|
Post by ultrasonic on May 24, 2019 7:11:10 GMT
I'll ask the question I logged in to first, and then go on to Hexspa's points which I'd be very interested in seeing discussed/debated. I haven't stuck any paper onto RFZ panels just yet as I'm making the frames. I've attached a photo of a work in progress to show I'm not just talk . (I appreciate that having the speaker that close to the room boundary is not ideal BTW but other furniture in my lounge forces an asymmetric layout. I'm hoping one benefit of the RFZ panels will be to reduce the downsides of this.) My question: For any panels that do have a foil/paper layer, is there likely any benefit to making the frames slightly deeper so that when the covering fabric is wrapped over it there will be a small air gap to the membrane rather than it touching? Or indeed would this actively be something to be avoided? Attachments:
|
|
|
Post by ultrasonic on May 24, 2019 8:12:55 GMT
1. FRK in front of panels is known to work - at least at 125Hz. Absorption is incident-dependent which means it works best when parallel to a sound source. The sound isn't coming from behind the panel but reflected against any surface behind it or diminished through the absorbent. Therefore, any FRK behind a panel will be less effective at any frequency. Surely at low bass frequencies there will still be significant sound incident on the rear of the panel though, since (unfortunately) at these frequencies the panels are relatively poor absorbers? 2. The RFZ is for mid-to-high targeting. By targeting bass, you're mixing uses. There's almost always a downside to any compromise - even if we don't know what it is yet. Agreed, which is why I specifically asked about any compromise above. If there isn't one though then the appeal is a little extra low frequency control without taking up any more space. (I have a fairly small room BTW, at roughly 4.7 m long by 3.5 m wide.) 3. The gap is being partially negated. The gapping of panels relies on that gap being unobstructed. Again, since RFZ panels are targeting a certain range, we can expect FRK of any kind to interfere with that process across the gap. This point I'm really interested in. I guess this comes down to how the frequency content not absorbed passing through the panel interacts with the paper/foil layer. That which is transmitted would leave the situation as before but if there was significant reflection (or other effect) then we could be changing things for the worse. 4. RFZ has little to do with bass. Even in headphones, stereo information below 100Hz becomes less meaningful. You will gain no stereo clarity by using FRK but in fact may harm it. Agreed. My own interest in adding a membrane to the back of an RFZ panel would have nothing to do with further improving imaging, just for added low frequency control as from bass traps. 5. Beginner status. It's a classic pitfall to over-think your first two years of any endeavor. I, and others experienced in a variety of areas, usually recommend novices to follow proven methods before attempting innovation. Rear-facing FRK is experimental, generally unused, mostly unproven, and totally unnecessary for treating a room in probably nearly all cases. Agreed, which is specifically why I asked about this here rather than just doing it . Note that it was seeing Ethan recommend this in the old GS post I quoted that made me take the idea seriously. FWIW, although I am a total novice in the area of room treatment I do actually have some background in wider acoustics. I worked at the Institute of Sound and Vibration Research at the University of Southampton (UK) for several years, and I have a PhD for research in medical/underwater ultrasonics. I've been out of the field for about 10 years though, so I'm much rustier than I'd like! I am also very well aware of the mistakes that can be made when someone strays out of their area of expertise, which again is why I'm posting to get the opinions of real experts here. I do very much appreciate you taking the time to post on this subject . With my research background in mind I'll say that I was really hoping their might be some good published data on this subject, but sadly it appears there isn't. There could be a paper in it for anyone with the time/resources I'd have thought... Finally I'll just add that I do realise the potential benefits in terms of low frequency control are minimal, and so if there are real potential downsides then this is probably not worth the risk.
|
|
|
Post by Hexspa on May 24, 2019 11:12:23 GMT
I'll ask the question I logged in to first, and then go on to Hexspa's points which I'd be very interested in seeing discussed/debated. I haven't stuck any paper onto RFZ panels just yet as I'm making the frames. I've attached a photo of a work in progress to show I'm not just talk . (I appreciate that having the speaker that close to the room boundary is not ideal BTW but other furniture in my lounge forces an asymmetric layout. I'm hoping one benefit of the RFZ panels will be to reduce the downsides of this.) My question: For any panels that do have a foil/paper layer, is there likely any benefit to making the frames slightly deeper so that when the covering fabric is wrapped over it there will be a small air gap to the membrane rather than it touching? Or indeed would this actively be something to be avoided? The fabric can be gapped so long as it's acoustically transparent i.e. you can breathe through it. Speakers close to a boundary is not inherently bad - it's a matter of managing Speaker Boundary Interference. Indeed, you typically want your speakers 'all the way in' or 'all the way out'. The former so that the interference is high enough as to be manageable by absorption and the latter so that the interference is below the speaker's output - as in the case of 5" monitors.
|
|
|
Post by Hexspa on May 24, 2019 11:34:44 GMT
1. Depends how thick you plan to go. Care to share the absorption coefficients of your chosen material and thickness with and without FRK? The main point is that the FRK will receive less energy but still act as a boundary and that creates an unknown. Will it be somewhat like placing the panel flush with the wall and somewhat like a gap? Only tests will confirm this. Also, poor bass absorption relative to what? Ethan has shown that broadband absorption is most effective when used as 'wide coverage' i.e. lots of surface area and thickness and FRK. I'm also convinced that broadband absorption, when correctly used, is more than sufficient for virtually any reasonable home or small studio environment.
2. Again, since I haven't seen A/B tests, I can't tell you exactly what the downside might be - if any. I'm just saying that usually there is a compromise somewhere no matter what you do. You want your RFZ panels to absorb as much mid and high frequencies as possible and I don't see how FRK is going to help with that - bass effect notwithstanding. At about 1,418ft3, your room isn't tiny - indeed workable - and that gives you plenty of surface area to target modes with conventional FRK use.
3. There's going to be some reflection, of course. What bandwidth and to what degree, I don't know. One thing's for sure: it'll include the frequency band you're trying to fully absorb with RFZ panels. It reminds me of the principle of decoupling when doing sound isolating construction - mass, air gap, mass. You may be minimizing the opportunity for that air gap and that, in my estimation, will have some, possibly negative, effect. Again, it might be better but I do not know.
4. But RFZ is all about imaging and clarity. You have the whole room to use FRK panels. Typically, you'll have 6 or less RFZ panels whose only real job is to minimize early reflections in the mid-to-high range. My feeling is just let them do what they're supposed to and use all your other surface area for bass.
5. That's awesome. I've been playing guitar since I was 10 and just picked up keyboard a few months ago - I'm still a lackluster keyboardist. With a brain like yours, you can make a valuable contribution to this matter by testing and publishing your findings. However, if you just want to get on with the business of enjoying your room and getting a baseline experience whence to explore, you may want to take a preexisting road.
Let me be clear: I'm happy that you're inquiring. The field of small room acoustics, as I call it, should make large strides in the coming years with the mass adoption of home recording and more people with backgrounds like yours applying themselves toward understanding their acoustic issues. My main concern is twofold: that you find success with minimal headache, wasted effort, or counterproductive activities and, secondly, that others reading this thread understand that this is experimental, not widely and conclusively established, and quite possibly not the best way to start.
By all means, I want you to try it - it'll save me the hassle - but I caution against purely using reason without testing and publishing your hypothesis. I believe Ethan when he says that the bass response is better. What I want to know - but not so much as to try it myself - is how does 'reverse FRK' compare to 'forward FRK' and unfaced options. Who knows - maybe it really is the Holy Grail - but, until then, we need to be clear about what's actually going on regarding this matter.
The bottom line is that if you don't test, you won't know. Also consider asking yourself, "What do I want - a clear path to victory or an exploration in the weeds of acoustics which may reveal fruit or thorns?" I respect your decision regardless.
|
|
|
Post by ultrasonic on May 24, 2019 11:50:53 GMT
The fabric can be gapped so long as it's acoustically transparent i.e. you can breathe through it. My question was actually whether it might be better with a gap, thinking in terms of what effect having the fabric touching the foil might have (and noting that Ethan's density report tests were with the panels uncovered). Obviously the point of bonding the foil to the absorber is so the membrane motion is damped but I was just wondering if fabric touching from the front might change things. I'd only deliberately add a gap if I thought it might be beneficial though, and of course only for panels that would be mounted with the front face vertical. Speakers close to a boundary is not inherently bad - it's a matter of managing Speaker Boundary Interference. Indeed, you typically want your speakers 'all the way in' or 'all the way out'. The former so that the interference is high enough as to be manageable by absorption and the latter so that the interference is below the speaker's output - as in the case of 5" monitors. Of relevance to this I'll just mention that my speakers cross-over to a sub at 120 Hz (using 48 dB per Octave Linkwitz-Riley crossover filters in a miniDSP 2x4 HD). The tweeters are about 1 m from the front wall, with their current positions having been optimised as best I could based on measurements using REW. Whether different positions might be better with RFZ panels in place is a point that had occurred to me but which poses its own challenges. In particular as I don't want my walls and particularly ceiling having extra holes from having to move panels about!
|
|
|
Post by Hexspa on May 24, 2019 12:02:11 GMT
To the first point, I doubt you'll see a difference in your SPL or decay graphs either way. I have fabric touching my rigid panels without FRK and gapped fabric on half-FRK'd super chunks and I never noticed anything special. Regarding positioning, I think as long as you're greater than 1/2 wavelength from the crossover, you're far enough. 1m is definitely not 1/2 wavelength distance at 120Hz but we work with what we have to the best of our ability.
The reason for this is that, at 1/2 wavelength, the reflection from the boundary constructively interferes at the listening position with the direct signal. If the distance is greater than 1/2 wavelength of the crossover, this won't occur at or above where you expect the response to be flat. There will be some other interference but it will be due to 3/4 destructive and 1x constructive which is weaker than 1/2 constructive and 1/4 destructive hence easier to manage.
In either case, 120Hz is right about where FRK starts to make a positive difference. You can certainly place FRK forward panels at these boundary points - SBIR points, not RFZ - to mitigate the effect.
|
|
|
Post by ultrasonic on May 24, 2019 12:24:03 GMT
1. Depends how thick you plan to go. Care to share the absorption coefficients of your chosen material and thickness with and without FRK? The main point is that the FRK will receive less energy but still act as a boundary and that creates an unknown. Will it be somewhat like placing the panel flush with the wall and somewhat like a gap? Only tests will confirm this. Also, poor bass absorption relative to what? Ethan has shown that broadband absorption is most effective when used as 'wide coverage' i.e. lots of surface area and thickness and FRK. I'm also convinced that broadband absorption, when correctly used, is more than sufficient for virtually any reasonable home or small studio environment. I'm making the panels from 100 mm (~ 4") thick Rockwool RW3 and I intend to space the RFZ panels about 4" from the walls/ceiling. I believe RW3 has a density of 60 kg/m^3. The only absorption data I've seen is in the following link but the test conditions are unclear, and I'd guess the numbers are for 2" thickness: www.studiospares.com/Studio-Gear/Sound-Insulation/Rockwool-RW3_401982.htm#tabs-rev|rtabs1The point I was making about poor absorption was simply that at low frequencies a single pass through the panel doesn't absorb everything and so reflected sound coming back through the panel is very much a relevant factor. There's also scope for sound to bounce around the edges of a gapped panel to be incident from the rear. So I don't think possible benefits should be dismissed based on which way a panel is facing. This is something that will be easy to test for bass-trap panels, and I will do so when get chance. 2. Again, since I haven't seen A/B tests, I can't tell you exactly what the downside might be - if any. I'm just saying that usually there is a compromise somewhere no matter what you do. You want your RFZ panels to absorb as much mid and high frequencies as possible and I don't see how FRK is going to help with that - bass effect notwithstanding. At about 1,418ft3, your room isn't tiny - indeed workable - and that gives you plenty of surface area to target modes with conventional FRK use. A very real factor for me is that my room is a general living space not a room solely dedicated to sound reproduction. I'm actually very much at the stage of trying the panels I'm making to see if I decide it's worth the aesthetic trade-offs to keep them long term. The more panels used, the greater this trade-off becomes so maximising the effectiveness of each panel very much appeals. As does saving money . Equally if there were no downside then it would seem silly not to do it anyway. I'm very much taking your concerns seriously though and right now am minded not to stick anything to the back of my RFZ panels. I'll be interested to see if Ethan has any thoughts or experiences re. your concerns that he has time to share. My main concern is twofold: that you find success with minimal headache, wasted effort, or counterproductive activities and, secondly, that others reading this thread understand that this is experimental, not widely and conclusively established, and quite possibly not the best way to start. By all means, I want you to try it - it'll save me the hassle - but I caution against purely using reason without testing and publishing your hypothesis. I believe Ethan when he says that the bass response is better. What I want to know - but not so much as to try it myself - is how does 'reverse FRK' compare to 'forward FRK' and unfaced options. Who knows - maybe it really is the Holy Grail - but, until then, we need to be clear about what's actually going on regarding this matter. The bottom line is that if you don't test, you won't know. Also consider asking yourself, "What do I want - a clear path to victory or an exploration in the weeds of acoustics which may reveal fruit or thorns?" I respect your decision regardless. My personal goal is to make some panels to make a fair test of whether they're worth sticking with long term. While I'm doing this it is logical to find out what I can easily to inform their design for optimum performance. Once I've made them I don't intend to keep tinkering with them though, and doing my own tests of having backing layers on RFZ panels or not is not really a route I can easily go down as I don't want to end up trying to remove layers I've added. Hence trying to find out first what might be best . It would be very interesting to know whether any commercial RFZ panels have a membrane on the back. I've seen it said that Real Traps HF panels do but I've no idea if this is true or not (and I appreciate that Ethan may well not want to comment directly on this).
|
|
|
Post by ultrasonic on May 24, 2019 12:35:10 GMT
In either case, 120Hz is right about where FRK starts to make a positive difference. You can certainly place FRK forward panels at these boundary points - SBIR points, not RFZ - to mitigate the effect.
The complication I have there is that the points I 'd want the panels would be where my AV rack and TV are. I've attached my approximate room layout FWIW (colours not representative!) The corner behind the door has a lower, angled ceiling that runs under my stairs. The walls are all solid and consequently (I assume) I have some stonking room modes which I am using some EQ to manage (and particularly for the 36 Hz mode expect to continue to long term). Attachments:
|
|
|
Post by Hexspa on May 25, 2019 5:48:34 GMT
Let me just address one thing at a time because this is getting to be too much. RW3, according to the Bob Gold's resource, may perform better than your posted stats. Plus, if you look at the IIG metrics, simply doubling the thickness about triples the absorption at 125Hz. www.bobgolds.com/AbsorptionCoefficients.htm Although we're not sure if this is flush mounted or with a gap. If flush, then the gap will help even more. Yes, some bass is going to bass through and that's why you may or may not want FRK on the back. Since you're reducing its energy, wouldn't it be possible that it just bounces off the paper more easily rather than passing through? I don't know exactly but it's worth considering. From what I've read, any sound coming from behind the panel - or even the sides or at an angle - doesn't get absorbed as much as if it's straight-on. Incident-dependent: matters from which direction sound hits it. But again, I'm not going to argue for or against this. My whole thing was, for the last time, to highlight the lack of comparative data and, certainly, I'm open to this being a worthwhile option for certain circumstances.
|
|
|
Post by ultrasonic on May 25, 2019 9:20:26 GMT
Sorry you sound a bit exasperated there Hexspa. Don't worry, I've had no doubt what your personal opinion is from the start. I've just been responding to the points you raised, and I had hoped this would be a subject that would attract wider interest and discussion. Thanks for the absorption data link. I should apologise I was actually already aware of this but I forgot the RW3 data was in it when you asked above. I was happy it was a sensible looking material, so when the panels I have popped up cheaply on ebay locally to me I bought them. I stuck the FRK-style layer on the 6 Rockwool panels I'll be using for bass absorption yesterday. As it sounds like you may be interested I'll probably do some quick tests with the layer on the front vs the back for you, although in proper use they will have the FRK facing forwards for higher frequency reflection. My prediction is that at low frequencies it won't make any difference but I'll see. Six panels isn't a large number of course but it could at least show if there is a big effect. I have a busy day of building wooden frames ahead of me now .
|
|
|
Post by ultrasonic on May 25, 2019 10:52:03 GMT
First FRK-style panel done bar the fabric covering. The wooden frame is thicker than the freestanding one I showed before, but this timber is about half the price. Attachments:
|
|
|
Post by Hexspa on May 25, 2019 12:25:56 GMT
I'm not exasperated but was starting my day - piano first, acoustics later. Again I'm going to disagree about something: aesthetics. On the contrary, acoustic treatment makes the place more aesthetic. We tend to get hung up on a room's visual appeal but few realize how much of a qualitative improvement acoustic treatment creates. You can go so far as to hide the panels behind a false wall - probably wouldn't even cost that much. Even raw insulation, in my opinion, would be worth the visually aesthetic trade off of not living in a treated room - really, I can never go back. This is my point of contention right here - that you want to optimize the treatment and save money but not perform comparative testing. These two, in my opinion, are conflicting motives. The thing is this: you can't know what's best until you install it and test it; acoustics are too complex. There are premier acoustic designers but I assume that even they move forward in a general direction and are surprised when the final measurement is taken. I know it's not for everybody but I spent at least three months setting up my current room and I learned a lot. Mainly what I learned is that if you experiment, measure, and have patience then you're going to optimize your room to a high degree - and it's worth it. The only thing is that these are your first six panels and that's why I advise you, or anyone in your position, to keep it along the well-trodden path, enjoy the results and forget the last 20% of the details. So, my view is that it's a spectrum between keeping it simple and forget the details or going all out and get science-y with it. Just know where you stand and commit because I think getting semi-science-y but also forgetting the details is just conflicted.
The panel looks good, by the way. I think a semi-thick fabric with the hem folded behind would look awesome and you could cover the thin edges with some black mesh. It would truly come as a surprise to me if you built these, installed them, came to appreciate the difference they make only to get rid of them.
|
|
|
Post by Hexspa on May 25, 2019 12:37:59 GMT
In either case, 120Hz is right about where FRK starts to make a positive difference. You can certainly place FRK forward panels at these boundary points - SBIR points, not RFZ - to mitigate the effect.
The complication I have there is that the points I 'd want the panels would be where my AV rack and TV are. I've attached my approximate room layout FWIW (colours not representative!) The corner behind the door has a lower, angled ceiling that runs under my stairs. The walls are all solid and consequently (I assume) I have some stonking room modes which I am using some EQ to manage (and particularly for the 36 Hz mode expect to continue to long term). Naturally, I have to remind you that it's better to center your listening couch laterally but you probably already know that. It's also best to keep the room from your ears forward as symmetrical as possible. Again, do what you can.
You can put SBIR panels anywhere near the speakers, if possible. Measuring will confirm ideal placement.
36Hz room modes don't matter as much as what's above 63Hz or so. My room has a 41Hz mode but it hardly matters. If you can meet the targets even from 120Hz and up you'll be doing well but above 63Hz is the ideal focus.
To piggy back off what I said before: keep the panels, sell the extra couch and make more panels.
|
|
|
Post by ultrasonic on May 25, 2019 12:53:18 GMT
Again I'm going to disagree about something: aesthetics. On the contrary, acoustic treatment makes the place more aesthetic. We tend to get hung up on a room's visual appeal but few realize how much of a qualitative improvement acoustic treatment creates. You can go so far as to hide the panels behind a false wall - probably wouldn't even cost that much. Aesthetics are a personal thing, but I know for absolute certain that if I currently had a female partner (I'm a single guy) the chances of trying what I am right now would be negligible! One first reflection point panel will sit bang in the middle of the patio doors going out to the garden for example. I've got another propped up in the corner now it's covered and it's looking pretty big/imposing to be honest, but I'll see how it goes. This is my point of contention right here - that you want to optimize the treatment and save money but not perform comparative testing. You've misunderstood me there. I will be doing a lot of testing to investigate the effects of the panels I make. What I'm not going to do is to start making and dismantling panels to test out sticking paper on the back of them or not. I have spent a considerable amount of time making measurements to optimise my current setup, including optimising the sub optimisation. On HiFi forums where I also post I suspect I'm very much thought of as a 'measurement guy'! The only thing is that these are your first six panels and that's why I advise you, or anyone in your position, to keep it along the well-trodden path, enjoy the results and forget the last 20% of the details. I'm actually making twelve panels: six for first reflection points and six as bass absorbers. Or at least that's what I have the materials for and what I was planning to. Now I've got a couple of them in my room I'm worried that 12 may be a bit much! The panel looks good, by the way. I think a semi-thick fabric with the hem folded behind would look awesome and you could cover the thin edges with some black mesh. It would truly come as a surprise to me if you built these, installed them, came to appreciate the difference they make only to get rid of them. Thanks. For initial testing I've gone for the cheapest acoustically transparent material I could find, which is some white voile (net curtain material). If I keep them long term I'll then invest in some decent looking fabric, and I'd be doing this as part of redecorating the whole room so I'd decide on colour(s) then. I'm certainly hoping at least some of them will stay long term given the effort involved, but I'm trying to keep an open mind.
|
|
|
Post by ultrasonic on May 25, 2019 13:04:28 GMT
36Hz room modes don't matter as much as what's above 63Hz or so. My room has a 41Hz mode but it hardly matters. If you can meet the targets even from 120Hz and up you'll be doing well but above 63Hz is the ideal focus. I take your point that higher frequency peaks are more problematic but the 36 Hz peak definitely is detrimentally noticeable on music. I'll post some comedy graphs of my untreated room response later. To piggy back off what I said before: keep the panels, sell the extra couch and make more panels. Ha! The couch you want me to sell hasn't actually been bought yet, but it will be . There are two battered old chairs in that location at the moment though. My room layout is very much dominated by getting a good AV layout, but I do need some seats for friends too . My choice to buy my current house was actually influenced by being able to get a layout as 'good' as I have. You'd be surprised how hard finding even this was (in a three bedroom house).
|
|