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Any difference with wedge and egg shell sound treatments??


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Hi everyone

 

Im about to do some sound Treatment in my home theatre set up...Iv notice there's different types of shapes (wedge n egg shell))...Is there any difference, as in performance wise??

 

Also with bass traps is thicker the better??

 

Regards

 

Obama

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IMHO foam needs to be damn thick to achieve broadband absorption. I steered away from it for this very reason. It's also expensive doe what it is.

Have you considered something like the Polymax XHD? I use this in 50mm thickness and it outperforms any foam product of equivalent thickness. It's also available in 75mm and 100mm with increasing low-end performance as you increase the thickness.

You'll need to go quite thick if you're constructing bass traps. There are a few design possibilities.

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Not only "thicker the better" but overall size the better and denser (within reason) the better too.  Theory says that dimensions of absorptive material needs to be in the order of 1/4 wavelength.  So for low frequencies the treatment needs to be huge - not always practical.  So little and/or flimsy things in corners won't do anything useful for bass control.  They might improve mid-range and highs a bit, however.  Absorption is more effective placed where the velocity of the sound waves is greatest (away from walls) rather than where the pressure is greatest (against walls and in corners), however this is rarely ever a practical solution.

 

I am using 6 x full sheets (1200 x 2400) of 32Kg/cu.m at 2 deep (ie 200mm thick) of semi-rigid fibreglass.  Then there are the 5 x 2m high x 500mm diameter x 64Kg/cu.m tube traps as well.  Some might say that all that is over-kill, but it still only has an average effect near 80Hz and little or no effect down low.

 

Wise placements of speakers and the listening chair with respect to peaks and nulls in the room is something useful to work on.  Then when all those avenues (positioning and treatment) are exhausted, lop the peaks off with EQ.

 

@@Malcolm recently posted a link to a respected acoustics textbook.  Its all in there.  Link to M's post             #58              

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I am using 6 x full sheets (1200 x 2400) of 32Kg/cu.m at 2 deep (ie 200mm thick) of semi-rigid fibreglass.  Then there are the 5 x 2m high x 500mm diameter x 64Kg/cu.m tube traps as well.  Some might say that all that is over-kill, but it still only has an average effect near 80Hz and little or no effect down low.

No such thing as over-kill when it comes to bass traps. My front corner ones are much like yours. Floor to ceiling 500mm TubeTraps with an additional 300mm thick 32kg/m poly again floor to ceiling covering the sides facing into the room.

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+1 for Polymax XHD. It's a bugger to cut, though. I have used an electric carving knife with razor sharp blades.

Or you can compress it with a piece of 90x45 timber until its only about 30mm thick. Then run a 9" circular saw with a steel blade along the line and its cut in seconds.

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I haven't tried cuttig the polymax as yet, as the factory here in Sydney has cut my 800mm x 1200mm panels for me on their industrial equipment. However, if I ever decide to cut it into smaller pieces for whatever reason, I'll keep these options in mind.

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You can build a relatively simple, but large, bass trap by acquiring a sheet of mass loaded vinyl (either 4kg/sq.m. or 8kg/sq.m.) and hanging it on 45 degree braces across the corner of the room in question, from floor to cwiling so that you create a triangular "chamber" in each corner. This material is about 1cm thick sheet and therefore the density is around 400 kg/cu.m! This can act as a pretty effective bass trap down to quite low (<30Hz) frequencies, especially if you use two layers of the stuff!  Putting foam on the facing of this such as the BT-400 trap from Sound Acoustics in Melbourne will give you a true broad band absorber down to at least 30Hz.  It is essential that you have a broad band absorber, wherever you place treatment in the room or you will end up having a room that is "bright" or "dull" depending on which end of the spectrum the devices have discriminated.  Diffusers will assist as well, but it is the bass frequencies that need cleaning up as they accumulate in each corner of the room (multiple standing waves).

Edited by garyjac
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@@garyjac - your story about the mass loaded vinyl curtains sounds good, but it will not significantly increase the energy decay rate of discrete modes.  Loading a fibrous absorber with a limp mass membrane increases low frequency absorption, but a limp mass membrane alone doesn't do much at all.

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Agreed, but I have not suggested a limp mass mebrane on its own. I say that the mass loaded vinyl has to have a BT-400 bass absorber directly in front of it. The BT-400 on its own goes down to 50Hz or so when placed across a corner of a room. The mass loaded vinyl just extends the bottom cut off a little. The volume behind the mass loaded vinyl can also be packed with acoustic abosorption material to prevent wall reflection, or a least slow it down:-)

Edited by garyjac
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@@garyjac - without having done the maths, my suspicion is that the presence of such a heavy layer behind an elaborately shaped piece of foam will not increase the low frequency absorption.  In fact, the heavy layer causes a pronounced impedance discontinuity causing a high reflection coefficient and thus lowers the absorption coefficient.

 

Have you tested the vinyl arrangement you recommend?

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Yes, mathematics involved, or not, I use an arrangement similar to that described and it does work, but I use a single layer of MLV, not two. There are other absorbers in the room, so I find measurement to be the only satisfactory way to assess the outcome in place of a series of differential equations. The room measures pretty "flat" at the listening position to under 40Hz and that is good enough for my needs. I never pooh-pooh the value of predictive models but they only take us so far. I am not claiming some magical process or similar nonsense, and this does not require any "faith" as anyone can do the same and confirm or deny the result.

P.S. "Flat" in these terms might be 4dB plus or minus in this situation, we're not talking amplifiers here...

Edited by garyjac
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It would be interesting to compare the modal decay in your room with and without this mass loaded vinyl assembly in place.

 

I agree with your comment that prediction accuracy for an entire room may be limited, however; standard acoustic models are very accurate when it comes to judging the random incident absorption coefficient for such a multi-layered absorber.

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Yes, it would be interesting and since I'll be moving shortly ( oh the pain) I will have the opportunity to do that, so I think I will, since it will be of benefit for the new room, which is similar in layout. For what it may be worth, the Sydney Opera house and the Berlin opera house, as originally designed, stand as models of our state of ignorance when it comes to modelling spatial effects with applied mathematics. As usual, this probably means we're measuring/calculating the wrong things, but eventually it will be worked out if anyone can make a dollar out of it.

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Hi everyone

 

Im about to do some sound Treatment in my home theatre set up...Iv notice there's different types of shapes (wedge n egg shell))...Is there any difference, as in performance wise??

 

Also with bass traps is thicker the better??

 

Regards

 

Obama

 

The differences are more cosmetic than performance related. A 50mm thick panel should have reasonable absorption down to around 300 Hz. As such, the foam panels, especially the flocked ones which have a nice finish, are a pretty simple easy solution. They are easier to mount than DIY panels that tend to get much heavier.

 

With resistive broadband traps, bigger, wider, deeper, thicker, located in many corners is always better. In a large number of rooms, however, using the available space and sizes that are domestically acceptable, the result can at times be discouraging.

 

Don't get me wrong. When you go to town on it, the result can be quite spectacular, but before you make that decision, you want to know what problems you are dealing with ie how good your room is to start with.

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For what it may be worth, the Sydney Opera house and the Berlin opera house, as originally designed, stand as models of our state of ignorance when it comes to modelling spatial effects with applied mathematics. As usual, this probably means we're measuring/calculating the wrong things, but eventually it will be worked out if anyone can make a dollar out of it.

 

I'm not sure what you are implying with the above. The earlier discussion was in regards to acoustically small rooms whereas your examples are acoustically large rooms.  

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