Bass traps and Risers. Does it work? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 19 Old 07-29-2018, 04:47 PM - Thread Starter
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Bass traps and Risers. Does it work?

I have read a lot about using your riser as a bass trap. I'm not so sure I believe what I have read or that it is truly worth it. I wouldn't be surprised if the science behind it is above most of our heads and it only works if you have a PhD level understanding of it as well. So I though I would give it a try. I setup an infinite baffle subwoofer in my 16'x11'x11' room and two boxes stuffed with pink beside the subwoofer. I haven't screwed the lid on the boxes yet so I could pull them back and create a "vent" to the pink for trapping bass at the back against the wall. Each box measures 40"x60"x30" So would I expect to get much reduction in decay or smoothing of room modes? Well I just setup my UMIK-1 to see. I measured with the lids closed, open 10", open 18" and with the lid closed and the door open. below are the waterfall graphs. I do see a difference, but not much and wonder if its significant. Also I was surprised that there was an improvement in ULF when the door was open as opposed to having it closed. So let me know, where did I screw up my theory and what should I do differently? Should I buy the grills and make the plunge cuts after I screw down the lid? I'll have to think on it some more.
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post #2 of 19 Old 07-29-2018, 07:58 PM
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Were your sweeps done with HDMI 4 and with room correction bass boosts (like Dynamic EQ) on or off? Try extending your waterfall to 450ms.

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post #3 of 19 Old 07-30-2018, 03:04 PM
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you would also need a test gear setup to measure the room response as an average over all the seating positions, not a single position. THX/HAA audio advanced setup and calibration class, 4 mic setup.


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post #4 of 19 Old 07-30-2018, 03:39 PM
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Bass builds up most in corners (don't forget where the ceiling meets the walls). You are mostly damping the resonance of the riser which is a good thing. For effective bass trapping in the room, you need to be working in the corners.


Caveat, I'm not an acoustician, but hired one who designed acoustic treatment for recording control rooms I worked in.
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post #5 of 19 Old 07-30-2018, 03:44 PM
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I had similar thoughts as Jeff. Bass traps don't strictly "trap bass" and reduce the bass level. You easily could do that with equalization. What they do is help minimize standing waves and make the frequency response SMOOTHER and more predictable throughout the room. When standing waves collide from different directions they either "stack up" or (partially) "cancel each other out". This leads to some areas of the room where the bass is louder and boomy, and other areas where it is shallower and lacking in volume. What you want is for a more consistent frequency response anywhere in the listening area, to which you can apply equalization as necessary for the proper overall frequency response.

I would love to have the opportunity to take a class like the one that Jeff took.
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post #6 of 19 Old 07-30-2018, 03:48 PM
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Quote:
Originally Posted by Rex Anderson View Post
Bass builds up most in corners (don't forget where the ceiling meets the walls). You are mostly damping the resonance of the riser which is a good thing. For effective bass trapping in the room, you need to be working in the corners.


Caveat, I'm not an acoustician, but hired one who designed acoustic treatment for recording control rooms I worked in.
In the example shown, the bass trap vents ARE in the corners! Between the wall and the top of the riser is a corner, Between the side walls and the rear walls is a corner. The example is right where you want it, in a three axis corner.
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post #7 of 19 Old 07-30-2018, 04:02 PM
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You need to try to flatten the response out before traps. Im sure you have some ringing going on but it could be placement, room modes as well as the box moving. Letting the rear wave and the front of an IB hit each other is not how a IB is supposed function. You would be better served to build individual boxes and find better placement. I do applaud the exercise

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post #8 of 19 Old 07-30-2018, 04:39 PM
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Quote:
Originally Posted by DaveClement View Post
In the example shown, the bass trap vents ARE in the corners! Between the wall and the top of the riser is a corner, Between the side walls and the rear walls is a corner. The example is right where you want it, in a three axis corner.

There are still a lot of untreated corners up the wall to the ceiling and across the ceiling from wall to wall.
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post #9 of 19 Old 07-31-2018, 12:22 AM - Thread Starter
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Thank you all for your responses. I picked the worst time ever to start this little experiment up. Worst work week ever coming up so I don't know when I will be able to make adjustments and get setup for more measurements!
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Originally Posted by Soulburner View Post
Were your sweeps done with HDMI 4 and with room correction bass boosts (like Dynamic EQ) on or off? Try extending your waterfall to 450ms.
I'm not sure if HDMI 4 is something other than just HDMI, but yes I have my laptop hooked up to my Marantz 6011 via HDMI. I had not run Audyssey yet so I'm pretty sure DEQ was not engaged, but I will run it and then make sure it is disabled as well as increase my next graphs up to 450ms. I did have the LF adjust knob turned all the way up on the subwoofer amplifier as well. I think I will have that turned up most of the time in the future so would you think I'm better off doing this experiment with the LFA turned up like I will normally use it or turned all the way down??

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Originally Posted by BIGmouthinDC View Post
you would also need a test gear setup to measure the room response as an average over all the seating positions, not a single position. THX/HAA audio advanced setup and calibration class, 4 mic setup.
That seems awesome, but there is only 1 position that matters. I guarantee that my entire family, extended family and friends think I've lost my marbles with this HT project. Anybody sitting in the other positions will be more than happy with what they experience and will roll their eyes if the words frequency response come out of my mouth.

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Originally Posted by Rex Anderson View Post
Bass builds up most in corners (don't forget where the ceiling meets the walls). You are mostly damping the resonance of the riser which is a good thing. For effective bass trapping in the room, you need to be working in the corners.


Caveat, I'm not an acoustician, but hired one who designed acoustic treatment for recording control rooms I worked in.
I am planning to add some corner bass traps up the back corners, but that will be after I finish the subwoofer riser and this experiment. For now I'm just focused on figuring out is there really much to be gained by venting the riser? I could have built the IB completely different using the entire space of the riser at less than the ideal suggested 10:1 VAS ratio and saved a lot of hassle not venting the IB out the back of the room into the garage, but I wanted to do this experiment and hopefully improve my FR while doing it. With regards to extra hassle and pain, check out this video showing the venting system for the IB

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Originally Posted by DaveClement View Post
I had similar thoughts as Jeff. Bass traps don't strictly "trap bass" and reduce the bass level. You easily could do that with equalization. What they do is help minimize standing waves and make the frequency response SMOOTHER and more predictable throughout the room. When standing waves collide from different directions they either "stack up" or (partially) "cancel each other out". This leads to some areas of the room where the bass is louder and boomy, and other areas where it is shallower and lacking in volume. What you want is for a more consistent frequency response anywhere in the listening area, to which you can apply equalization as necessary for the proper overall frequency response.

I would love to have the opportunity to take a class like the one that Jeff took.
Yes I am looking to see the waterfall plots show reduced decay time and some of the peaks and valleys get smoothed out on the SPL graphs as I add or increase the venting. I see some, but I am a novice at this and so my initial less than impressed feelings are likely are result of me misunderstanding how to setup the testing scenario or misinterpretation of the significance of the change in the graphs. I'm hoping you more experience guys can give me some suggestions on how to make my little experiment more useful or maybe this leads to some evidence that the idea of venting your riser has an insignificant effect on your bass response after all.

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Originally Posted by DaveClement View Post
In the example shown, the bass trap vents ARE in the corners! Between the wall and the top of the riser is a corner, Between the side walls and the rear walls is a corner. The example is right where you want it, in a three axis corner.
That's what I was aiming for in a way that I can easily vary the "ventilation" area for testing.

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Originally Posted by Gorilla Killa View Post
You need to try to flatten the response out before traps. Im sure you have some ringing going on but it could be placement, room modes as well as the box moving. Letting the rear wave and the front of an IB hit each other is not how a IB is supposed function. You would be better served to build individual boxes and find better placement. I do applaud the exercise
I don't think I explained the IB setup well enough above, but the back wave is exiting the room through the back wall as shown in the video above in this reply. The side boxes are completely isolated from the IB box. I am planning another IB manifold in the ceiling, which is a whole different project in trying to determine the best location to flatten the response out just like you said. Maybe I should work on the project of determining the best location of the second IB before experimenting with the bass trapping ventilation options...These are the kind of suggestions I'm looking for!

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Originally Posted by Rex Anderson View Post
There are still a lot of untreated corners up the wall to the ceiling and across the ceiling from wall to wall.
Yes I know, but unfortunately the ceiling corners will be staying untreated, gotta do the best I can down here.

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post #10 of 19 Old 07-31-2018, 08:01 AM
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Quote:
Originally Posted by Tyborg View Post
Yes I am looking to see the waterfall plots show reduced decay time and some of the peaks and valleys get smoothed out on the SPL graphs as I add or increase the venting. I see some, but I am a novice at this and so my initial less than impressed feelings are likely are result of me misunderstanding how to setup the testing scenario or misinterpretation of the significance of the change in the graphs. I'm hoping you more experience guys can give me some suggestions on how to make my little experiment more useful or maybe this leads to some evidence that the idea of venting your riser has an insignificant effect on your bass response after all.
You are confusing standing waves and flat frequency response. They are very different concepts. You need to understand the standing waves before you can understand the purpose of a bass trap, and therefore understand what you are testing for and what the results mean.

This video is a very good visual demonstration of standing waves. It uses a string to visualize sound waves. Sound waves behave in much the same way, you just can't see them.


If the video does not play properly, click on where it says "YouTube" near the bottom to watch on a separate YouTube.com page.

Take a look at 50 seconds in. It shows the results of standing waves, where the sound bounces back and creates areas that stack up (on either side) and where they cancel out (in the center). In this case there are two standing waves. In the spots where the sound stacks up, it is much louder than it should be. In the spots where it cancels out, it is much quieter than it should be. In this demonstration where nearly 100% of the wave is reflected, there would be NO sound at that frequency at the null in the center.

Every frequency will have a different wave pattern, depending on the size of the room. At 2:14 you can see a single standing wave at half the frequency that was demonstrated earlier. At 2:54 you can see three standing waves at triple the frequency of the single standing wave. Between 3:40 and 3:50, when he goes between the 3rd and the 6th harmonic you can see the waveforms of the 4th and 5th harmonics, when there are 4 and 5 peaks. At the 6th harmonic you can see all six peaks.

The point of a bass trap is to minimize the reflections that cause the peaks and valleys. In a theoretical perfect world, you could eliminate all reflections and not have any standing waves. Since that is not possible, the best that you can do is to minimize the standing waves. That is what a bass trap is for. It "traps" some of the low frequency bass so that it does not reflect back and cause standing waves.

Do not confuse any of this with a frequency response plot. A frequency response plot is only valid for the specific location that the microphone was placed. If you are measuring where a null is, you will get a very low reading at the frequency of the null. If you move the microphone over to where a peak is (which can be only a few feet away), you will get a much higher reading. Again, the purpose of a bass trap is to minimize the peaks and nulls, to make the frequency response more consistent throughout the room. More consistent is not the same as a "flatter" frequency response.
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post #11 of 19 Old 07-31-2018, 08:21 PM
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Here is my take on all this. I may be off base in some of my thinking, but this is my view on RT60 decay measurements, room modes, standing waves, peaks and nulls, and how it is all affected by a riser filled with insulation.

RT60 is a measurement of sound decay in a room (the time it takes for a given frequency to drop by 60db). Usually the RT60 of higher frequencies is short because they are easily absorbed by soft materials like carpet, couches, people, and of course room treatments. So we typically look at the RT60 of bass as this is something that tends to stick around for a while and muddy up the dynamic quality of a recording.

A waterfall graph is just a frequency response plot taken over and over after one burst of sound is created. The RT60 isn't based on how that waterfall graph LOOKS, its just the measurement from one end to the other of a given frequency. Perhaps in one location of the room, one frequency might decay faster or slower as the room changes, but over the average of several locations, it should be more consistent. We look at a waterfall graph because looking at a spreadsheet with dozens or hundreds of frequencies and measurements for each at each millisecond is not an easy thing to decipher.

When we can improve the RT60 of bass in a room, the recording sounds better. Bass is tighter and less muddy and sound effects are more dynamic sounding. "Bass Traps" in my opinion, are the tool to reduce the RT60 of bass.

Any absorption in a room can affect both the RT60 measurement and the frequency response measurement (or plot, whatever you want to call it) for a given location. But they are NOT the same thing, and not even affected by all the same things. How one is affected by a treatment has no bearing on the other. But they ARE both affected by absorption.

Many experts have claimed that RT60 is not affected in any way by diffusion. Diffusion is very effective at treating standing waves and reducing or eliminating them. If an RT60 measurement can't be affected by diffusion, then that would suggest that sound decay is not influenced by standing waves. This makes sense from a physics standpoint as it is all just energy. You release energy into the room as sound energy, it get converted to heat and kinetic energy as it gets absorbed by everything in the room. Where that energy bunches up in the room is not relevant to sound decay, nor are the locations in the room where the sound energy is close to zero. But if sound IS bunching up in certain locations, that would be a good place to absorb them and convert the energy into heat or kinetic energy, which would reduce the decay time and give a better RT60 measurement. We know that sound energy will bunch up in the corners of the room, so you put absorption there and you reduce the RT60 measurement.

Keep in mind, standing waves don't happen in corners, they happen between two parallel surfaces. Corners are not parallel. So why treat corners to improve decay times if sound decay is inhibited by standing waves? And why do experts not say to treat the peaks of standing wave locations in a room with absorption to reduce RT60 measurements?

One other interesting thing, you can reduce the RT60 with DSP for an entire room, but you can't reduce a standing wave with DSP. You can compensate for the standing wave for one specific spot in the room using DSP, but raising or lowering the volume of a given frequency will affect the whole room, and outside of the one spot you are trying to improve, it usually adversely affects the rest of the room. Again, if you can't stop standing waves with DSP but you can reduce decay times with DSP, I would say that the relationship between standing waves and sound decay is minimal at best.

Based on all this, I would consider a "Bass Trap" to be a treatment that absorbs sound energy with the intent of reducing the RT60 measurement, specifically in the bass frequency range, most commonly used in the corners of the room.

Now, just because it is built to reduce RT60 doesn't mean it isn't also effective at smoothing out frequency response by reducing standing waves (peaks and nulls). It is because two opposing corners still have parallel surfaces, and the bass trap is absorbing those sound waves, reducing the effectiveness of the energy to cancel and multiply as the waves cross back and forth. Those corners can also be a second, third, or fourth reflection point for sound, and by absorbing it before it gets to your ears you increase the quality of the sound (psycho acoustically speaking).

But a "bass trap" is not the same as an absorber placed on reflection points or between parallel surfaces for the purposes of smoothing out frequency response at a given listening position or reducing reflections that cause phase and delay issues. Those are just acoustic treatments. And since diffusion and absorption can both be used to treat frequency response in a room by reducing standing waves, we use them both for that purpose, but nobody puts diffusion in the corners. So while some acoustic treatments can help with RT60, unless they are specifically used to treat corners, I wouldn't call them Bass Traps. This could all be semantics though.

Now how does this relate to the riser? Well, if you have a big absorber in a corner, it is a bass trap. But it is also a big absorber that can reduce standing waves, so it affects BOTH the RT60 and the measured frequency response of the listening position.

I think the question on the table is HOW MUCH it affects the sound decay in particular, and that has yet to be measured. If the measurements here included more data (extend to 500+ms) so we can see how much of the sound decays beyond 300ms, and measurements from different points in the room were taken so we can get a better picture of how each frequency is affected by the bass trap riser, I think we could get some definitive data.

Furthermore, it is clear that moving the plywood off the corner and closer to the middle of the room is affecting the response at the measured location, so perhaps starting with the plywood slid out and then using a second piece of plywood to cover the opening would give us more consistent measurements?

Anyway, this is a cool experiment, something I am sure a lot of us who have built risers with the intent of using them as a bass trap have wanted to do but never tried. Kudos for the effort, and I hope you can turn it into some definitive conclusion. And after this, start measuring the difference between sand in a stage and an empty stage, lol.
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post #12 of 19 Old 07-31-2018, 08:46 PM
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Quote:
Originally Posted by Rex Anderson View Post
There are still a lot of untreated corners up the wall to the ceiling and across the ceiling from wall to wall.
Pressure is high in the corners, but the air velocity is not. Resistive absorbers like this work best at locations of maximum air velocity, which is 1/4 wave length from the wall. Corners are just convenient since nobody wants to put bass traps in the middle of their rooms.
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post #13 of 19 Old 08-01-2018, 09:40 AM - Thread Starter
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@dkersten thank you for your response, it’s super informative and really got me thinking. The comment about plywood affecting the frequency response when slid out was spot on, I’ll definitely repeat and adjust for that. This really is the worst week ever for me at work. Wish I could jump in more and add to the discussion, but I am planning to take advantage of the opportunity to learn a lot about how I can affect my audio experience in the lower frequency with the available space in the riser. Just need to wait for a bit more time probably next week to start rerunning measurements, but I want to figure out what I’m going to do differently in advance. Also for my first measurements I used the 0 degree calibration file from cross spectrum, should I be using the 90 degree file? Does it matter for low frequency as much as the higher frequencies? If doing multiple positions will placing a tripod on the riser affect the measurements since the whole thing shakes like a transducer? I don’t have a boom for the mic to do it otherwise
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post #14 of 19 Old 08-01-2018, 11:58 PM
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If you want to go crazy with basstraps substantial improvement is possible.
A link from the Acoustic Frontiers site. Before and After waterfalls near the bottom are most telling. Not sure what they mean by "extensive bass trapping" but I assume they are using more than standard absorption.
Certainly more than a couple boxes up front with some insulation in them.

http://realtraps.com/art_measuring.htm
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post #15 of 19 Old 08-03-2018, 11:48 PM - Thread Starter
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I’m a sucker for this hobby. Just placed the order for this mic stand with a boom to get measurements over the riser for those seating positions. Should be here Tuesday.

LyxPro SMT-1 Professional Microphone Stand Heavy Duty 93” Studio Overhead Boom Stand 76” Extra Long Telescoping Arm Mount, Foldable Tripod Legs & Adjustable Counterweight https://www.amazon.com/dp/B075ZHTHD5..._J2tzBbD8DFBC4
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post #16 of 19 Old 08-05-2018, 10:33 AM
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Quote:
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Pressure is high in the corners, but the air velocity is not. Resistive absorbers like this work best at locations of maximum air velocity, which is 1/4 wave length from the wall. Corners are just convenient since nobody wants to put bass traps in the middle of their rooms.

Lots of great info in this thread. This one got me thinking.


Is it better then to have absorption placed 1/4 wavelength (of whatever frequency you want to target) away from the walls or corner?


It makes sense that against the wall the air movement will be near zero. Then I thought that well, I've personally put myself in the corner with an 80Hz tone playing in my room and wham, it got really loud in that corner so *something* is happening. Then I thought that pressure and air (particle) movement are not the same thing. For me to observe the 80Hz sine wave, my eardrums are being physically vibrated by the sound source. However, since the pressure behind my eardrum is relatively constant, all it takes is *pressure* to make this happen.


So then for corner traps, rather than just filling them with as much absorptive material that you can, would it be better to place a malleable but impenetrable (by air) object in the corner? Imagine a balloon for example. If it was just filled with air, the oscillating pressure outside of the balloon would just transfer to the inside of the balloon and no energy would be absorbed. What if the balloon was filled with something that resisted the contraction of the balloon's walls due to the external pressure? Like absorptive material? The external *pressure*, not air movement, would be converted to heat and essentially absorbed.



Makes sense to me but I've been wrong before Has anything like this ever been done or experimented with?
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post #17 of 19 Old 08-05-2018, 10:37 AM
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Sorry, one more to follow up on the previous post.


Thinking about this more and trying to come up with a real-world way to test my thoughts above, rather than using something like a balloon, what about closed-cell foam? Would it not resist pressure changes and make a good bass trap for walls or corners?
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post #18 of 19 Old 08-05-2018, 11:32 AM
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Quote:
Originally Posted by jcmccorm View Post
So then for corner traps, rather than just filling them with as much absorptive material that you can, would it be better to place a malleable but impenetrable (by air) object in the corner? Imagine a balloon for example. If it was just filled with air, the oscillating pressure outside of the balloon would just transfer to the inside of the balloon and no energy would be absorbed. What if the balloon was filled with something that resisted the contraction of the balloon's walls due to the external pressure? Like absorptive material? The external *pressure*, not air movement, would be converted to heat and essentially absorbed.

Makes sense to me but I've been wrong before Has anything like this ever been done or experimented with?
You are describing a membrane absorber! Also called a diaphragmatic absorber/resonator. Typically you would determine the frequency in your room that you are having resonance issues with, then you would tune the resonant frequency of the absorber to match. These can go right against a wall for maximum effectiveness.

Here's a nice summary article: http://www.acousticsinsider.com/why-...ps-in-corners/
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post #19 of 19 Old 08-05-2018, 04:09 PM
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Originally Posted by bochoss View Post
You are describing a membrane absorber! Also called a diaphragmatic absorber/resonator. Typically you would determine the frequency in your room that you are having resonance issues with, then you would tune the resonant frequency of the absorber to match. These can go right against a wall for maximum effectiveness.

Here's a nice summary article: http://www.acousticsinsider.com/why-...ps-in-corners/

Thanks you sir! Good and interesting article. Too short though I'll research membrane absorbers.


One reason this piqued my interest is that I just got some new speakers and ran REW to see what I have. There's some low frequency problems with them (subs turned off, mains set to large, so it's just the mains I'm having trouble with). Although a riser is a possibility, I want to understand what's going on and how to fix it.Whether it's a rider trap, corner trap, or something else.


Thanks again.
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