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Post by indranil on Jan 26, 2018 17:58:03 GMT
Hello, I am currently using a combination of velocity type bass traps (wall corners) and broadband absorbers (1st Reflection Points) in my relatively small music room (12ftx15ft) and got into some issues after adding a plastic membrane to bass traps. Here is the exact description of the issue.
All bass traps/absorbers in my room are made of Knauf insulation material and without any membrane it made the room sound too dull and heavy. So I went through a lot of articles on internet regarding this subject (how to balance absorption and reflection/diffusion in a small room) and decided to bond a plastic membrane to all bass traps such that they reflect high/mid frequencies and absorb more bass.
I have two corner bass traps at the two corners of the front wall (behind speakers), two bass traps at the center of two side walls (where wall meets the ceiling) and two bass traps at the center of front wall-ceiling corner in the room. I took a 6mil plastic sheet and glued it to the Knauf insulation to complete this enhancement. After the installation, the sound opened up really well but the voice became a tad hazy/out of focus. The overall sound appears to be highly diffusede lacking the punch which was present before. Not sure what caused it but I have an observation to share. The plastic sheets that are bonded to the insulation had uneven surfaces because of multiple folds and I am wondering if this is causing the diffusion effect if what I am hearing is really a diffused sound.
I would love to hear your comments/feedback on this issue before I start making further changes like installing a relatively smooth/hard surface i.e. Kraft paper or a very thin ply on the insulation.
Thanks.
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Post by rock on Jan 26, 2018 20:32:48 GMT
I'm not sure what might cause the diffusion but to one thing that stands out to me is that you mention 6mil plastic. IIRC, Ethan's recommendation for plastic is "regular garbage bag thickness" and I'm pretty sure that's less than 1 mil. The thinnest plastic sheeting I have found at Home Depot that is 0.7 mil.
I don't know if that's your problem but it's my best guess that it at least might be.
The center front wall/ceiling is not considered a reflection point but since it's in front of you and angled toward you, there is a chance it could have something to do with what you're hearing too but I'd experiment with thinner plastic first.
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Post by indranil on Jan 26, 2018 20:47:01 GMT
I'm not sure what might cause the diffusion but to one thing that stands out to me is that you mention 6mil plastic. IIRC, Ethan's recommendation for plastic is "regular garbage bag thickness" and I'm pretty sure that's less than 1 mil. The thinnest plastic sheeting I have found at Home Depot that is 0.7 mil. I don't know if that's your problem but it's my best guess that it at least might be. The center front wall/ceiling is not considered a reflection point but since it's in front of you and angled toward you, there is a chance it could have something to do with what you're hearing too but I'd experiment with thinner plastic first. rock- Thanks for your feedback. I wasn't aware of the thicness recommendation of Ethan and i bought thicker plastic to make sure that it is not acoustically transparent which doesn't absorb high/mid frequencies. I can try 0.7 mil plastic on the other side of the insulation but if it doesn't work i would not be able to use it in any other way... Thanks.
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Post by indranil on Jan 26, 2018 23:48:37 GMT
I'm not sure what might cause the diffusion but to one thing that stands out to me is that you mention 6mil plastic. IIRC, Ethan's recommendation for plastic is "regular garbage bag thickness" and I'm pretty sure that's less than 1 mil. The thinnest plastic sheeting I have found at Home Depot that is 0.7 mil. I don't know if that's your problem but it's my best guess that it at least might be. The center front wall/ceiling is not considered a reflection point but since it's in front of you and angled toward you, there is a chance it could have something to do with what you're hearing too but I'd experiment with thinner plastic first. rock- Thanks for your feedback. I wasn't aware of the thicness recommendation of Ethan and i bought thicker plastic to make sure that it is not acoustically transparent which doesn't absorb high/mid frequencies. I can try 0.7 mil plastic on the other side of the insulation but if it doesn't work i would not be able to use it in any other way... Thanks. Another question for the folks here regarding trying 0.7mil plastic. Instead of using glue could i just use tape to attach the plastic to the insulation such that i can remove it later if required? Would it not have the benefit of additional low frequency absorption? Also could some explain the role of thickness in this entire process? Thanks.
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Post by rock on Jan 27, 2018 0:14:20 GMT
Spray glue like Scotch 77 but not to heavy, there are similar gules with light or medium tack, I believe that's enough. You may be able to remove the 6 mil by "sawing" it off with and abrasive cord or wire. You'll probably need a helper.
The plastic sheeting improves LF performance at the expense of reflecting mids and highs.
Ethan has a graphic that shows the relationship of the wave frequency to the thickness. The thicker the lower the freqs. Spacing off the wall is also explained in this topic.
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Post by indranil on Jan 27, 2018 18:58:06 GMT
Spray glue like Scotch 77 but not to heavy, there are similar gules with light or medium tack, I believe that's enough. You may be able to remove the 6 mil by "sawing" it off with and abrasive cord or wire. You'll probably need a helper. The plastic sheeting improves LF performance at the expense of reflecting mids and highs. Ethan has a graphic that shows the relationship of the wave frequency to the thickness. The thicker the lower the freqs. Spacing off the wall is also explained in this topic. rock- Thanks for your reply. I am not too good with tools and not certainly familiar with abrasive tape and the saw off technique that you suggested. But i am willing to learn and remove the 6 mil plastic. Could you tell me the tools that i have to buy for this work and explain the process if possible? I would appreciate it. Regarding the thickness i think your comment was for the thickness of the insulation and not for the thichkness of the plastic. Correct? If true do you know why a thinner plastic works better for this application? When i opened up the triangular shaped corner bass traps i found three layers of insulation. The width of each piece decreases as you move from outside to inside to fit the triangular area. These individual pieces are not bonded to each other. Is it a standard design for corner bass trap or do i have to bond these individual pieces to increase the thickness for better low end absorption? Comnents/suggestion from you and other members are always appreciated. Thanks
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Post by rock on Jan 28, 2018 2:12:03 GMT
I never had to remove plastic from insulation so I'm just spitballin', not rocket science but I imagine, if I had to do it, I'd lay the panel on a table, get a friend and get something like a piece of wire and each of us (me and my friend) would pull the wire taught and try to slice the plastic off the insulation like slicing a thin slice of cheese.
Sorry, I do no know why or how the plastic works.
I don't know why the pieces get smaller but I don't think it's important, probably they are that size because that's what fit. (I'm confused, who made those "triangular shaped corner traps" and why did you take them apart?)
Right, the layers of insulation should NOT be glued together.
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Post by indranil on Jan 28, 2018 5:57:30 GMT
rock- Thanks for taking time to elaborate on the unglue process and it makes sense. In fact I have several unused bass trap hanging wires which could be used to peel off the plastic as you explained. I bought these bass traps long time back when I had very little idea on room acoustics and on bass traps. These traps including the corner traps did not have any membrane inside the frame and as I explained it made the room sound dull and heavy. So I read up several articles on internet regarding velocity bass traps with membrane which reflect high/mid frequency to preserve the liveliness of a room and decided to go the surgery, which is why I opened up all these bass traps. Ideally I would have bought or built these traps with a membrane but I was not that lucky. However I am excited to find that my music room has got back its liveliness after adding the plastics. So the concept works and I have to just get it right so that the music doesn't become so laid back or lacks the natural punch. I am uploading a couple of pictures of my music room to show how I have currently placed my bass traps. Thanks.
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Post by Hexspa on Jan 28, 2018 6:51:51 GMT
rock - Thanks for taking time to elaborate on the unglue process and it makes sense. In fact I have several unused bass trap hanging wires which could be used to peel off the plastic as you explained. I bought these bass traps long time back when I had very little idea on room acoustics and on bass traps. These traps including the corner traps did not have any membrane inside the frame and as I explained it made the room sound dull and heavy. So I read up several articles on internet regarding velocity bass traps with membrane which reflect high/mid frequency to preserve the liveliness of a room and decided to go the surgery, which is why I opened up all these bass traps. Ideally I would have bought or built these traps with a membrane but I was not that lucky. However I am excited to find that my music room has got back its liveliness after adding the plastics. So the concept works and I have to just get it right so that the music doesn't become so laid back or lacks the natural punch. I am uploading a couple of pictures of my music room to show how I have currently placed my bass traps. Thanks. Looks cool. Remember that if you widen your RFZ coverage then you contribute to a larger "sweet spot".
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Post by Ethan Winer on Mar 1, 2018 20:03:05 GMT
I got your email and I'm glad to add my two cents. I never compared (by measuring trap performance) 1 mil plastic to 6 mil plastic, but my guess is either is fine. I certainly wouldn't bother to replace what you have. I've used thin paper, thick paper, thin cardboard, and even thick corrugated cardboard used for shipping boxes, and they all seemed more similar than different. I can't see how anything you do that's out of the way of reflections would change anything other than the overall ambience in the room. So adding plastic might reduce some beneficial mid/high frequency absorption, but it can't add "diffusion" of any type because those panels are out of the way of the speaker. I do see some other problems: You have two RFZ Panels on each side wall. They should be adjacent, and just forward of halfway between you and the front speakers. It looks like the front panels are too far forward, and the rear panels are too far back, so you have bare wall where you need absorption. But I can't really tell much from the photos other than the panels need to be adjacent and located front-back properly. I also disagree with having surround speakers so high up on the walls. Yes, I know a lot of people recommend that, but it's wrong. From my Audio Expert book:
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Post by indranil on Mar 2, 2018 4:19:01 GMT
Ethan- Thanks for your feedback. The broadband traps along two side walls are placed at the 1st reflection points (identified using the mirror technique). I have a couple of 2ft traps which could be used to replace the 1st set of 1ft traps near the speakers. In this arrangement there would be no bare wall between the two traps. I would test this change and comment. The term diffusion might not be exactly accurate to describe what I am currently hearing. The sound now is much more laid-back and less sharp than before when the bass traps at corners and on side wall-ceiling joints didn't have any plastic membrane. A little more sharpness and speed would make it perfect....
Although I have a 5.1 channel system in the room but I mostly listen to 2 channel music. That is why I am not too worried about surround speakers. I would agree with you that surround speakers need to be at the ear-height and perhaps be placed at the same distance as your front speakers are from the listening spot. However I don't have enough space in the room to go for such set up. In any case I an not a big HT user so this compromise is ok for me.
Once again thanks for going through my post and proving some helpful recommendations.
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Post by indranil on Jun 3, 2018 2:49:38 GMT
Hello, I experimented with changing the position of bass traps and it worked to resolve the out-of-focus issue. However I uncovered another issue which might be relatively tricky to handle. I have two large openings in that room, one is the half wall behind the couch and the other one is on the left side wall just next to the side of the couch. These openings lead to a similar sized room and a kitchen. While experimenting I found that the sound consistently gets better and clear when I put bass traps in front of those openings. It could be the loss of sound through these openings or reflections coming back from the rooms causing the issue. I don't know how openings like what I have in the room affect sound but it is clear to me that closing them improves the sound. Do you guys have any recommendation on how to seal these openings with portable items or bass traps or something such that I can continue to use this room as my music room? From a room acoustics perspective is it absolutely necessary to completely seal the openings? I ask this question because any portable items like standing bass trap will be able to cut down reflection but will not bed able to prevent sound loss. I appreciate your feedback on this issue. Thanks.
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Post by Michael Lawrence on Jun 3, 2018 13:44:16 GMT
Indranil- This is a situation where we can sit here and postulate all day, but what you actually should do is take some measurements. Room EQ Wizard is free, and Ethan's book explains how to use it and what to look for. You can take measurements from the listening position with and without the treatment in place and we can see the data and help you make a decision as to how to move forward.
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Post by rock on Jun 3, 2018 16:53:49 GMT
Yes, I absolutely agree. Following Michael's suggestions will give you info to make a decision.
But here's the postulation anyway : ) FWIW: In general it can be considered that an opening into another room is analogous to absorbing the sound at the same point, the idea being the sound is not reflected back into the room just as if there were an absorber there. That's the theory but in practice there may be reflections and resonances in the other space and that sound may bounce back into your listening space if there is no absorber there. If you do put an absorber in the opening, it does not need to be sealed or even fit tightly; the relatively small fraction of sound that goes around into the other space will bounce around and most that does come back to the opening will be absorbed at the rear of the absorber. Also, your absorber does not necessarily need to be flush in the opening; it could be a little in front or behind. Let your measurements guide you.
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Post by indranil on Jun 3, 2018 19:57:16 GMT
@michael/Rock, Thanks for your feedback. I have been taking REW measurements from time to time as I was moving the bass traps from one location to another. Here is a set of REW measurements with nothing in front of these two openings. Although I have made some changes in the bass trap arrangement since then but this will give you an idea of the FR and Waterfall graphs for each front speakers individually and both engaged. As you can see the left speaker measurement has several dips at 55 hz, 84 hz, 95hz, 225hz, 270 hz, etc compared to right speaker. The combined graph looks better on the paper but even with that improvement the sound appears to be relatively harsh and feels somewhat compressed. I don't have the measurements with bass traps in front of the openings but I hear more natural sound when traps are placed in front of the openings. I don't think bass traps would address issues at the very low frequency range i.e 55 hz-100 hz but I will try to get the REW measurement and share it with you shortly. My only concern is that if the resolution turns out to be either close these openings permREW-Measurements.zip (750.67 KB)anently or move to a different location then it might not be feasible immediately. But as always I would love to hear your recommendations. Thanks. P.S: I could not attach the entire REW project file because of the size limitation. In the attached JPG file the colour green represents Left speaker, pink is the right speaker and yellow is for both speakers together. These graphs don't have any smoothing. Attachments:REW-Measurements.zip (750.67 KB)
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