Bag End E-Trap

[MidnightSensi]

I've been beginning to look at the treatment of my room. I have some acoustic panels a friend hooked me up with since he is redoing his studio. For the low-end, however, he reconfirmed my assumption that I would require some pretty thick panels to tame the room. In his old studio his bass traps are over three foot thick, and his new one is going to include an entire false wall in order to tame down to 20Hz. This just isn't possible in my room.

EQ helps to an extent for my 50Hz room mode, but it just lowers the level, it doesn't fix the ringing and time problems.

I was at Bag End picking up some gear last year and they showed me their E-Trap.

http://www.bagend.com/

My peak is 5dB without EQ at 50Hz, and this review said he got 4dB at 33Hz and 4.75dB at 50Hz without any trouble.

What are your guys thoughts on this? My room is pretty small, so I don't want to go crazy on bass traps. I can get one of these for under a grand, and when I've been looking at decent bass traps they all seem to end up being significantly more. Is this worth a go?

Here is my system thread, the last couple pages have the current setup: http://forums.audioholics.com/forums/showthread.php?t=53920&page=8

 

[nibhaz]

Just my 2 cents…

IMO, you could do better by going a DIY corner absorber route. You’ll get absorption over a much broader range of frequencies. If you were to do a simple Corner Chunk Absorber using 703 I would estimate that it would cost you around about $300-400 to treat 4 corners floor to ceiling assuming 8ft ceilings and using a cut pattern that results in a 2’ face. This price could be driven down even further using mineral wool, without any loss in performance. You could use Roxul AFB for constructing SCAs and I’m guessing you could cut my estimated cost in half.

For your reading and consideration:

http://forum.studiotips.com/viewtopic.php?f=8&t=535

http://forum.studiotips.com/viewtopic.php?f=8&t=534

Here is a product for easy nested corner builds.


Check out this forum if you not already familiar with it. It has tons of free knowledge about acoustical treatments, and DIY builds.

Just my 2 cents…

 

[MidnightSensi]

I've been reading about bass traps, DIY and budget style. The problem I've seen is that, with the thickness that works in my room, bass traps will handle the bass but really fall off in the sub-bass. I probably will do some corner traps (thanks, btw, for those links, that stuff is better than what I found googling...that little bracket is pretty trick also!), but for the 20-60Hz range it seems like this E-Trap may work better for the size.

By the time someone considers an E-Trap, they probably have a dedicated room where they would consider treatments, but in order to treat for deep bass, it seems like in my room I'd have to make a helmholtz resonator...which, the E-trap looks a lot more compact and seems to do about the same thing but with two frequencies and adjustable.

But, I don't know. This stuff is pretty far above my head.

IMO, you could do better by going a DIY corner absorber route. You’ll get absorption over a much broader range of frequencies. If you were to do a simple Corner Chunk Absorber using 703 I would estimate that it would cost you around about $300-400 to treat 4 corners floor to ceiling assuming 8ft ceilings and using a cut pattern that results in a 2’ face. This price could be driven down even further using mineral wool, without any loss in performance. You could use Roxul AFB for constructing SCAs and I’m guessing you could cut my estimated cost in half.

For your reading and consideration:

http://forum.studiotips.com/viewtopic.php?f=8&t=535

http://forum.studiotips.com/viewtopic.php?f=8&t=534

Here is a product for easy nested corner builds.


Check out this forum if you not already familiar with it. It has tons of free knowledge about acoustical treatments, and DIY builds.

Just my 2 cents…

 

[jostenmeat]

For the low-end, however, he reconfirmed my assumption that I would require some pretty thick panels to tame the room. In his old studio his bass traps are over three foot thick, and his new one is going to include an entire false wall in order to tame down to 20Hz. This just isn't possible in my room.

Is your friend's room with such thick panels made of concrete?

EQ helps to an extent for my 50Hz room mode, but it just lowers the level, it doesn't fix the ringing and time problems.

hmm

It's difficult to get substantial absorption below about 80 Hz using a fiberglass-based trap. Fortunately, as explained earlier, most rooms don't need much absorption below 80 Hz because sheet rock walls pass and absorb those very low frequencies. So unless you have walls made of cement or block, or have two layers of sheet rock for increased isolation between rooms, fiberglass bass traps are an ideal choice. Further, a parametric equalizer can be used to minimize resonance or modal problems at the low frequencies. You should always try to minimize bass problems passively, with a combination of speaker and listener position and appropriate bass traps before using a parametric. However, there may be practical limitations in the room, whereby you can not put in as many bass traps as you would like, or you may not be able to move the speakers into their optimal position. In cases like these, parametric equalizers, are ideal.

from this AH article by the owner of Real Traps:
http://www.audioholics.com/education/acoustics-principles/bass-traps-not-just-for-fisherman

that I originally learned about from this thread,
How to treat a concrete room.
http://forums.audioholics.com/forums/showthread.php?t=59572

 

[MidnightSensi]

Is your friend's room with such thick panels made of concrete?

Not sure, I know his rear wall of the new studio uses quarter wavelength absorbers.... I think the largest is up to 10ft between the entrance and the absorbtion material. There are a couple difference lengths. I know the formula is F=1130/4D (D = distance in feet). So to handle my 50Hz issue it would require a almost 6ft deep chamber. That's why the panels mounted off of the wall do a better job, but it still isn't low frequency.

My room is constructed of reinforced concrete on the front wall and left wall, with a 3" spaced drywall layer. The rear and right walls are concrete. The doors are solid wood. The floor is concrete slab, ceiling is drywall but heavily insulated. The room is so tight, that if I leave a door half open, the subwoofers will shut it. At high volume levels with a open door, you can feel the pressure waves.... literally enough to move women's hair. Also if you shut the door quick enough, people in the room can feel it in their ears. It's that tight.

I can't complain too much though, as far as I can tell this room is the flattest of any room I've had. The problems are nearly all only in the deep bass. I have a nearly silent noise floor (even the A/C noise is inaudible, and what fan noise my amps have is mostly muffled by the rack), and my walls don't really 'talk' as I tend to call it (where the wall vibrates and colors the mids and highs), and I have so many irregular surfaces that standing waves aren't too bad till very low.

The downside of it is I have almost no low frequency absorbtion. Near the listening positions, they are all peaks, which is handy because a dip in this room would be very hard to deal with.

So, as of now, I use EQ... but that doesn't solve any time issues or ringing. Everything I read always starts talking about deep bass but they just start talking about bass traps...which would be great for 100Hz but doesn't help me down low. I need a 'sub-bass' trap.

Where I go curious in the E-Trap is because it seems to work down low where I need it the most and it is pretty small. As long as it works.

 

[bpape]

First, I would completely and utterly disagree that there isn't much absorption and bass control needed below 80Hz.

That said, I've heard both ways on the ETrap. For very very deep bass, it's supposed to work pretty decently. For 50Hz? I'm not sure I'd spend the money over some more standard approaches. You can have an impact at 50Hz with something around 8" thick.

Bryan

 

[MidnightSensi]

First, I would completely and utterly disagree that there isn't much absorption and bass control needed below 80Hz.

Me too, especially in my room.

That said, I've heard both ways on the ETrap. For very very deep bass, it's supposed to work pretty decently. For 50Hz? I'm not sure I'd spend the money over some more standard approaches. You can have an impact at 50Hz with something around 8" thick.

Bryan

Not trying to be arguementative, but, your Monster Bass Traps are 7.5" thick, and from the report I read on your website they are great at 90Hz and then have basically no absorption below 60Hz, even when offset from the wall.

Also, the ETrap is like 800 bucks, so I'm not sure it really is more expensive than even buying bass traps for the same money... and it sounds like it is more effective down low.

<embed id=VideoPlayback src=http://video.google.com/googleplayer.swf?docid=302675073313804539&hl=en&fs=true style=width:400px;height:326px allowFullScreen=true allowScriptAccess=always type=application/x-shockwave-flash> </embed>

<embed id=VideoPlayback src=http://video.google.com/googleplayer.swf?docid=-3361769025189369598&hl=en&fs=true style=width:400px;height:326px allowFullScreen=true allowScriptAccess=always type=application/x-shockwave-flash> </embed>


So, so far this looks like my best option... but it bothers me that it seems to be the only option (obviously beyond converting my rear closet and bathroom into a giant bass trap). Also, if it is another 'speaker' per say, would it induce distortion? And with my twin Danley subs, would its little driver be able to keep up or since its such a thin band would it be alright?

Hm, I need to do more research.

 

[jostenmeat]

Midnight, forgive me if you had already told me that your room was mostly concrete. I now have this nagging feeling you might have at one point, but Iono.

That E trap is fascinating! How the heck does that work, especially when it is so diminutive? Wow.

I like the Helmholtz idea, but when I looked back at your build, not only do you not have a lot of room for a second row (muahaha) which also blocks access to the doors in the rear, but your present seating as it is doesn't lend itself to a riser either, unless it perhaps had an enormous footprint. But, I guess some people make their risers to double as Helmholtz resonators. That stuff is still beyond me, but if it ever interests you, I would maybe PM Savant.

He did his best in helping me understand if I could modify my riser to act as such a resonator, but, um . . . I'm leaving things alone! lol, not that I feel like much choice in the matter . . .

http://forums.audioholics.com/forums/showthread.php?t=27511&page=4

But, if you built one from scratch, your odds of success would be much greater than mine.

E trap is looking interesting. What I want to know is how effective it is, and if it happens to be more effective at a particular range of freq than another, or you know, things like that . . .

 

[MidnightSensi]

Midnight, forgive me if you had already told me that your room was mostly concrete. I now have this nagging feeling you might have at one point, but Iono.

I may have mentioned it in my theater thread at some point, not sure. At any rate, I wouldn't expect you to remember my house construction.

That E trap is fascinating! How the heck does that work, especially when it is so diminutive? Wow.

I'm not 100% sure, but I think it works about the same way as a bass trap does. It absorbs the energy/sound. So, to absorb sound a traditional bass trap will flex and translate acoustic energy into heat energy. The E-Trap does the same thing, but instead uses a speaker cone to do the dampening at a 'tuned' frequency. Then there is a microphone that tells it how much dampening to apply. So, basically you set it to your two primary modes in that range and it is supposed to provide dampening.

Where I get lost is this: If it only dampens a tuned frequency, how much can it dampen? I have enough subwoofer horsepower to kill small animals, is this little box going to even make a dent when I'm watching a Blu-Ray at reference?

If I get a chance tomorrow I'm going to call Jim at Bag End. I've spoken to him before and he is good at explaining these sorts of things. If I get a hold of him, I'll let you know what he says.

 

[bpape]

Me too, especially in my room.



Not trying to be arguementative, but, your Monster Bass Traps are 7.5" thick, and from the report I read on your website they are great at 90Hz and then have basically no absorption below 60Hz, even when offset from the wall.

Also, the ETrap is like 800 bucks, so I'm not sure it really is more expensive than even buying bass traps for the same money... and it sounds like it is more effective down low.

<embed id=VideoPlayback src=http://video.google.com/googleplayer.swf?docid=302675073313804539&hl=en&fs=true style=width:400px;height:326px allowFullScreen=true allowScriptAccess=always type=application/x-shockwave-flash> </embed>

<embed id=VideoPlayback src=http://video.google.com/googleplayer.swf?docid=-3361769025189369598&hl=en&fs=true style=width:400px;height:326px allowFullScreen=true allowScriptAccess=always type=application/x-shockwave-flash> </embed>


So, so far this looks like my best option... but it bothers me that it seems to be the only option (obviously beyond converting my rear closet and bathroom into a giant bass trap). Also, if it is another 'speaker' per say, would it induce distortion? And with my twin Danley subs, would its little driver be able to keep up or since its such a thin band would it be alright?

Hm, I need to do more research.

Not sure what you're referring to. In a typical A mount, the Monster is .47 at 50Hz. In a straddled corner mount, it's .58. No, it's not a 1.0, but it's still definitely doing something.

Even with a solid 8", it's not going to be 1.0 or close to it, but it is having an impact.

Bryan

 

[nibhaz]

Faulty Logic...

Sensi,

I’m no expert in this field, but it is my understanding that absorption measurements below 100 Hz tend to be inaccurate do to the physical size of the waves involved in relation to the test chamber. So if you are reading the absorption reports on the GIK website, I don’t believe that you are getting a real # to properly assess the low frequency performance of their Monster Bass Traps, especially when corner mounted.

In an attempt to get a # for you to look at that might be meaningful I took some leaps of logic which may or may not hold water, but here goes. If you built a solid corner absorber using 18”X18”24” wedges 50% of the absorber would be at least 9” thick. So I then used the porous absorber calculator to calculate the effectiveness of a 9” panel mounted flat on the wall. Keep in mind that this tool is crude, and I am severely simplifying what’s going on in a corner absorber. Also keep in mind that you are only looking at a single angle of incidence per graph. The real world performance of the entire corner absorber should be much greater than these graphs portray.

Using OC 701



Using OC 703






Really looking forward to what you hear from Bag End !

 

[MidnightSensi]

I talked to Jim at Bag End today. He was pretty honest with me, and said that before I buy it, I need to test my room using a oscillator. He said that my dbX and Audyssey don't really have the resolution to find where a ETrap would need to be set for it to be perfect, but it would be close. He said with a oscillator I could get dead on.

Sine wave oscillators are harder to find than I thought. I want one with a knob where I can 'play' and find the trouble frequencies, and be able to move the knobs on the ETrap to get it to lock in.

He said the maximum SPL output for one ETrap is 110dB. But, it's not making 'bass', it's just dampening, so that doesn't mean it would run out of steam in a typical home theater application because its just trimming peaks. He said that if I do run it out of steam, it will go into protection and flash a red light.

Not sure what you're referring to. In a typical A mount, the Monster is .47 at 50Hz. In a straddled corner mount, it's .58. No, it's not a 1.0, but it's still definitely doing something.

Even with a solid 8", it's not going to be 1.0 or close to it, but it is having an impact.

Bryan

I guess I'm too much of a novice to understand what that means for my room. It looks from that graph that, relatively speaking, it does basically nothing for the last octave. It looks like it would be great for controlling my midbass though.


....

So, the search continues. I'm going to look for an oscillator to play with.

 

[MidnightSensi]

Sensi,

I’m no expert in this field, but it is my understanding that absorption measurements below 100 Hz tend to be inaccurate do to the physical size of the waves involved in relation to the test chamber. So if you are reading the absorption reports on the GIK website, I don’t believe that you are getting a real # to properly assess the low frequency performance of their Monster Bass Traps, especially when corner mounted.

In an attempt to get a # for you to look at that might be meaningful I took some leaps of logic which may or may not hold water, but here goes. If you built a solid corner absorber using 18”X18”24” wedges 50% of the absorber would be at least 9” thick. So I then used the porous absorber calculator to calculate the effectiveness of a 9” panel mounted flat on the wall. Keep in mind that this tool is crude, and I am severely simplifying what’s going on in a corner absorber. Also keep in mind that you are only looking at a single angle of incidence per graph. The real world performance of the entire corner absorber should be much greater than these graphs portray.

Using OC 701



Using OC 703






Really looking forward to what you hear from Bag End !

Well it looks like their testing stopped for the last octave, so your probably right.

What does a 1 mean versus a .5? I'm thinking of a transfer function... like take the magnitude and multiply it by the absorption coefficient...

So if it is a .5 it will take half of the bass floating away? Does that mean on the second pass it will take another .5 ... and then another .5 .... so exponential decay?

 

[bpape]

I talked to Jim at Bag End today. He was pretty honest with me, and said that before I buy it, I need to test my room using a oscillator. He said that my dbX and Audyssey don't really have the resolution to find where a ETrap would need to be set for it to be perfect, but it would be close. He said with a oscillator I could get dead on.

Sine wave oscillators are harder to find than I thought. I want one with a knob where I can 'play' and find the trouble frequencies, and be able to move the knobs on the ETrap to get it to lock in.

He said the maximum SPL output for one ETrap is 110dB. But, it's not making 'bass', it's just dampening, so that doesn't mean it would run out of steam in a typical home theater application because its just trimming peaks. He said that if I do run it out of steam, it will go into protection and flash a red light.



I guess I'm too much of a novice to understand what that means for my room. It looks from that graph that, relatively speaking, it does basically nothing for the last octave. It looks like it would be great for controlling my midbass though.


....

So, the search continues. I'm going to look for an oscillator to play with.

Look at the graph of the Monster carefully. The very bottom of the graph is a 1.0 (theoretically perfect) coefficient. Due to the side area exposed and the way lab measurements are done, they just do that much more above a 1.0 higher up.

Bryan

 

[nibhaz]

What does a 1 mean versus a .5? I'm thinking of a transfer function... like take the magnitude and multiply it by the absorption coefficient...

Great question Midnight!

My simple caveman mind has always interpreted that Y axis to represent the materials sound absorption coefficient ,which is the fraction of energy striking the material or object that is not reflected. So I’ve just viewed .5 as 50% absorption, I know simple right…maybe to simple.

So if it is a .5 it will take half of the bass floating away? Does that mean on the second pass it will take another .5 ... and then another .5 .... so exponential decay?

I see your engineering mind cranking on this and if you go to much further I probably won’t be able to follow.

I know that the bass doesn’t float away, but rather is converted to heat energy The exponential decay seems very logical to me…but I don’t know the definitive answer sorry.

 

[bpape]

In THEORY, yes, it would equate to percentage. In the real world, not quite so.

Actually, the BEST way to see what you're getting is to look at the amount of Sabines absorbed per unit area. That can then be directly multiplied by the number of units and get a total number of Sabines.

Bryan

 

[MidnightSensi]

First off, I appreciate you guys discussing this with me, it's interesting!

....

Let's say I play a 50Hz sine wave with no acoustic panels and I measure 100dB on my SPL meter.

Now lets say I add some bass traps (say 3 sq meters worth, since I think Sabin is a metric unit) that have a .5 coefficient at 50Hz, what will the SPL meter read (in theory)?

Maybe I'm overthinking this, but, I guess the concept of these units is difficult for me to understand in usable form.

Basically, will bass traps be good enough at absorbing low frequencies to tame my deep bass modes....and how do I calculate how much I would need?

SPL is logrithmic, but if I just multiply by .5, that's on a linear scale.

My simple caveman mind has always interpreted that Y axis to represent the materials sound absorption coefficient ,which is the fraction of energy striking the material or object that is not reflected. So I’ve just viewed .5 as 50% absorption, I know simple right…maybe to simple.

I see your engineering mind cranking on this and if you go to much further I probably won’t be able to follow.

I know that the bass doesn’t float away, but rather is converted to heat energy The exponential decay seems very logical to me…but I don’t know the definitive answer sorry.

That's how I see it too, but, if 50% of the energy isn't converted, then when it reflects back again I wonder if it reduces another 50% and so forth. So, 50% of original the first cycle, 25% of the original the second, 12.5% of the original the third, and so forth.... So, spread over time the absorption with panels would be greater by a square than without at a .5 absorption coefficient.

....

I guess I say that to say this: I have no professional equipment. I have a SPL meter, some sweep CDs and my ears. I can 'hear' my room resonate at some frequencies and read it as an increase on my SPL meter. This happens to translate with more or less what should mathematically be the room modes for my dimensions. I can also 'hear' that after I EQ the sound increase, the time delay problems/ringing still are there, making my bass lose some definition, but I don't have the equipment or know-how to measure it. Well, yet. The one thing I have learned is while EQ is holding me over for a while, it really is a poor bandaid.

In THEORY, yes, it would equate to percentage. In the real world, not quite so.

Actually, the BEST way to see what you're getting is to look at the amount of Sabines absorbed per unit area. That can then be directly multiplied by the number of units and get a total number of Sabines.

Bryan

 

[jostenmeat]

I don't think Sabins are metric. From an RT article:

You can convert sabins to an equivalent absorption coefficient by simply dividing the sabins by the square feet of front surface area.

http://www.realtraps.com/art_measure.htm

 

[bpape]

The SPL meter may still say 100db. Might say more, might say less. Depends on where you are and if 50 happens to be a modal issue. The big thing is how LONG it reads higher.

If I play 50Hz at 100db and it takes 3 seconds to decay to 40db, that's a problem. Then, if I add some bass control and now it only takes 1 second to decay to 40db, that's much better (in a residential sized room) but still not good enough.

As for taking sabines and dividing by the area, that's how the measurements are made to go into a coefficient. That's why you can have a number bigger than 1.

For instance, the Monster is 7.5" thick and the sides are exposed. So, that's 8 sq ft each PLUS 7.5"x24"x2 PLUS 7.5"x48"x2. However, when the standard (and the labs) calculate, they only take into account the square footage of the face (8 sq ft) even though there is more than 50% more area exposed and absorbing sound. And, in reality, that's what it will really do in your room.

Sabines is actually a more accurate way to look at things as it looks at a relative difference between an empty room and a room with a proper sample size in it.

Not a perfect standard but at least everybody does it the same.

Bryan

 

[MidnightSensi]

I don't think Sabins are metric. From an RT article:



http://www.realtraps.com/art_measure.htm

Not sure why they say that, because I can't confirm that anywhere else. From what I have found, one m^2 of totally absorbing material has a value of one metric Sabin, which keeps the units m^2 sabin. Now, if Real Traps is talking about finding the coefficient, then as long as the units cancel it doesn't matter...

A=sum(S * a)

So, A (sabines) = (Surface area of my carpt)(absorption coefficient of my carpet) + (surface area of my wall)(absorption coefficient of my wall) + (surface area of my doors)(absorption coefficient of the wood) + and so on and so on.

where:
A= Sabin absorption of the room
S= Surface area
a=absorption coefficient of the room


... but Sabin seems to be in m^2 everywhere I took, so if we use m^2 for the Sabins and then ft^2 for the surface area it will throw us off. A lot of places just say Sabin though, and don't specify



As far as the SPL drop/reduction, I kind of answered my own question sniffing around the journals database at work. The equation is:

SPLlisteningposition

SPL = SPL@Source + log[D/(4*Pi*r^2)] + (4/r)

where:
SPL = The estimated SPL at a given frequency in dB
SPL@Source = The SPL at the speakers in dB
D = Directivity coefficient (1 for a typical sweet spot, closer to walls is 2), unitless
R = Room Constant (m^2 sabine) .... the added room constant, which you use that formula on Real Traps website)
r = Distance to source (basically, the radius if your speakers were positioned in a circular fashion) in meters.

So to see how much a given set of treatments would help, I would do this for my untreated room and then again with the new room constant factoring the surface area of the fabric/panels.

Looks like if I took GIK's panel testing numbers, for example, at each frequency, I would set up a spreadsheet and it could estimate what the change would be.

That seems to only be for magnitude though, not sure how time factors in.

I haven't tried the formula yet, because I need to calculate my room's constant...which is kind of tedious, but may be interesting to compare a ETrap versus a GIK Bass Trap or something.

...

I'm left with more questions though... like, if surface area is the how the coefficients are determined, then is that why some of the foam traps have the triangles all over them? And if a panel is made with 45-deg triangles, what is the thickness? Is it the average depth to the wall? What about the distance normal to the triangles? That would be much deeper... I wonder if that's why the triangle bass traps are popular, because a triangle would maximize your surface area and depth.


Ah, another thing to play with for my system. Just as I thought I was running out ....