Bracing subs (XO < 100 Hz) - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 64 Old 02-01-08, 10:10 AM Thread Starter
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Re: Bracing subs (XO < 100 Hz)

http://tech.groups.yahoo.com/group/L...s/message/3047

(The Nick referred to, was the owner of Lambda Acoustics, a woofer maker that made some of the best drivers around; but it seems that Nick was a much better engineer than businessman, and despite numerous raves on quality, Lambda went under).

"Nick & I played with bracings, and discovered round metal better than square wood.
Less space & stiffer bracings. But, labor cost prohibitive in consumer product."

I don't know why round metal required more labor than square wood, but they preferred the results of metal . .
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post #22 of 64 Old 02-01-08, 11:19 AM
 
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Re: Bracing subs (XO < 100 Hz)

Quote:
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> The midrange is a completely different beast when compared to a subwoofer. Exponentially more thought and bracing would go into making a fully inert midrange cabinet as compared to a subwoofer.

Can you point me to a source with more info on that?

> For most subwoofers if one connects the XYZ axes and places bracing horizontally every 15" or so the cabinet will have resonance bands far beyond passband.
For a completely inert midrange far more dense (2" spacing on the XYZ) bracing must be used while implementing a constrained layer of dampening material such as Peal N Seal.
*Please note these are just examples and should not be thrown in random builds as I have not outlined the method exactly.*

15" spacing for subs is a similar starting point to another post I read.
Linkwitz recommended 4" spacing, but I don't know in what context . .
Having said that, what is the method you refer to?
This is practical application of physics. You are focusing on pressurized force flex in your calculations and actual resonance differs from this.

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> If you properly tie together the axes you will make the use of metal in this application complete and utter overkill.

I don't think I'll ever be welding capable - for midranges, I'll ask a friend who's a metal worker if he knows of an off the shelf fitting that allows 2 (threaded rods) at 90 degrees to tie into each other.

> a steel matrix as this would be an extremely efficient way to brace a midrange (not in terms of weight)

I'll check what suitable moderately priced materials are stronger for their weight. Why do you think a metal matrix would weigh more than a wood matrix of the same strength?
If you are looking for equal strengths there aren't as many benefits for metal over steel. The real reason to use such a material is because of its superior rigidity allowing for an even more inert cabinet if properly implemented. Another upside to using a properly dense metal is that it will take considerably less volume allowing for a smaller cabinet while being more rigid than its wood counterpart.

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> You really do not need to use more than oak for your bracing

Here we don't have oak, though several other hardwoods.
However as outlined above, it seems to me that metal rods have net advantages, especially in being quicker to build & install.

That could allow more bracing in the same amount of time
Again, in a subwoofer there is no need for more bracing then already discussed. Intelligent design of your cabinet will make this complete overkill.

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rick57 wrote: View Post
> Post pictures of your plans

There are no CAD/plans (just Unibox results in a spreadsheet of box dimension scenarios). I'm attempting to weigh up and work out how to implement a basic appoach that I saw mentioned on the Lambda forum, with no details.

A drawing could show desirable spacing for the passband, which is becoming clear.

If the basic approach isn't clear, I'll redraw my sketch more legibly, scan & post, if that helps convey it.

Thanks
There is a free alternative to CAD called Google Sketchup. While I find it a huge pain to use and rely on CAD for those not interested in getting such an expensive program Sketchup might suffice.

Quote:
rick57 wrote: View Post
http://tech.groups.yahoo.com/group/L...s/message/3047

(The Nick referred to, was the owner of Lambda Acoustics, a woofer maker that made some of the best drivers around; but it seems that Nick was a much better engineer than businessman, and despite numerous raves on quality, Lambda went under).

"Nick & I played with bracings, and discovered round metal better than square wood.
Less space & stiffer bracings. But, labor cost prohibitive in consumer product."

I don't know why round metal required more labor than square wood, but they preferred the results of metal . .
I believe I have sufficiently covered the topic of metal versus wood bracing. If you want to use metal bracing in your subwoofer you can do it. If properly done there will be no adverse effects, but as I have said the subwoofer will perform no better than a properly designed one with wood bracing.

What are the frequencies you want this subwoofer to be playing once completed?
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post #23 of 64 Old 02-01-08, 11:41 AM
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Re: Bracing subs (XO < 100 Hz)

I'm still ruminating about how to brace big sub boxes. Your questions which include using metal tensioning rods is adding another level of intrigue to the equation. A box painted in "camo" with a bunch of metal rods sticking out of it might look completely natural in a scene from "The Matrix"! Now that I think about it my "living room" looks kinda like the matrix. I think I better start cleaning it sometime in the spring.
For your own build 9mm wood bracing in a 2.8 cubic foot box will work under most circumstances (if) it comes from a full panel of wood with material removed to provide full access to air/frequency stimulation.
The final result of course is still to reduce box noise to below audibility. Remember that higher Hz can be absorbed by foam and/or fiber stuffing. You don't need bracing to contain unwanted 15,000Hz material.

Last edited by ISLAND1000; 02-01-08 at 12:35 PM.
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post #24 of 64 Old 02-01-08, 11:41 PM Thread Starter
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Re: Bracing subs (XO < 100 Hz)

Andrew

Thank you, I'll give Google Sketchup a try.

(As above) I have a big backlog of bass projects:

Peerless XLS 12” in 40 litres sealed up to 100 Hz
Peerless 10 in 40 litre sealed up to 500 Hz
Peerless XLS 12 in an 80 litre EBS (externally 12 * 17 * 35") for LFE, I think that's up to 120 Hz.

PHL 15” vented midbass: 100 – 500 Hz
Rythmik 15”, sealed up to 100 Hz
The last two in identical net 80 litre boxes, 17 * 17 * 23"

ie that's going to add up to a lot of bracing . .

(and as this is a sub forum, I haven't mentioned my mid boxes "to do" . .)

You don't think that threaded rods could be as effective, but would be a lot quicker to build?

(Gotta get back to rebating some mids)

Thanks
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post #25 of 64 Old 02-02-08, 04:09 AM Thread Starter
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Re: Bracing subs (XO < 100 Hz)

In response to me:
"15 inch spacing for subs is a similar starting point to another post I read.
Linkwitz recommended 4" spacing, but I don't know in what context . .
Having said that, what is the method you refer to?

You replied:
"This is practical application of physics. You are focusing on pressurized force flex in your calculations and actual resonance differs from this."

Though I was pretty good at it at the time, I haven't studied physics since 1975 . . .
(Whether hardwood or metal) I'd guess the primary factors setting the useful bracing distance are the passband relative to the box panel's stiffness, and the desired SPL (being the trigger force that needs to coped with).

From what you know (and maybe planning for your 'mids' to accompany your subs) are you able to suggest some sensible spacing or 'intelligent design' for frequencies above 100 Hz (up to 100 dB)? or any reference other than the JAES paper I hope to track down, that would help me work it out?

Thanks again
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post #26 of 64 Old 02-02-08, 10:58 AM
 
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Re: Bracing subs (XO < 100 Hz)

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rick57 wrote: View Post
Peerless XLS 12 in 40 litres sealed up to 100 Hz
Peerless 10 in 40 litre sealed up to 500 Hz
Peerless XLS 12 in an 80 litre EBS (externally 12 * 17 * 35") for LFE, I think that's up to 120 Hz.

PHL 15 vented midbass: 100 500 Hz
Rythmik 15, sealed up to 100 Hz
The last two in identical net 80 litre boxes, 17 * 17 * 23"

ie that's going to add up to a lot of bracing . .

(and as this is a sub forum, I haven't mentioned my mid boxes "to do" . .)

You don't think that threaded rods could be as effective, but would be a lot quicker to build?
So it seems you have some very different projects planned. For the Rythmik driver I really wouldn't worry too much. bobgpsr previous example of his bracing would be more than sufficient for the application. If you really wanted to add some efficiency to what he did and add some insurance you could add a cross brace that ties the third axis into the situation making the bracing far more rigid. This would completely remove all resonance in the passband and likely well above too (200-250Hz).

As far as your mid bass module you will want to have a more dense bracing matrix for this. If you do a full XYZ hardwood matrix with 4-5" spacing it should be sufficient to remove all resonance. Insurance for a project like this would be using a smaller bracing matrix or even better a constrained layer between the cabinet and the bracing with something like Peal N Seal. This constrained layer will greatly decrease cabinet resonance.

Like I have said you can use metal, but you must remember while the metal is more rigid it will still need to be as dense or almost as dense as the oak. Resonance is in part a factor of the cabinets surface area. The more surface are the more bracing needed. For your application (before you get to the mid module) metal is probably overkill. Also, without proper welding abilities I doubt you will be able to get the custom sized pieces you would need to make an appropriate matrix, but if you do let me know as I will probably do the same.

Quote:
rick57 wrote: View Post
In response to me:
"15 inch spacing for subs is a similar starting point to another post I read.
Linkwitz recommended 4" spacing, but I don't know in what context . .
Having said that, what is the method you refer to?

You replied:
"This is practical application of physics. You are focusing on pressurized force flex in your calculations and actual resonance differs from this."

Though I was pretty good at it at the time, I haven't studied physics since 1975 . . .
(Whether hardwood or metal) I'd guess the primary factors setting the useful bracing distance are the passband relative to the box panel's stiffness, and the desired SPL (being the trigger force that needs to coped with).

From what you know (and maybe planning for your 'mids' to accompany your subs) are you able to suggest some sensible spacing or 'intelligent design' for frequencies above 100 Hz (up to 100 dB)? or any reference other than the JAES paper I hope to track down, that would help me work it out?

Thanks again
There is no set standard to create a resonance free enclosure. Very few bother taking the time and effort to achieve this despite its nature on sound quality via loudspeaker playback. The easiest test for determining cabinet resonance is use of an accelerometer and a controlled voltage, but even that is just a relative measurement as an anechoic chamber and multiple impulse response measurements at various angles need to be average for coloration due to resonance to show on a frequency response graph.

A rule of thumb is just that and from what I have learned they normally don't apply to anything well.
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post #27 of 64 Old 02-02-08, 12:14 PM
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Quote:
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Peerless 10 in 40 litre sealed up to 500 Hz
Andrew has made a valid point about resonances. However, with this specific situation(bandwidth up to 500Hz), it would be somewhat of a building challenge to prevent resonance in this band with a cabinet of this size. Due to the difficult nature of this particular application, please remember, the following construction method assumes you require transparent sound with very low resonance(most designs, retail and DIY have relatively high levels of resonances), for the highest quality sound reproduction. If you still think you need this, then follow this basic method using conventional materials:

1) External 3/4" shell, made from hardwood ply or MDF.
2) Apply 1/4"-1/3" Peel N' Seal on the inside of the shell.
3) Apply 1.5" oak(laminate two 0.75" pieces together with glue) inner shell. Apply the shell using a glue to bond to the aluminum facing of the Peel 'n Seal. Do not breach the constrain construction by running screws all the way through from the external to internal shells. This will decrease the damping efficiency of the constrained layer system.
4) Install a solid oak bracing system using 2" x 2" oak, in a criss cross fashion(and be sure to glue all crossed point where the overlap to increase bracing strength), overlapping in all axis(X, Y, Z), being ceratin no greater than 4" is exceeded from any brace point to another.

The above will result in a very low resonance system. The problem is cost(hardwood oak is not cheap) and complexity of build. Also weight. The weight may be almost 3x the normal weight of a conventional enclosure. The enclosure will also be larger than normal, due to the increased wall thickness and substantial displacement volume of the bracing.

For acoustic dampening, I recommend in this case, 4" of OC705 or #8 mineral wool board on the two longest dimensions ends, and 2" of the same material on other surfaces. Due to the bracing system used, this material has to be installed before you close up the subwoofer's last wall, and you will have to cut the material into blocks to fit into the bracing network.

Is the speaker system this is being used intended for serious hi-fi purposes? I think that is the question that must be answered before you decide to execute such a building method.


Quote:
Peerless XLS 12 in an 80 litre EBS (externally 12 * 17 * 35") for LFE, I think that's up to 120 Hz.
This is a much easier situation. Due to the limited bandwidth, you can push the resonances of these panels over the bandwidth. Standard shelf braces every 5" would be sufficient. Be sure the shelf braces themselves are braced to each other through another axis -- because after all - these can act as large radiation surfaces. I would suggest a high grade hardwood ply ; it's lighter and stiffer than MDF. It will ultimately be easier shift the resonances upward using this material. Due to the longest dimension here, you will have a substantial 190Hz half-wavelength mode, and 1/4 wavelength cancellations relative to the driver placement and wall distance from that point. These problems will occur less than half an octave past your intended crossover point. Unless you are using an 8th order crossover slope or steeper, you need to address this. To eliminate this as an issue, use OC705 or #8 mineral wool board in a minimum 4" thickness on the walls at the end of the longest dimension, and 2" minimum on all other walls.

-Chris

Last edited by WmAx; 02-02-08 at 12:21 PM.
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post #28 of 64 Old 02-03-08, 04:00 AM Thread Starter
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Re: If you are insane, you can do it....

Andrew and Chris

Thank you again very much.

Chris the method you suggest for a 40 litre sealed up to 500 Hz, do I understand you correctly that it's a *complete 1.5" box within an outer box? (Peel N' Seal, is that like dynamat)?
That'd be quite a project! That 'sealed up to 500 Hz' is not my highest priority, so I probably wouldn't do a box within an box, but I think I'll implement all your other points .. unless some 'co-depend' on the box within a box structure?

You have acquired a lot of specific knowledge, may I ask is it from more theory (eg AES papers?, experience and/ or other forum folk's experience?

I feel a lot more confident now about what to do, this thread has been enormously helpful.

Cheers
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post #29 of 64 Old 02-03-08, 05:02 AM
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Re: If you are insane, you can do it....

Quote:
rick57 wrote: View Post

Chris the method you suggest for a 40 litre sealed up to 500 Hz, do I understand you correctly that it's a *complete 1.5" box within an outer box? (Peel N' Seal, is that like dynamat)?
Yes, a complete box within a box, with layers of 'Dynamat' glued in the middle to bond them and effect a very efficient dampening core.
Quote:
That'd be quite a project! That 'sealed up to 500 Hz' is not my highest priority, so I probably wouldn't do a box within an box, but I think I'll implement all your other points .. unless some 'co-depend' on the box within a box structure?
It's quite unlikely you would need to build my above constrained layer suggestion. That suggestion assumes a specific bandwidth of use, and it assumes you are targeting a near perfect performing example for the highest of quality system that can not accept compromises. Statistically, this is rarely the situation.

Quote:
You have acquired a lot of specific knowledge, may I ask is it from more theory (eg AES papers?, experience and/ or other forum folk's experience?
Speaking of my general knowledge in the area of audio reproduction; primarily sourced through JAES peer reviewed perceptual papers, general physics, and my own extensive analysis/measurements and control/blind experiments.

-Chris
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post #30 of 64 Old 02-03-08, 07:47 AM
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Re: If you are insane, you can do it....

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Yes, a complete box within a box, with layers of 'Dynamat' glued in the middle to bond them and effect a very efficient dampening core.
Chris
Hey Chris how did your first attempt at a bonded Dynamat concentric enclosure turn out? What was it's final cost? That stuff's expensive!
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