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post #181 of 239 Old 12-22-2014, 11:18 PM
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Granroth:
Are you putting the holes where the modes are in your measurements?
(or are you hoping for results where there won't be any)
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post #182 of 239 Old 12-23-2014, 12:09 PM
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Quote:
Originally Posted by BasementBob View Post
Granroth:
Are you putting the holes where the modes are in your measurements?
(or are you hoping for results where there won't be any)
They are not targetted, no. But neither am I intending it to be completely arbitrary. In fact, I was mostly going off of your advice in Post 153:

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Originally Posted by BasementBob View Post
The vent placement makes no difference -- there's no practical point of looking for modal spots and putting tiny vents there. Put as many vents as you can along the wall, don't try to pick spots along the wall that are 'optimum'. If you're good enough to pick where they should be, you don't need to be reading this low-acoustical-science post.
The question I was trying to answer was: IF vent placement does not matter, then how much is "enough" and is any improvement linear? My results seem to suggest that placement does matter at least a little, but overall the improvement does appear to be linear.

I'm very curious about the possibility of placing vents to tackle specific modes, though! I have no idea how.

My room is 21'-6" x 13'-3" x 8'-1". If I plug that into your room modes calculator, I see that my three primary axial nodes are 26.3Hz, 42.6Hz, and 69.9Hz. Indeed, the overwhelmingly dominant room mode in my measurements is 52.6Hz (the one I've been incorrectly labeling 54Hz above), a second order length axial.

So now I know that... how do I use that knowledge to place the vents, other than just put as much linear feet of vents as possible?
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post #183 of 239 Old 12-23-2014, 12:19 PM
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Originally Posted by Laidback View Post
Here it is. ZERO vent costs.
That could absolutely work, but it wouldn't be the classic Erskine design as primarily detailed here. The Erskine design targets modal frequencies, while your design would likely be more broadband.

One of my tests might have approximated your design. See HERE. In it, I stuffed a riser with fiberglass but didn't put any top at all on it. This is definitely a broadband absorber and the graph showed that to be the case,

Notably, though, while it dramatically reduced the ringing for all frequencies, it did so in a more pronounced fashion the higher the frequency was. My primary problem room mode at 53Hz only went down a little, while higher frequencies were reduced by several times more. Also, compared to a sealed sub, the dB per frequency was far more peak+valley. Putting a top on reduced the highest dB peaks and notably evened out the rest.

Practically speaking, the biggest issue with your design is likely that you couldn't (easily) put carpet on it. It would need to remain a hard surface. That's at least partially a stylistic decision.

I say build it and test and report your findings!
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post #184 of 239 Old 12-23-2014, 12:43 PM
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Quote:
Originally Posted by granroth View Post
That could absolutely work, but it wouldn't be the classic Erskine design as primarily detailed here. The Erskine design targets modal frequencies, while your design would likely be more broadband.

One of my tests might have approximated your design. See HERE. In it, I stuffed a riser with fiberglass but didn't put any top at all on it. This is definitely a broadband absorber and the graph showed that to be the case,

Notably, though, while it dramatically reduced the ringing for all frequencies, it did so in a more pronounced fashion the higher the frequency was. My primary problem room mode at 53Hz only went down a little, while higher frequencies were reduced by several times more. Also, compared to a sealed sub, the dB per frequency was far more peak+valley. Putting a top on reduced the highest dB peaks and notably evened out the rest.

Practically speaking, the biggest issue with your design is likely that you couldn't (easily) put carpet on it. It would need to remain a hard surface. That's at least partially a stylistic decision.

I say build it and test and report your findings!
Granroth,

The drawing was just to show the framing plan under the 3/4" platform. I was just trying to solve the vent size/closest to the wall/usable volume issue. I will be building it. Right now I have nothing in the room other than scrap carpet. I am not sure what exact frequencies this would take care of, I guess that is room by room. My room has angled ceilings that flatten out at 12' and then come back down, so who knows where my problem areas are.

I have the carpet figured out if my carpet guy doesn't like the raw edge on the ends. I can install a 2x2 at the top of the framing and secure the tack strip to the 2x2 already in place when carpet goes in.

So I am guessing the Erskine model is more of a tune-able or specific frequency reducer.

I need to see if there is a basics thread on doing the sound measurements, what the pretty waterfall graphs mean and what test tones or audio sample you guys are using to get the sound samples.

Where should I start on the "control test"? No carpet, no riser, no treatments? I will have an AT screen with speakers behind it built into a false wall. So, do test like that and then go one item at a time to see what happens?

I am so stoked to get this thing going!!!
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post #185 of 239 Old 12-23-2014, 11:01 PM
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Originally Posted by HopefulFred View Post
Very cool. You could probably recover some of the holes with scrap, just temporarily to compare some other combinations.
I'm absolutely willing to do that, but I'm not sure what I'd even expect to find. I tend to like to have a hypothesis in place before doing my experiments, just to see how close I can predict results. I'm not sure what I'd be testing in this case.

Maybe if length axial modes react to back wall vents and width modes to side vents?
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post #186 of 239 Old 12-23-2014, 11:12 PM
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Originally Posted by Laidback View Post
I need to see if there is a basics thread on doing the sound measurements, what the pretty waterfall graphs mean and what test tones or audio sample you guys are using to get the sound samples.
I'll attest to acoustic measurements being very intimidating before you learn what they mean! Here are two resources that really clarified things in my mind:

RealTraps - Room Measuring Series

Acoustic measurement with the UMIK-1 and REW

The former is written by Ethan Winer, who used to be very active on AVS (and is still very active on other sites) and @Nyal Mellor , who is still very active here. It's a tiny bit old in that it still recommends a non-USB microphone (definitely get a USB one) but the concepts are all still rock solid and very well explained.

The latter is written by the makers of the UMIK-1 USB microphone, so it's slanted in that direction... but the UMIK-1 is an excellent microphone to get, so no harm done. The other excellent microphone is the Dayton Audio UMM-6. With either one, I'd highly recommend getting it from Cross Spectrum Labs to get them professionally calibrated.

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Originally Posted by Laidback View Post
Where should I start on the "control test"? No carpet, no riser, no treatments? I will have an AT screen with speakers behind it built into a false wall. So, do test like that and then go one item at a time to see what happens?
Start with some constant, where you know that nothing will change other than the variable that you are adding. In my case, when testing the riser, I already had my stage in place, but nothing else. I made sure that the ONLY changes that happened in the theater were the changes made to the riser. And that even means such things as microphone placement, where I stand during the tests, what volume the tones are played at and so on.
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post #187 of 239 Old 12-23-2014, 11:38 PM
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Quote:
Originally Posted by granroth View Post
They are not targetted, no.
Now that you have measurements, you can target them, and save money.

Quote:
Originally Posted by granroth View Post
I'm very curious about the possibility of placing vents to tackle specific modes, though! ... My room is 21'-6" x 13'-3" x 8'-1". If I plug that into your room modes calculator, I see that my three primary axial nodes are 26.3Hz, 42.6Hz, and 69.9Hz. Indeed, the overwhelmingly dominant room mode in my measurements is 52.6Hz (the one I've been incorrectly labeling 54Hz above), a second order length axial.



OK, so you have a ringing mode where you have that microphone, at 52.6hz (2,0,0 axial), and another at 85.3hz (0,2,0 axial). There might be one at 26.3hz and/or 42.6hz that your microphone isn't showing, perhaps your microphone is in a null node (remeasure to test hypothesis).
In any event your big issue is the length mode -- so, what can you do to increase absorption of that?


[/p>
- According to this image, you could extend the 3 vents along the top to cover the entire wall, and you could make those three vents twice as wide. Either, or both, of those would increase the absorption along that 52.6 (2,0,0) axial mode. Similarly, your seats riser at the other end of the room, same thing. Obviously leave enough deck for structural integrity, or remove the deck plywood and add 2x4 blocking and replace the deck plywood.
- It looks to me like from the pink holes you're already getting as much from this riser as you're going to get against the 85.3hz (0,2,0 axial) mode, although you could do the same with the seats riser if you haven't already done so.
- You have nothing in the measurement around 69.9 hz (0,0,1 mode), although again it could be microphone placement in a null node, nonetheless if there's nothing to treat then there's nothing to treat.

Quote:
Originally Posted by Granroth
The landscape fabric didn't impede the performance of the riser vents at all
No big surprise -- you're talking about bass. Landscape fabric is virtually acoustically transparent at low frequencies.
- To test covered vs uncovered - Try two layers of drywall or an inch and a half of plywood to 'cover' the vents.
- If you want to line the vents to separate the fiberglass from view without affecting low frequency absorption, I think you've just proved to yourself that this could be a viable material acoustically (may be non-acoustic issues)

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post #188 of 239 Old 12-24-2014, 05:53 AM
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Quote:
Originally Posted by granroth View Post
I'll attest to acoustic measurements being very intimidating before you learn what they mean! Here are two resources that really clarified things in my mind:

RealTraps - Room Measuring Series

Acoustic measurement with the UMIK-1 and REW

The former is written by Ethan Winer, who used to be very active on AVS (and is still very active on other sites) and @Nyal Mellor , who is still very active here. It's a tiny bit old in that it still recommends a non-USB microphone (definitely get a USB one) but the concepts are all still rock solid and very well explained.

The latter is written by the makers of the UMIK-1 USB microphone, so it's slanted in that direction... but the UMIK-1 is an excellent microphone to get, so no harm done. The other excellent microphone is the Dayton Audio UMM-6. With either one, I'd highly recommend getting it from Cross Spectrum Labs to get them professionally calibrated.



Start with some constant, where you know that nothing will change other than the variable that you are adding. In my case, when testing the riser, I already had my stage in place, but nothing else. I made sure that the ONLY changes that happened in the theater were the changes made to the riser. And that even means such things as microphone placement, where I stand during the tests, what volume the tones are played at and so on.
GREAT!
I have been reading a couple of other threads on the DIY acoustic panels and the like but couldn't make a lot of sense of the graphs being put up (although the waterfall ones were really cool looking).

Did you have carpet in when doing the tests or were you just doing the tests for JUST the riser at this time? I would hate to have to try to frame around my final carpet, even if I left it back from where the risers were going to go.

It sounds Like (no pun intended, haha) I will get the front of the room built out and place speakers before moving onto the room measurements phase. I plan on having two subs so I will make sure I have extra wire to move those around.

As far as the microphone, will the one that Marantz supplies for their room correction program work? It has "Audyssey MultEQ XT, Dynamic EQ, Dynamic Volume". I just know it is a usb mic and can fit on a tri pod. I haven't played with it yet.

What is the drawing program you used? The OSB hatching was funny to me for the top of the riser. Looked just like Advantec (I'm a home builder so I notice weird things).

Sounds like I am going to have to start a new "Custom Theater" build page soon. Just need to get back home and clean out the room!
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post #189 of 239 Old 12-24-2014, 11:02 AM
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Originally Posted by Laidback View Post
Did you have carpet in when doing the tests or were you just doing the tests for JUST the riser at this time? I would hate to have to try to frame around my final carpet, even if I left it back from where the risers were going to go.
Nope, carpet is going to be one the very last things installed in my theater. See the link in my signature to my build thread for the current state.

Quote:
As far as the microphone, will the one that Marantz supplies for their room correction program work? It has "Audyssey MultEQ XT, Dynamic EQ, Dynamic Volume". I just know it is a usb mic and can fit on a tri pod. I haven't played with it yet.
Hmm... not sure. I've seen references to using the older Audyssey mics with REW and those don't appear all that bad, relatively speaking. I'm not aware of any USB mics other than the UMIK-1 and UMM-6 working with REW, though.

Quote:
What is the drawing program you used? The OSB hatching was funny to me for the top of the riser. Looked just like Advantec (I'm a home builder so I notice weird things).
SketchUp. It's free, very easy to use, insanely powerful, and is practically ubiquitous around AVS.
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post #190 of 239 Old 12-24-2014, 12:53 PM
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Originally Posted by BasementBob View Post
OK, so you have a ringing mode where you have that microphone, at 52.6hz (2,0,0 axial), and another at 85.3hz (0,2,0 axial). There might be one at 26.3hz and/or 42.6hz that your microphone isn't showing, perhaps your microphone is in a null node (remeasure to test hypothesis).
In any event your big issue is the length mode -- so, what can you do to increase absorption of that?
I did a few quick new tests this morning, with the microphone in a new location. This time it's roughly in the middle of the riser. Running the same measurement test as before showed markedly different results, as we'd expect:



The 52.6Hz spike is much more muted in this location and most of the frequencies are a bit flatter. But what I'm really interested to see is how the different vents affect different modal frequencies. As in, do the back vents work on the length frequencies and the side vents work on width ones? In that case, what would work on the height modes?

You can see why I was hiding results below 30Hz before. It looks like garbage results.

There's something interesting about all of the peaks. For instance, here are my room modes up to 200Hz

Length Modes: 26.3 Hz 52.6 Hz 78.8 Hz 105.1 Hz 131.4 Hz 157.7 Hz 184.0 Hz

Width Modes: 42.6 Hz 85.3 Hz 127.9 Hz 170.6 Hz

Height Modes: 69.9 Hz 139.9 Hz

Now look at the frequencies where the ringing is. I can tell you from playing with REW that they are nearly ALL the length modes ringing! There's a cute little ringing at the 2nd order width mode of 85.3Hz and maybe the 3rd order 127.9Hz combines with the 5th order length mode of 131.4Hz. But otherwise, they aren't visible at all in the waterfall and neither of the height modes show up even a little bit.

Curious.

So for test one, I loosely covered the back vents with the cut-off pieces of the riser (two layers of 3/4" OSB glued together). It's not a perfect cover, since there are some air gap slivers roughly the width of the jig-saw blade kerf, but they are mostly covered. The results:



Ringing increased in pretty much all of the length modes, ranging from maybe 10ms on the low end to around 40ms on the upper end. The one known axial width mode didn't budge at all.

I then uncovered the back vents and loosely covered the side vents:



There aren't any strong width modes in play, so we can't expect much change there. I do note that the lower frequency length modes don't budge at all, while the upper ones do go a little. In fact, although the back vents seem to affect all of the modes, the side vents only really target the 4th, 6th, and 7th length modes. Whatever is at 200Hz seems to also be massively affected, but that doesn't correspond with any axial modes.

More info that's not in these screen captures -- if I increase my window to 300Hz, then I see that the side vents affect the 200-300Hz band quite a bit more than the lower frequencies. That implies its use as a broadband absorber in addition to one targeting modal frequencies.

My tentative conclusions at this point:

1. My length modes DOMINATE
2. The back vents have more impact than the side vents
3. The side vents do still help, but only a little on the notable modes and more on the higher frequencies

Quote:
- According to this image, you could extend the 3 vents along the top to cover the entire wall, and you could make those three vents twice as wide. Either, or both, of those would increase the absorption along that 52.6 (2,0,0) axial mode. Similarly, your seats riser at the other end of the room, same thing. Obviously leave enough deck for structural integrity, or remove the deck plywood and add 2x4 blocking and replace the deck plywood.
Yeah, I'll be messing with these later. I don't want to touch them now since my columns are going to be around there and I'm not yet sure what their size will be. I think I'll revisit the back vent size after my soffit and columns are done.

I'm not entirely certain what you mean by "seats riser on the other end of the room"? Only the stage is on the other end of the room.
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post #191 of 239 Old 12-24-2014, 03:06 PM
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I'm not entirely certain what you mean by "seats riser on the other end of the room"? Only the stage is on the other end of the room.
OK. Somehow I thought you were doing two -- one riser under the screen, and another riser for the seats.

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post #192 of 239 Old 01-06-2015, 09:11 PM
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The single most effective aspect in a pressure absorber is height. The higher the riser the more effective it is. Risers of at least 2 feet in height will yield an almost flat low frequency response. Further more, single mic testing is not the best to use in these situations. You should be spatial averaging your response plots. If you have already my apologies. I have just caught the last page of the thread.

Further, I still think you have a lot more testing to do than to declare the stage/sand mixture a complete farse and dogma. There is actually "real" research out there that completely negates your findings. Please be careful throwing around words such as dogma. At the most, your findings show that there is need for further study, nothing more. Just so you're aware, we don't show all of our tricks on AVS or in magazines with decade old articles. We've learned a few things. . Off my soap box.

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post #193 of 239 Old 01-06-2015, 09:42 PM
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The single most effective aspect in a pressure absorber is height. The higher the riser the more effective it is. Risers of at least 2 feet in height will yield an almost flat low frequency response. Further more, single mic testing is not the best to use in these situations. You should be spatial averaging your response plots. If you have already my apologies. I have just caught the last page of the thread.
Ah, that's good to know! I cannot have a 2 foot high riser, so that's not an option for me, but my minimal tests here showed me that even what I do have made a measurable difference. If height is the dominant factor, then that does imply that making my vents wider (as opposed to longer) will likely have little impact?

And yeah, my testing methodology could never be as comprehensive as that done by a learned acoustician (or similar). My attempt was just to add another small data point to this particular corpus. I know of my tests and those done by @mtbdudex . Both of our sets of tests showed that this design absolutely works. Both did tend to have similar riser heights, though. I'd love to see some data points for a higher riser to see how it compares.

Okay, I'd also really love to see some professionally done tests, but I recognize that those days are long gone.
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post #194 of 239 Old 01-06-2015, 10:00 PM
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Not gone. I have a mountain of things I'd love to test, but just no time. And your test was admirable.
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post #195 of 239 Old 01-06-2015, 10:06 PM
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Further, I still think you have a lot more testing to do than to declare the stage/sand mixture a complete farse and dogma. There is actually "real" research out there that completely negates your findings. Please be careful throwing around words such as dogma. At the most, your findings show that there is need for further study, nothing more. Just so you're aware, we don't show all of our tricks on AVS or in magazines with decade old articles. We've learned a few things. . Off my soap box.
I do try to be at least a little precise with my words. I have never said that the stage/sand mixture was a farce, but I do stand by it being dogma -- a principle or belief handed down from authority that is held to be incontrovertibly true by the believers, without any proof of said principle being offered (or needed). It comes from an authority (Dennis originally; and now you) and is believed without question (scores of stages have been built this way) with absolutely no proof that I know of ever offered that it actually works. It's a perfect match for the definition.

You have brought up the possibility of research that shows that it does work several times but have never actually produced any such research! I'm not saying it doesn't exist -- I'm saying that if it does, it's never been presented to the community and remains in the domain of the authority. Again, to be precise, the research that you have pointed to all show that sand makes for a very effective damping material for mechanical vibrations. That's indisputable, since it's easily proven. I've seen absolutely no link from that premise to the idea that it actually applies with a stage and a subwoofer on it.

I've also never said that my experiments were the be-all and end-all of this. FAR from it! I've been outright begging other people to do their own testing on this topic. There absolutely could be something unique about my setup or my testing methodology that gave non-representative results. It's impossible to say without further and more varied testing.

But.. as far as I know, my results are the only ones that exist now and that are publically available -- and they show that it doesn't work.

Shawn, YOU are the authority in question and are in the unique position to prove that it's not dogma and that the principle absolutely works. I'm not being even a little sarcastic when I say that I would love to see such research!
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post #196 of 239 Old 01-06-2015, 11:25 PM
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Quote:
Originally Posted by granroth View Post
I do try to be at least a little precise with my words. I have never said that the stage/sand mixture was a farce, but I do stand by it being dogma -- a principle or belief handed down from authority that is held to be incontrovertibly true by the believers, without any proof of said principle being offered (or needed). It comes from an authority (Dennis originally; and now you) and is believed without question (scores of stages have been built this way) with absolutely no proof that I know of ever offered that it actually works. It's a perfect match for the definition.

You have brought up the possibility of research that shows that it does work several times but have never actually produced any such research! I'm not saying it doesn't exist -- I'm saying that if it does, it's never been presented to the community and remains in the domain of the authority. Again, to be precise, the research that you have pointed to all show that sand makes for a very effective damping material for mechanical vibrations. That's indisputable, since it's easily proven. I've seen absolutely no link from that premise to the idea that it actually applies with a stage and a subwoofer on it.

I've also never said that my experiments were the be-all and end-all of this. FAR from it! I've been outright begging other people to do their own testing on this topic. There absolutely could be something unique about my setup or my testing methodology that gave non-representative results. It's impossible to say without further and more varied testing.

But.. as far as I know, my results are the only ones that exist now and that are publically available -- and they show that it doesn't work.

Shawn, YOU are the authority in question and are in the unique position to prove that it's not dogma and that the principle absolutely works. I'm not being even a little sarcastic when I say that I would love to see such research!
Hmmmm. Well, I guess I'll need to start writing some grants. I actually did produce a paper and I believe posted it. That was just with 5 minutes of searching on Google. You just have to ante up and pay for it.

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post #197 of 239 Old 03-07-2015, 06:55 PM
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Granroth: What drove the size of the vents in the riser? Did you use that modecalc.xls spreadsheet?

I'm thinking that you could lay the joists for the riser in certain patterns to create certain size cavities, and coupled with specific size ports into those cavities, you would create Helmholtz Resonators for the trouble frequencies.

I've done a little reading on HRs, but I'm still not clear on the best place within the room for the port opening. I think it should be at high-pressure places (near walls/corners) versus high-velocity places (middle of the room), but I'm not sure. Hopefully someone will hear my plaintive cry for help and answer up.
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post #198 of 239 Old 03-07-2015, 10:46 PM
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Quote:
Originally Posted by crossrh View Post
Granroth: What drove the size of the vents in the riser? Did you use that modecalc.xls spreadsheet?

I'm thinking that you could lay the joists for the riser in certain patterns to create certain size cavities, and coupled with specific size ports into those cavities, you would create Helmholtz Resonators for the trouble frequencies.

I've done a little reading on HRs, but I'm still not clear on the best place within the room for the port opening. I think it should be at high-pressure places (near walls/corners) versus high-velocity places (middle of the room), but I'm not sure. Hopefully someone will hear my plaintive cry for help and answer up.
I did not use any spreadsheet nor any calculations at all in determining the size and placement of the vents. I do know what the modal frequencies are in my room, but there's no indication that I know of that this type of riser addresses specific frequencies.

Instead, Erskine has been pretty clear over the years that this design is not a Helmholtz Resonator and is not a bass trap -- he describes it as a "broadband modal frequencies trap".

Given that, I wanted to know what the dominant factors were in creating the vents. Does placement matter more or size? Or do both play an equal role. This required creating somewhat random sizes and places since I've never seen any published info dictating those factors.

Now, you absolutely can create an HR from a riser. @mtbdudex did exactly that and had some very good results. That was out of the scope of the tests I wanted to run so my riser doesn't work that way.

If you decide to do that, then please post the results of your efforts here! The more examples we have of using risers as traps of any sort, the better
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post #199 of 239 Old 06-01-2015, 05:13 PM
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Here's a test.
Cover all your "vents"...solid wood...no air flow, no vibration in your covers (no more so than the platform itself).
Place your microphone (isolated from the floor and walls) in a right tri-corner (junction of two walls and the floor, or two walls and the ceiling).
Place a sub in the corner of the room, send it full spectrum pink noise.
Measure frequency response ONLY, 1/24 octave.
Make a record of all peaks.
Save the plot.
Expose the back wall vents.
Repeat.
Cover the back wall vents.
Uncover side wall vents.
Repeat.
Uncover all vents.
Repeat.

Overlay all 4 plots.

Have fun.

(Oh, send me an email when you're done and I'll take a look.)

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post #200 of 239 Old 06-08-2015, 06:23 AM
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Too much work, eh?
Quote:
Are they still 2" from the back wall, and thus partially or entirely covered by the bass trap? Or do we now consider the bass trap to be the "wall" and put the vent flush with that?
The 'vents near the wall boundary' are not for the purpose of broadband absorption but rather to reduce the amplitude of modal response. At 80Hz the wave length is on the order of 14'... in other words, 80Hz doesn't give a krap about 2".

Basement Bob's posts are spot on. By definition modal frequencies peak (are at the lowest velocity and highest pressure) at the walls. Placing a microphone at a wall will provide the modal frequencies ... placing the microphone in a right tri-corner (boundary of floor, length wall, width wall) will reveal all modal frequencies. Placing absorptive material and the wall (fiberglass, cotton, etc.) won't do anything for low frequencies (unless you can make it >4' deep). The purpose of the vents near the side/rear walls is to reduce the amplitude of modal frequencies. The degree to which that works will depend upon the volume of the riser (filled with fiberglass batts). This is a brain dead, easy method to help with modal response which requires little knowledge and zero math ... plus you're putting otherwise wasted space to work doing something actually helpful.

I've seen risers used as low pass filters and helmholtz resonators ... sometimes with success and only when done by a professional very, very experienced in making the theory work in real life. In the DIY world, you might get lucky.
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post #201 of 239 Old 06-08-2015, 08:01 AM
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Quote:
Originally Posted by Dennis Erskine View Post
Too much work, eh?
Sorry, I'm a wife-mandated break from theater related work this summer, to try and knock off all of the scores of other projects around the house that I've been neglecting. I'll definitely do the tests you suggested as soon as I'm able.
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post #202 of 239 Old 06-08-2015, 08:31 AM
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Tests aren't for my benefit. No worries.

Have fun this summer.
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post #203 of 239 Old 06-08-2015, 09:02 AM
 
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Quote:
Originally Posted by granroth View Post
Sorry, I'm a wife-mandated break from theater related work this summer, to try and knock off all of the scores of other projects around the house that I've been neglecting. I'll definitely do the tests you suggested as soon as I'm able.
Kurt, I have not followed this thread very close, but I am curious to what you are trying to accomplish or test?
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post #204 of 239 Old 06-08-2015, 06:39 PM
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Quote:
Originally Posted by Mfusick View Post
Kurt, I have not followed this thread very close, but I am curious to what you are trying to accomplish or test?
My earlier tests confirmed what others had also shown; that the riser vents absolutely work very well for bass frequencies. The open questions I had were all involving the size and positions of the vents. What combination of vent size or location provides the biggest bang? Do certain configurations affect different frequencies?

Dennis suggested a few tests which will hopefully lead towards some additional data and maybe actual answers.
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post #205 of 239 Old 11-09-2015, 04:45 AM
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Since my riser was not located against the rear wall and only against the left wall, I have constructed a riser with an open design on the front, rear, and right side, with the intention of broadband absorbing / shotgun-taming room modes.
It might be a bit too late for such a question, but am I missing something in my expectations of the acoustical effect the riser will have?

front view


Side view


Front view with fiberglass insulation


cheers,
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post #206 of 239 Old 11-09-2015, 12:17 PM
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Quote:
Originally Posted by Jacob B View Post
front, rear, and right side, with the intention of broadband absorbing / shotgun-taming room modes
I would imagine the front won't do much for room modes.
But the rear and in your case the right, both being near a wall, should help with room modes
because room modes have local maxima at the room boundaries

As for the magnitude of the effect, I can't predict that. It should take a tiny bit out of even very low hz modes.
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post #207 of 239 Old 09-23-2016, 07:30 PM
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Digging up an oldie but goodie and have a question. I'm trying to finish my riser this weekend. Do stairs need to be filled with insulation? I'm assuming they don't since they're isolated from the rest of the riser. Here are pics of my riser for reference.


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post #208 of 239 Old 09-24-2016, 07:03 AM
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A cavity is a cavity I fill them up so they don't resonate and reduce footstep noise.
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post #209 of 239 Old 09-24-2016, 10:28 AM
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A cavity is a cavity I fill them up so they don't resonate and reduce footstep noise.
Thanks Big.
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post #210 of 239 Old 10-03-2016, 07:22 PM
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I'm going to be starting to build a riser for my theater. The riser will be roughly 8'x6'. I'm not going to be making this an acoustically perfect room, but if I'm making a riser, I figure I might as well see if I can have it serve an acoustic purpose. What are some rough guidelines for size and placement of openings to give me a decent shot of having it act as a bass trap?

Also, is 6' deep enough to have room for a reclining rear row?
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