Explain function of "ported" bass trap

Sandersd · Post by **Sandersd** » Fri Jan 13, 2006 10:58 am

So far I've found five different types of bass attenuators:

Type 1 - Absorbent Corner Trap, corner absorber, fabric faced over insulation, no resonate panel, hanging absorbant panels. (like first drawing)
Type 2 - Resonate Corner Trap, corner absorber, fabric faced over insulation, resonate panel on outside face, sealed cavity, .
Type 3 - Ported Corner Trap, corner absorber, fabric faced over insulation, resonate panel on inside face dampened with insulation, sealed cavity, ported on bottom, hanging absorbant panels. (like second drawing - section thru corner soffit)
Type 4 - Resonate Wall Absorber, Like RealTraps, sealed rectangular cavity of specific depth with resonating panel and insulation spaced 1/2" from panel, frequency depends on depth and panel rigidity.
Type 5 - Dampened Panel Absorber, Like RealTraps, sealed rectangular cavity of specific depth but with resonating panel dampened by insulation glued to panel, frequency depends on depth and panel rigidity.

Below are two drawings:

- a corner structure with hanging bass absorber panels that could be modified to create one of the corner absorbers listed above,

- a section thru a corner mounted soffit with a bass trap on the bottom.

It seems the "ported" corner trap is something of a hybrid of the type 1, 2, and type 5 traps listed above: a cross between a purely absorbent trap and a purely resonate trap.

Analyzing the section below thru the soffit, I understand the location of the port at the point of greatest pressure- near the floor and ceiling. I understand a sealed cavity allows the air inside to work as a spring. I understand the insulation on the front absorbs reflections from the rear of the mixing console.

Here is the million dollar question: Why put a dampened panel on the front of a corner bass trap? If one wanted an absorbent face on the trap, wouldn't it work just as well without the plywood panel? A plywood panel 5/8" thick is very stiff and will not resonate that much, especially dampened with insulation.

Am I correct in my analysis of the design and functionality of the ported bass trap?

Post by **kendale** » Fri Jan 13, 2006 11:21 am

Aloha,

Why put a dampened panel on the front of a corner bass trap?

My understanding of the "dampened panel" is to absorb reflections. http://www.johnlsayers.com/phpBB2/viewt ... &start=435

The "port" is to assist in the cooling/ventilation of powered monitors mounted in a soffit.

Hope this helps.

Aloha

Sandersd · Post by **Sandersd** » Fri Jan 13, 2006 12:24 pm

Are you sure the function isn't similar to the ports on a monitor, or like that of a bass (guitar) cabinet?

If your explanation is correct, the plywood wouldn't be necessary for LF or HF absorbsion or ventilation for a powered monitor. Surely it has something to do with the air pressure in the cavity and hence the need to seal the space from air leakage.

Post by **kendale** » Fri Jan 13, 2006 7:27 pm

Aloha,

Here's a post that may help shed some light

: http://johnlsayers.com/phpBB2/viewtopic ... sc&start=0

Hope this helps,

Aloha

Sandersd · Post by **Sandersd** » Sat Jan 14, 2006 12:00 am

I'v read that post while researching these questions. It makes statements regarding function but provides no technical explanation from which those claims can be verified. They are also in direct contradiction to the referenced drawing above, which is why I am asking these questions and requesting anyone provide a technical explanation.

I can build anything, I want to know why I should build it a certain way.

knightfly · Post by **knightfly** » Tue Jan 24, 2006 5:23 pm

The ports being at floor and ceiling is right, that's where maximum SPL for all modes occurs so the bass trapping part will work best that way; however, with what I've recently learned about baffle sizing for soffited monitors, I would NOT build with absorbent up front except probably for a surround install - for stereo, it's better that you have a HEAVY, solid baffle extension for as far as you can get it in every direction from the center of the woofer -

Check my first post in this thread for more -

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=5426

And don't forget to check the link in that post for even more...

More on why surround is different -

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=3336

HTH... Steve

Sandersd · Post by **Sandersd** » Tue Jan 24, 2006 11:08 pm

Thanks for your reply Steve. So what is the difference in function and result between a corner absorbent face trap and a corner ported trap in regards to bass attenuation.

Obviously the face of the ported trap will be reflective of mids and highs whereas the other will absorb all frequencies, but I limit my inquiry to the bass frequencies.

Does the ported trap work on the "open window" principle? Please feel free to be as long winded and technical as you desire.

Also, the whole purpose of my research is to confirm the design of my "studio". Please take a look and tell me your thoughts.
http://www.johnlsayers.com/phpBB2/viewtopic.php?t=5389

Thanks

knightfly · Post by **knightfly** » Wed Jan 25, 2006 4:44 am

Thanks for your reply Steve. So what is the difference in function and result between a corner absorbent face trap and a corner ported trap in regards to bass attenuation.

A corner absorbent trap won't go as low in frequency as a ported one, for the same reason a folded horn Klipsch speaker goes lower in frequency with the same size driver than a normal speaker box - a longer sound path equates to lower frequency cutoff point. IOW, the port at the bottom forces sound to only travel in at the bottom and then up through the absorbent, and to get BACK into the room it must travel the same, longer path - this lowers the resonance of the chamber so it affects longer wavelengths.

However, the absorbent trap would affect MORE of the mids (and highs, unless the FACE of the ported version were absorbed as is the design you asked about.

Obviously the face of the ported trap will be reflective of mids and highs whereas the other will absorb all frequencies, but I limit my inquiry to the bass frequencies.

Does the ported trap work on the "open window" principle? Please feel free to be as long winded and technical as you desire.

Mostly covered above; also, a ported trap with HANGERS adds another absorption method that's just slightly different; besides normal absorption, I believe hangers work better for low bass absorption because they are free to move, and because they present a labyrinth path that is absorbent all along its length - it takes energy to move a hanging, padded board so that energy will be removed(re-directed, actually - I'm not saying we are capable of destroying energy) - this works the same as using a series of chains hanging down into the water to shield the area behind the chains from turbulence (as in some marinas exposed to wave action from a breakwater channel ) the water on the other side of the chains is markedly calmer.

And no, it's not as good as an open window, especially at the lower frequencies; it would take a path length of over 8 feet to be optimised at 30 hZ (1/4 wavelength of 30 hZ is approx. 9.5 feet)

Also, the whole purpose of my research is to confirm the design of my "studio". Please take a look and tell me your thoughts.
http://www.johnlsayers.com/phpBB2/viewtopic.php?t=5389

The first problem I see is a square room - your inner walls will help that some, but I'd think you would still have some modes piled up (unevenly distributed) at the low mids and lows, especially with the relatively small size. Only way to completely avoid that is a different room ratio.

The second is the 90 degree speaker layout - I know John has used this, and I've also tried; I didn't like that the phantom center was so "iffy" with speakers that wide apart - it was too distracting to have to "keep my head in a vise". (YMMV)

A possible third - speakers angled downward can cause as many problems as they cure - reasons FOR angling are to redirect early reflections off desk, etc, to somewhere OTHER than your ears - another reason is to get the nearfields out of the direct path of sound from the mains.

Reasons why NOT to angle, at least not too MUCH, are your ears; the pinnae cause frequency response of your ears to attenuate highs when sounds arrive from above. It's thought that this is part of the "fight or flight" legacy - not sure if I believe that part, but the treble attenuation is real - Newell, in at least one of his books, mentions this with a few examples of real life situations about mixes not translating even with $50k speakers (each speaker) - some people don't mind tilting their heads up to listen, down to adjust, up to listen, etc - just something to be aware of.

Fourth - if you can get your triangle back to 60 degree, that will increase the baffle area around your speakers which will widen the sweet spot and smooth out the LF response - placing the speakers a bit closer together should help with the "too far back" problem with mix position -

http://www.quarter-wave.com/General/BSC_Sizing.pdf

From your plan, am I correct in assuming you're NOT concerned with 5.1 surround? This would add a couple more considerations to the mix, such as more absorptive space (early reflections from rear speakers), subwoofer location (even MORE modal problems in a square room)

From the "are my speaker in a null" thread, the Harman spreadsheet (while useless for most of your purposes) can at least give some good starting points for front-to-back location of the mix position (keeping your head out of peaks/nulls) as you examine alternate speaker position widths/angles.

If there is any way possible, I would reconsider using like dimensions in your room; it takes a LOT less treatments when a room has evenly distributed modes. Jeff Szymanski's ModesV2 spreadsheet from Studiotips shows this well, especially when looking at Bonello distribution graphs. IF you've not already joined and downloaded that, I recommend it. (granted, an angled wall room with a 5/12 pitched ceiling can't really be translated, but it's still a good tool for education)

Gotta go for now; BTW, "bumping" a post is just posting again, even if you only say "bump", so it won't "fall off the page" or get missed by whoever you're hoping to get feedback from. Normally I don't recommend it, because I tend to start at the bottom (oldest first) and work my way to the top when I get a chance to do more; however, I sometimes STILL miss things - if it seems I have, then a "bump" makes sense... Steve

Sandersd · Post by **Sandersd** » Wed Jan 25, 2006 10:45 pm

Designing a space with the proper ratio was the first thing I tried. If the room ratio were corrected to a preferred ratio, it would make the width so narrow as to be unusable - well, for me anyway. I have to do what I can in a 12'x12' room, so have tried to make the most of it.

Edit: I reran a room mode calculation and I can live with (maybe) a 1:1.28:1.54 ratio which gives room dims of 7'-9 x 9'-11 x 11'-11.

As for the speaker angle, I've attached a plan with the angle at 60 degrees and with optimized room ratio.

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=5389

Question:

What is the difference between filling a space with the hangers and simply stuffing it with insulation? If the goal is the conversion of sound energy to heat via friction, what is the advantage of the hangers?

Question:

Regarding the slat resonators, will sound striking the narrow end of the wall be attenuated at a different rate than sound striking at the deep end, or will it simply see the space by its average depth? (The same question could be asked regarding the sloped ceiling.)

Thanks again for your comments.

knightfly · Post by **knightfly** » Fri Jan 27, 2006 3:47 pm

Question:

What is the difference between filling a space with the hangers and simply stuffing it with insulation? If the goal is the conversion of sound energy to heat via friction, what is the advantage of the hangers?

Hangers were used first (I think) in the 70's when overpowering bass was the norm in recording; theoretically, they "wave in the breeze", which requires energy. According to John, they do a better job of reducing bass in the room than plain absorption. I've not had the chance to compare the two methods side by side, sorry -

Question:

Regarding the slat resonators, will sound striking the narrow end of the wall be attenuated at a different rate than sound striking at the deep end, or will it simply see the space by its average depth? (The same question could be asked regarding the sloped ceiling.)

This would be wavelength dependent; at typical small room situations, lower frequencies operate in "pressure" mode not "wave" mode, and the size of the trap would need to be much larger in order to not average the absorption... Steve