Bass trap kraft paper - Home Theater Forum and Systems -

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post #1 of 4 Old 10-26-11, 08:11 AM Thread Starter
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Question Bass trap kraft paper


I'm trying to get a definitive answer on a question if poss.

I am building bass traps and superchunks for the corners of my room and I want to know if I should use brown kraft paper on the front face of the traps and chunks to increase their efficiency.

I've read so much stuff across the web that I'm in a spin about what is the right thing to do - Nothing on, or thin paper / thick paper, pulled tight / not pulled tight to avoid reflections?

Has anyone done this and have some concrete ideas?

I start to build this Friday and at this point I'm not going to put any Kraft on.

All ideas would be appreciated,

Cheers, Ste.
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post #2 of 4 Old 10-26-11, 08:25 AM
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Re: Bass trap kraft paper

It depends on what you want to accomplish - though the kraft isn't going to increase efficiciency as much as stopping some upper mid/high frequency absorption. If it is going on a flat board on the front and is fully bonded, it will produce a 'hump' in the absorption curve at some frequency based on the material behind, space behind to boundary, mass of kraft, etc.

Overall, in the front, don't face it. You want the front to be more dead in the mids and highs on the front wall. In the rear, your choice pending the above and how much other high frequency absorption you will have elsewhere in the space.


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post #3 of 4 Old 10-26-11, 09:00 AM
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Re: Bass trap kraft paper

Porous corner traps are broad band absorbers, that are increasingly efficient the higher the frequency.

In most small rooms this can present a problem, as it is a very common even that when such corner traps are used in conjunction with porous pane absorbers that the room becomes over damped - dead.

Ideally we only want to use broadband (~300 Hz and up) absorbent panels surgically to control anomalous specular energy - meaning to ONLY use them to address actual anomalous high gain early reflections. We want to retain the remaining none destructive lower gain energy in the room.

To this end we want to make the porous corner traps function as LF absorbers. One way to do this is to face the unit with a surface that is reflective to specular energy.

In order to do this a variety of materials can be used. The lightest material that you want to use is 6 mil plastic sheeting. This will reflect the mid-high energy while functioning effectively as a high pass filter, absorbing the low frequency energy. It can be 'heavier' or thicker, such as paneling or MDF sheet, but you do not want to go thinner, as test indicate that such material is not exhibit sufficient reflectance.

Actually, in testing, a 6 mil covering will exhibit a decline in absorption of energy (meaning it will become reflective) beginning at ~630 Hz and up. Any thinner and the reflective performance is simply inadequate.

One additional thing that one will want to be reflecting the energy, you want to be sure that you do not reflect it into the listening area. Instead you want to redirect it outside of the listening area toward the back of the room where it can then be diffused and returned laterally to the listening position.

This can be done relatively easily by making the corner traps slightly asymmetric, such as making the front wall union 2'4" and the side wall union 2' long. This will angle the face out slightly towards the back of the room and will redirect the reflected energy slightly to the side of the listening position.

If one has questions about the actual direction of the reflected energy, the ETC response will easily indicate the effect of the reflective surface and whether it is directed toward the listening position.
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post #4 of 4 Old 10-31-11, 11:40 PM
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Re: Bass trap kraft paper

Note that most anything said about any configuration that doesn't have good quality (from a lab such as Riverbank) measurements published is somewhat of an educated guess (even if based on loads of experience). Plus there are vagaries with each room, so unless you contract with a pro acoustician using advanced modeling software there will be some level of install-and-test involved. With that bit out of the way:

I built in-room bass dampers the outside of which is fluffy with the regular kraft facing out into the room and I'm very happy with the result. Ethan Winer and Glenn Kuras often say that if you are going to use a membrane facing on your bass dampers, one of the keys is having it bonded to damping material rather than just "over the front" of the device - which is why fiberglass batts or faced 70x panels are convenient. Otherwise you may consider testing it with the membrane free and with it spray-glued to your damping material.

With acoustics, though, it is never so simple. If you look at absorption data of faced devices, they get a ~100Hz absorption "bump" that indicates it has a presumably greater absorption within a band about that value. However if you play with enough tuned devices, thickness and spacing of absorbers etc in e.g. Whealy's porous absorber calculator, you will find that often when a device exhibits that response, it has a bit lower absorption (than a device that doesn't exhibit the "bump" such as an unfaced thick fluffy bass damper) below some value ~50Hz. Note that the 100Hz bump can be a feature even of unexpected configurations such as the unfaced superchunk setup, which Eric Desart has noted unexpectedly has what appears to be a response curve resembling a device with a membrane (below 200 Hz at least - at HF it should response as an unfaced device). The performance <50Hz would be a non-issue in a music environment but in an HT it might be a concern.

How you use that information depends on your situation. I'd suggest for most in-room LF damping devices you want them faced unless they are at reflection points you have identified with the mirror method or an ETC analysis, in order to maintain HF ambience in the room. In my situation, however, there is a closet in the back of my room with hollow-core doors in it and testing reveals it is essentially part of the room at LF; therefore I have thick fluffy bass dampers in there without any facing, as my in-room faced devices take care of the higher part of the modal region and the unfaced ones in the closet (which have a sort of "natural" membrane in the closet doors anyway) should more effectively reach the bottom-most modes.

My vote generally would be to (kraft or plastic as SAC suggested) face bass damping devices that are in-room. If they are found to be a reflection point through a mirror-point or ETC analysis, just treat them by mounting whatever type of device you are using at reflection points over top the bass damper. However, the desirability of facing might depend on your personal preferences (I don't like a room that is very dead _or_ one that is very live for example), whether you are using the room for two-channel as well as multichannel, what other acoustic devices you will have in the room, and whether the room is carpeted / has tapestries / has an absorptive drop ceiling, to name a few other possible considerations.
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