Thursday, February 24, 2011

So many traps, so little time...

[This post is under construction.  I am trying to consolidate and summarize the difference between competing bass traps here, but it takes time to gather all the information.]

I have become convinced of the need for bass trapping, and basically agree with the assertion by Real Trap's founder Ethan Winer that it's almost impossible to have too much bass trapping (and this is much different than HF absorption, where you can definitely have too much and maybe don't even need any).

Maybe the walls themselves should be designed as bass traps.  Thicker, with solid masonry outer wall, stuffed with fiberglass and lossy plastic membrane surface instead of gypsum board.

I'd love to get the high performance Megatraps from Real Traps, but they are monsterously big and I can's see right now even where I could put them.  (The price is very reasonable for the performance provided.  I just meant I don't want to buy something I might not ultimately be able to use because it's so big.)

Here are the comparison measurements of various Real Traps units compared with a few others.   Well, it turns out that doesn't include the megatraps, all that's available for them is on the Megatraps page linked in previous paragraph.  It shows, per pair of megatraps, 8 Sabins reduction at 50Hz, 28 Sabins (!) between 70-125Hz.  Sabins/dollar at 50Hz is 0.016, and 0.056 at 70-125Hz.

Somewhat comparable to Megatraps are the RPG Modex Corner, it's more expensive, slight different form factor, and comes in 3 quasi-tuned versions.  Similar membrane-fiberglass design.

Real Traps brag about how Megatraps is the best.  OK, but on a cost-effectiveness they're not that much (well, a factor of 2-3x) different from acoustical foam traps, mainly because acoustical foam corner traps are much cheaper.  At most, they might be 2x or therabouts more effective per dollar spent than foam (fiberglass is about 2x as dense as foam also), and actually, the numbers I have (from promotional websites which may be misleading) suggest that some foam traps might even be better on a cost basis.

Of course, the long run issue for me may not be the cost per unit of absorption, but the quantity/quality of bass reduction per unit volume.  In the long run, there is only so much room in the room for traps, and one would want to make the best use possible of whatever space can be made available.

But I am considering the cost factor as I'm doing this very experimentally.  I don't really know what I can or need to do.  It might be worthwhile to buy some cheaper traps that I can fit now.  If I replace them later with something better, I can re-use the traps somewhere else.  So I'm looking at the cost factor to see how much I'm paying for the experiment vs how much good it might do.

Yesterday I finally started looking at foam traps.  One of the best well known is the Auralex LENRD, been around for a long time, gets good reviews, you can buy 1 for $40 and it will have between 1/10 and 1/20 the performance of a pair of Megatraps according to specs.  For the price of two Megatraps you could buy 12 or more, and as I said, it looks like that would be in the same ballpark of performance, maybe 1/2 as good, maybe not, maybe better.

Lets look at 50Hz, where I seem to have some of my worst modal problems (though that too requires more analysis), 100 Hz, and 125 Hz.

Here are the measurements on the Auralex LENRD traps. measuring a total of 48 (!) traps, they found total absorption of 59.62 Sabins at 100Hz, 76 Sabins at 125 (no measurement for 50).  That amounts to about 1.24 Sabins per unit at 100 Hz.  Per dollar that is 0.03 Sabins/$, and one could expect about 1/3 that at 50Hz (so estimate less than 0.01 Sabins/$ at 50hz).

The measurements done by Real Traps (they test a 4 ft section of 2 LENRD's) and find it has 0.52 Sabins at 50Hz.  That would be 0.26 Sabins per unit, even less than I estimated above.  So at $40 cost, that means 0.0065 Sabins/dollar, about 2.5x less effective per cost than Megatraps.  (Before I did this calculation, I had been all set to get some).  They show 5.2 Sabins/pair at 100Hz, which is 2.6 Sabins per unit, or 0.065 at 100 Hz.  So at 100Hz, Real Traps measurement is 2x what Auralex itself shows.

GIK makes Tri Traps, big foam traps for tri-corners.  According to these measurements, 8 GIK tritraps have a total of 52.2 sabins at 50hz, 98.5 sabins at 100Hz, and 102.92 sabins at 125 Hz.  Per trap, this is:


They sell for $250/2 is an excellent value, 0.05 Sabins/$ at 50Hz.  Each is 4 ft high and "2 feet wide" which probably means the open face, more like 17 inch extension on each side.

Looks to me like GIK Tritraps are one of the best values, much better value than Auralex.  A poster named Shadome at Audiogon has posted a comparison of Auralex and GIK corner traps and notes that the GIK were tested in corner, while Auralex was tested against wall, giving GIK about a 2x advantage. In his estimation, GIK is only about 1.6x more effective.  In my estimation, his estimation looks a bit low, I'm thinking corner has more than 2x advantage, so I'd guess more like 3x advantage.  (Note if he was right about 1.6x advantage, Auralex would be the better value, because GIK traps cost about 2x as much).

Anyway, this analysis suggests Auralex isn't as bad as I first thought, though I wouldn't trust it for deepest bass (50Hz) and I still think GIK is the best value...if you have room for the 17" sides vs 12" sides.

GIK doesn't actually show the side dimensions on their web page.  Here's an interesting review that includes that and other information.

Previously, I purchased These S.T.O.P melamine foam bass trapsfor about $90 each.  They don't look to be quite such a good value anymore; deep bass absorption isn't even specified, only 125Hz, for which they have 0.036 S/$.  One might expect the performance at 50Hz to be a fraction of that, 1/3 or less.  Anyway, the white color (and Class A flame resistance) are appropriate for the kitchen above the refrigerator, helping to suppress refrigerator noise (which is surprisingly high between 60-180Hz even though it sounds like a high frequency noise).

The Foam Factory has corner bass solutions, looks like performance is similar (maybe slightly less) than Auralex, but at less than 1/2 the cost.  They have several different varieties too, including the usual Auralex style wedges, blocks, and cubes, all about the same price per pound of foam.  But on further investigation, this looks like IS IDENTICAL to the very mediocre "Foam By Mail" foam that tests extremely poorly (as in not worth bothering) below 100Hz.

Tube Traps (from Acoustical Sciences Corporation and designed by Art Noxon) have been around for a long time.  They are said to be good by competitor Ethan Winer (maker of Real Traps) but just rather expensive for the performance provided, make somewhat inefficient use of corner space (otoh, very easy to install or move), and the diffusion grid concept is not as good as purpose-built diffusers.  And he says, for bass trapping, be sure to get the 20" versions.  He thinks they are fine for corners and look nice.  He thinks his own panel absorbers are a better choice (and far cheaper!) for flat walls.  ASC on the other hand sometimes doesn't put them in corners, but creates whole walls of them.  Note that such walls of Tube Traps can be astronomically expensive, one guy spent $38,000.

Now that I see that they are completely sealed (cardboard surrounds fiberglass, then further inclosed in structure and cloth) I feel comfortable with putting them near HVAC intake.  They may actually be better "sealed" than Real Traps (!) which rely on cloth cover to prevent fiberglass leakage (though there is also a "membrane" which prevents that also).  I suspect in either case, actual fiberglass leakage may be less than what is simply tracked into the house from the garage.  Ar

Here is Art Noxon's original AES paper describing the design of Tube Traps.

The price is formidable indeed.  A 4 foot 20" round tube trap costs $854 on the factory direct pricelist.  As a 4 foot high corner section, that's equivalent to the 2 GIK Tri Traps you can buy for $259, or the pair of Megatraps you can buy for $500 (though most likely the Megatraps have the highest performance of these options because they are the biggest...almost value comparable with the GIK...the GIK still looks like the best value, especially since I can't see how I could fit the larger units).

What would really be cool, IMO, would be an 8' tube would fill up the whole corner (like by my HVAC intake) pretty nicely (though...even with curvature there would be some blockage to HVAC intake.  The price for taller models isn't listed, one can guess it's probably at least twice the price of the 8 foot section, so $1708.

The best option for that corner might be to forgo trapping on the very bottom, then start trapping just above the HVAC intake.  Thus there would be no blockage right in front of the HVAC, but there would be blockage of intake *above* the vent, not sure how crucial that is.

Another option would be to have tube trap in the bottom section, then GIK on top, less blockage below.

One idea occurs to me now: cover up existing HVAC intake and turn water heater (beneath HVAC) door in kitchen into new HVAC intake.  (I've now spent days thinking about this.  It *could* work, there is plenty of space on door for needed cutout, unless done very skillfully with some kind of wood or metal grille, it would be very ugly.  I will probably need cabinet maker to do the needed work.

Another company that makes traps is Primacoustic.  Their bass traps include two thicknesses of flat panel, a solid foam wedge (nicer looking than Auralex but also way more expensive), and a flat triangle tri-trap that looks lighter than the Real Traps version, but probably easier to mount.

As a unique solid (not sculpted) foam, the Australis looks especially interesting to me.  I think solid foam should be better than sculpted for low frequencies.  (Though, one could also get Auralex cubes...)  You get 6 linear feet of corner damping for $299.  You get the much bigger fiberglass-based GIK traps for slightly less, and get 8 linear feet.  So the Australis does not look like the value leader.  But the small and relatively light traps can easily be hung...they are expressly designed for hanging while the GIK are designed for stacking.  Though the specs suggest the Australis works competitively, a 12" extension foam trap just cannot be as good as a 17" extension membrane/fiberglass/airgap/panel trap.

The beauty of the Australis is it's easy mounting.  You can always use them somewhere, even at ceiling wall corners.

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