everything below 150hz, it just exponentially drops away with the settings I have available, I don't have a small enough incremental change at my disposal. It must be said that in this room the bass peak at 50 doesn't sound as bad as the graph looks; it doesn't appear to run away at higher volumes, maybe the speakers are nearing their lower frequency limits, providing a 'mechanical' solution to at least part of the problem. This is the only reason I haven't taken a little bass out of the mix. A second reason I think it doesn't sound too bad is that the peak isn't due to a sub that may have some phase discrepancy to the mains, so you don't get that 'voice boom' effect, it behaves itself rather well for a 12db boost - the decay time and nulls are my main concern. I'm thinking of becoming the test dummy for this Fletcher product - I'll price some up next week and we'll see if its worth it or not. Considering that the specific density of this Mineral Wool Slab is the same as OC 705 (96kg/m3), but made from a slightly different material, it has the potential to perform brilliantly. Time will tell...
Lawsy Posted September 15, 2012 Author Posted September 15, 2012 Thanks Paul, your input is definitely appreciated. A lot of the theory I already know, it is the specific verification of trap types with empirical evidence, or lack thereof, that causes the confusion. And just to provide a little fill in for some of my idea's, I stuck the sound meter in the corner and there is a huge amount of low frequency energy down there, and not much of anything else. My thought process was to have 100mm of high density material with 100mm of air behind it (as depicted in the image). But now that you mention it, your idea for a more targeted trap does sound like something I should very strongly consider. Do you have any suggestions on a tuned trap design to target a centre frequency of say, 80hz? Assuming it has a little effect an octave below and above, it might help with the 50hz node and also with some of the nulls up to 160. I was reading about an absorber with specific hole sizes for a specific volume of sealed enclosure with some compressed fb inside. Was this the sort of trap you were thinking of?
Paul Spencer Posted September 15, 2012 Posted September 15, 2012 The broadband trap that is only 200mm out from the wall is not going to do the job. The make matters worse, you then have an uphill battle with your wife! Making an invisible trap that works earns more points than a visible one that doesn't. It then becomes hard to justify further such projects! Have a look around the Gear Slutz forum and you might find a tuned trap that works in the range you want, otherwise you will have to tweak and measure until you hit your target. Regarding tuned traps there are various designs, some are Helmholz resonators, others are a sealed box with a thin front membrane, others have holes.
Lawsy Posted September 20, 2012 Author Posted September 20, 2012 The broadband trap that is only 200mm out from the wall is not going to do the job. The make matters worse, you then have an uphill battle with your wife! Making an invisible trap that works earns more points than a visible one that doesn't. It then becomes hard to justify further such projects! Have a look around the Gear Slutz forum and you might find a tuned trap that works in the range you want, otherwise you will have to tweak and measure until you hit your target. Regarding tuned traps there are various designs, some are Helmholz resonators, others are a sealed box with a thin front membrane, others have holes. It appears that, after reading during basically every spare moment I've had (wow, sounds like I have no life.... damn) for the last few days, that its near on impossible to guarantee any result, of any kind, with any-thing - what a pain in the ass this is! It seems that without spending a fairly serious amount of cash, and that without being able to completely treat a room, lackluster results, or results of little significance in the bass region, are to be expected. I just don't believe I'll be able to spend enough, or cover enough surfaces with the wife's approval, to achieve my results. Enough of my contemptible whining... So I think I've nutted out a few things, and if anyone can shed some light on this with your own experience, correct or comment on my perception of any of the below, it might turn my currently not so optimistic viewpoint. As an engineer I'm usually fairly good at interpreting results, but I'm afraid my brain just can't decipher anything further after reading what I believe to be the majority of 90 odd pages on Gearslutz, as well as a baker's dozen of papers and engineering entries on the topic of bass treatment... Helmholz, Helmholz arrays or perforated resonators Pro's Good effect on response at targeted frequency, at any bass frequency Relatively small Doesn't effect HF Cheap (ish?) - construction semi-complex if you can't work with wood Con's Hit and miss results Narrow band, if you get it to work Doesn't appear to help decay much, if at all, for most who've actually got one to work effectively Seems to push energy from the tuned frequency to another (though I can't remember where I read this, could be completely wrong) Placement sensitive (possibly less though than limp mass?) Limp mass membrane (sealed design; open seem irrelevant) Pro's Wider response than above Can absorb quite low frequencies Smallish size for its performance Keeps HF absorption to a minimum as well Reduces decay fairly well at target frequencies Cons Also very difficult to get right it seems - Even with "Tim's guide" only a couple of successful attempts have been made on GS forums Still fairly limited frequency range Also appear to be not only placement, but angle sensitive Material selection appears to be far too critical for limp mass Enhanced porous absorber (modified VPR, 2" PU foam, 1mm steel, 2" PU foam) Pro's G.E from GS seems to have killed it with this - the best showing of any bass trap Widest absorption range, largest absorption Easiest construction if you're really lazy - straddle corner with absorber, lay 1mm steel over it, place another layer of absorber, walk away.... Cheap ish if you know someone in the steel industry Gap in corner can be filled with light media, improving results even further If material availability permits, highest absorption potential per corner (a crude measurement, I am sure) Con's Unfortunately, no one else seems to be able to attain the same results Can't get the specific absorption material here as far as I can tell (some PU based foam it seems) Expensive if you don't know anyone in the steel industry Working with 1mm steel can be a hazard - I've worked with possible a thousand sheets of it, with gloves on - much blood was spilled when gloves weren't used... It does appear that, compared with the smaller absorbers, that it needs to be wall to ceiling to be most effective Furthermore, that makes it hard to conceal as it needs to remain wide as well as tall What does appear to be constant, across the entire world wide tin-ta-net, is that absorption will always work - if only a little, or even by nearly immeasurable amounts, it will usually do something... The more you can stuff in a room without covering every surface (without killing the room), the better - I'm being drawn more and more to my originally idea, but with a thinner high density layer, and then stuffing the remaining cavity with something lighter, and then planting on top of that a corner straddling (as I will be able to breach the top of the lounge) full trap. Then I'll attack the back and front walls, and lastly the corner under the stairs; I'll just pack the living daylights out of that space with something, anything, and forget about it... Bloody acoustics, what the hell is with this stuff that we care about it so much? The fact that I care pisses me off, the fact that I can't do all that I want to, does so even further... And yet I cannot stop... What a stupid addiction - I can't imagine what a cocaine habit would be like...
Lawsy Posted September 20, 2012 Author Posted September 20, 2012 Oh, and by the way Paul, your effort with horn loaded drivers is, quite simply, awesome. Fantastic work mate... Looking forward to you completing your dual 12" horn loaded bass enclosure too.
Paul Spencer Posted September 20, 2012 Posted September 20, 2012 Lawsy, you are beyond help! Bahahaha The bass horn is built and installed. It has been for some time. I'm doing some work on the room and the sub isn't running (amp install issue) but the horn itself is done.
svenr Posted September 20, 2012 Posted September 20, 2012 Hi lawsy, from what you wrote I see Helmholtz absorbers under the stairs as the best way forward. The design is straightforward, and fine tuning is easy if you include an adjustable port. But first things first - I'd recommend you identify the modes that bother in more detail. The FR plot shown has a number of effects in them that are not all modes. Do a couple of more measurements. Firstly place the mic flush against the rear wall above the couch. Analyse the impulse response using say a 600ms waterfall plot or better a burst decay plot. Try to normalise the decay plots to the IR at t=0ms. Any frequency of delayed decay will show up very clearly. These are the bad boys. Then measure the same responses at the face of your stair cavity to see which ones actually exist in this area and can be treated there. Hopefully the 50Hz mode shows up there sufficiently strong. Another thread "How I am killing a 36Hz room mode" by berun discusses a Helmholtz design that worked well in the 50Hz range. This can be your guide for your design. Porous absorbers would need to be very deep to be effective at 50Hz, and membrane absorbers are hard to design due to the lack of technical data on available sheet material. Tuning them is a pain in the neck as one has to vary the size of the panel - new prototype at each design stage. Adding mass to lower the resonance also increases the mechanical impedance of the absorber, i.e. lowers the absorption efficiency. Don't go there...
Lawsy Posted September 21, 2012 Author Posted September 21, 2012 Thanks Sven, appreciate that - you have lost me somewhat regarding the methodology of measurement. The part of REW I don't fully understand yet is the Impulse response and its filtered counterpart. I've read the help, and I've still got a blank look on my face. I've no idea what I'm meant to get from this graph.... Help?
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