I was reading an article describing how the Linkwitz Transform circuit works at ESP and I saw something that really confused me:
"Bracing is usually not needed, since the frequencies are so low that panel resonance is unlikely if the unit is a self contained subwoofer."
You can read the article for yourself here. I've always heard bracing was VERY important to speaker enclosures ESPECIALLY subwoofers, is this wrong? I'm about to start construction on a sub, but I don't want to waste my time on bracing if it isn't important. Anyone else have insight they'd like to share?
"Bracing is usually not needed, since the frequencies are so low that panel resonance is unlikely if the unit is a self contained subwoofer."
You can read the article for yourself here. I've always heard bracing was VERY important to speaker enclosures ESPECIALLY subwoofers, is this wrong? I'm about to start construction on a sub, but I don't want to waste my time on bracing if it isn't important. Anyone else have insight they'd like to share?
What he means is that if you use a Linkwitz Transform or EAS circuit then the chances are very high indeed that you will be using a small box. A small box does not have the large panel areas to vibrate and are inherently more rigid anyway, the result being that bracing is not always strictly necessary.
Consequently, if your box was larger than about 30 litres, bracing would be required.
Consequently, if your box was larger than about 30 litres, bracing would be required.
Hi,
in general, there are two reasons for bracing an enclosure, both have to do with damping. While the first use for braces is damping the standing waves inside the enclosure, the second reason for braces is damping the resonating enclosure walls.
For subwoofer (or bass box) applications, damping the internal box volume is useless, while stiffening the box enclosure is essential.
Both damping purposes are done by bracing the box internally, but the braces for the two purposes differ in the way they are constructed.
I agree that a very small box volume (made of reasonably stiff material) doesn't really need bracing. It would ruin the effect of having the smallest possible enclosure anyway...
Sebastian.
in general, there are two reasons for bracing an enclosure, both have to do with damping. While the first use for braces is damping the standing waves inside the enclosure, the second reason for braces is damping the resonating enclosure walls.
For subwoofer (or bass box) applications, damping the internal box volume is useless, while stiffening the box enclosure is essential.
Both damping purposes are done by bracing the box internally, but the braces for the two purposes differ in the way they are constructed.
I agree that a very small box volume (made of reasonably stiff material) doesn't really need bracing. It would ruin the effect of having the smallest possible enclosure anyway...
Sebastian.
sek said:
that's right about small box, but anyway wood will be resound.
that's because I've use material with large number of damping factor. it's to answer the purpose of first reason.
second one - standing waves - very serios problem and we can't leave it out, using bracing.
form of box, damping materials inside...should be effective
ok. good luck
Could you clairify this statement? By "damping the internal box volume" do you mean using damping material to line the walls and stuff the box? I was using the impression this was useful. The enclosure I will be using is about 3.7ft^3, pretty large, I believe bracing would be appropriate then?
The usual damping materials (bitumised felt, etc) aren't really effective at sub frequencies. In that sense, damping is not that useful--the fill is hopefully enough to absorb higher frequency low amplitude harmonics, suspension noise, etc.
Open cell foam is a little different.
Extreme mass loading, such as concrete or other heavy lining, is probably in a different ballpark though.
Open cell foam is a little different.
Extreme mass loading, such as concrete or other heavy lining, is probably in a different ballpark though.
Bracing is critical for a sub, more so than other speakers due to the high internal pressures involved. A good rule of thumb is to have all panels fully shelf braced so that at least one plane has no more than 8 inches of clear span. That pretty much limits maximum box size without braced panels at 1/3 cu. ft or 30 liters.
Internal damping is only required when any internal dimension measures at least 1/4 wavelength; for a sub with a 100 Hz low pass function that translates to about 2.8 feet.
www.billfitzmaurice.com
Internal damping is only required when any internal dimension measures at least 1/4 wavelength; for a sub with a 100 Hz low pass function that translates to about 2.8 feet.
www.billfitzmaurice.com
Damping standing waves or resonances inside the box is usually done with soft, absorbant materials (to line the walls and stuff the box). I just wanted to say that this is almost useless in bass applications, as the damping effect of e.g. foam, cotton or other soft materials is frequency dependent - and due to the long wave lengths of bass frequencies, the structure of the soft damping materials doesn't absorb much energy (because the tissue fibers or foam bubbles don't "resonate" with the waves).
Damping with soft, absorbant materials comes into play from the mid-bass upwards and has it's strongest effect from the upper-mid frequencies on.
That's pretty large, right. You should apply bracing to the walls as Bill recommends. A beautiful method is to build a matrix of boards so that the boards develop "tiny" cubic volumes, which are cut out. This would then result in a 3D-frame structure filling the entire box volume (don't forget that the speaker opening needs some spare volume for the speaker back side and magnet).
This gives the two advantages that the outer box walls are covered with stabilizing elements while the pairs of opposite walls are stiffly connected. Thus, the pressure can't expand or collapse (simultaneously push out or in) the enclosure walls. The smallest possible matrix size would of course be 2, one big, two medium and four small boards cut out and glued in... I usually make them out of thin MDF, as it's easy to work with and you don't need anything more strong here. But stick with Bill, your box size probably needs more than one brace per wall dimension. What board material and thickness do you use?
Hope this helps,
Sebastian.
okay im confused now too....
bracing? no bracing?
damping on the walls? bare wood?
stuffing? no stuffing?
I just finished a sub of my own - 3.5 cubic feet. The entire insides of the box is lined with BH5 and there are no braces..
but it sounds great!
Standing waves? .... seeing as there are no internal braces im assuming i'll get them. What do they sound like?
Stuffing a box always seemed counter productive to me.. we work so hard to get the internal volume correct (subtract port size, subtract speaker cone displacement, subtract bracing, etc...) - and then said volume gets reduced by some arbitrary amount of stuffing.
someone edumacate us please!
bracing? no bracing?
damping on the walls? bare wood?
stuffing? no stuffing?
I just finished a sub of my own - 3.5 cubic feet. The entire insides of the box is lined with BH5 and there are no braces..
but it sounds great!
Standing waves? .... seeing as there are no internal braces im assuming i'll get them. What do they sound like?
Stuffing a box always seemed counter productive to me.. we work so hard to get the internal volume correct (subtract port size, subtract speaker cone displacement, subtract bracing, etc...) - and then said volume gets reduced by some arbitrary amount of stuffing.
someone edumacate us please!
The design I'm discussing is a sealed box design, a lot of these suggestions may not apply to your ported (bass reflex) sub. Just wanted to make sure to reiterate that. Also, stuffing actually increases the apparent box volume by causing air to move more slowly throughout your ported enclosure.
Motion,
great website! Nice to see someone who does just about everything diy ... would be great to see a write up on the amp and speakers as well.
...
1/3 cu ft = 30L
>>> That should be 1 cu ft = 28L
...
I don't see how standing waves have any relevance inside any speaker box.
The rear wave that is trapped inside the box will do the following:
1. be absorbed ie. converted into heat inside the box
2. re-enter the room via the speaker cone or passive radiator (the driver is a weak part in the system as far as transmission through the enclosure is concerned)
3. re-enter the room via a port
4. cause vibration of the box and thus be re-radiated into the room with a change in frequency response due to the stiffness and damping of the box and resonances in the transmission characteristics of the box ... thus this involves "coloration"
As damping is not effective below 100 Hz, the best thing you can do is make the box as stiff as you can. The walls should be reasonably thick and well braced as Bill mentioned. Another technique not commonly used is called constrained layer damping (CLD), which is in effect building a box within a box. The driver will be attached to the inner box and the two boxes will be connected via a flexible adhesive like liquid nails.
I know it has been said that bracing or internal baffles to deal with standing waves is not for small subs. However, I don't see how standing waves have any real effect on box coloration in any box.
The question of bracing and box thickness - there is a point where you keep making your box thicker and adding more bracing where the box just becomes bigger and there is no noticeable improvement.
great website! Nice to see someone who does just about everything diy
... would be great to see a write up on the amp and speakers as well....
1/3 cu ft = 30L
>>> That should be 1 cu ft = 28L
...
I don't see how standing waves have any relevance inside any speaker box.
The rear wave that is trapped inside the box will do the following:
1. be absorbed ie. converted into heat inside the box
2. re-enter the room via the speaker cone or passive radiator (the driver is a weak part in the system as far as transmission through the enclosure is concerned)
3. re-enter the room via a port
4. cause vibration of the box and thus be re-radiated into the room with a change in frequency response due to the stiffness and damping of the box and resonances in the transmission characteristics of the box ... thus this involves "coloration"
As damping is not effective below 100 Hz, the best thing you can do is make the box as stiff as you can. The walls should be reasonably thick and well braced as Bill mentioned. Another technique not commonly used is called constrained layer damping (CLD), which is in effect building a box within a box. The driver will be attached to the inner box and the two boxes will be connected via a flexible adhesive like liquid nails.
I know it has been said that bracing or internal baffles to deal with standing waves is not for small subs. However, I don't see how standing waves have any real effect on box coloration in any box.
The question of bracing and box thickness - there is a point where you keep making your box thicker and adding more bracing where the box just becomes bigger and there is no noticeable improvement.
A few weeks ago, I wanted to see what sitting on a small commercial sub would feel like. We played the explosions from U571 (the movie). The vibrations were so intense it was almost painful, I had to clamp my jaw to keep my teeth from chattering.
My friend said the apparent bass in the room was reduced by about 10dB. This ponderous smeared cabinet resonance was a major contributor to the sub's output.
My subs at home are built with a box inside a box, with damping material in between, and very well braced. You can not feel any vibration on the exterior box.
Damp, brace, mass load, and spike-couple them to the floor. You will get clean non-smeared bass as a reward.
My friend said the apparent bass in the room was reduced by about 10dB. This ponderous smeared cabinet resonance was a major contributor to the sub's output.
My subs at home are built with a box inside a box, with damping material in between, and very well braced. You can not feel any vibration on the exterior box.
Damp, brace, mass load, and spike-couple them to the floor. You will get clean non-smeared bass as a reward.
A standing wave is essentially a resonance with a high Q (low damping factor). As all parts in the box interact (acoustically), the Q of the box resonances have influence on the response of the speaker in it's box.
A completely undamped standing wave inside the box will - once energized - have an easily perceptible influence on how the speaker's cone movement at that very frequency decays. It just takes longer for the cone to stop, once the wave is "standing".
That's why damped boxes sound less "boomey" than before damping them...
They don't if the box is too small for them to develop. In the pre- T/S days cabs with at least one internal dimension of 3 feet or more were common and standing waves could be real problematic, as they will occur when that distance is 1/4 wavelength or more, and at 3 feet that means about 100 Hz. It took a lot of fiberglas or wool or whatever to damp out the resulting resonant peaks and valleys at 100 Hz, or even 200 for that matter. Nowadays with smaller box sizes standing waves are seldom an issue; the midrange frequencies where they can develop are quite easily damped with an inch or so of polyfill batting on the box interior walls.
Don't confuse damping with vibration control. Vibration control devices, be they braces or asphalt sheets, control physical vibration of the cabinet walls, while damping materials control wave reflections inside the box.
Sorry about the English/Metric comversion. I still prefer Henry VIII to Napoleon.
I haven't heard a lot about this technique (probably because it involves a LOT more work than a traditional cabinet). I'd definately be interested in hearing the details of this construction and pics would be great as well. I like the idea though and I'll give it some serious thought. If nothing else could you give a brief description of your particular implementation?
I've made an animation showing the construction of a sub using CLD (a box within a box). The driver is attached to the inner box which is braced using matrix bracing in 3 planes. The vibration of the driver will transfer to the inner box, and this will be damped by the flexible adhesive layer before being transferred to the outer box, with a magnitude that is significantly reduced.
Shown in the animation:
450 x 450 x 450mm sealed box
18mm MDF
driver: Stryke AV12
Box volume: ~50L
Shown in the animation:
450 x 450 x 450mm sealed box
18mm MDF
driver: Stryke AV12
Box volume: ~50L
marsupialx said:
I can't see any acoustic virtue in spike coupling a sub to the floor. What this will do is transfer vibrations through to the floor, and if the floor is not very solid (such as concrete), but is in fact timber, then the vibrations will cause the floor to make a contribution to the sound that will be less true to the original as it will add its own resonances. This *might* result in a sound that you like more, but as far as accuracy goes it works against the other measures you have taken. To be consistent, I think you would be better to use a flexible coupling to the floor which reduces the transfer of vibration.
sek said:
A completely undamped standing wave inside the box will - once energized - have an easily perceptible influence on how the speaker's cone movement at that very frequency decays. It just takes longer for the cone to stop, once the wave is "standing".
Sek, based on your description, I'm not sure if this is true or not. Do you have any links to other information to back this up?
BillFitzmaurice said:
very good point
...
As I'm in Australia (metric), and there are so many people from the US in audio discussions and with websites, I have to be good at metric/imperial conversion. Generally, I find imperial units a nuisance, but when it comes to audio, often imperial are more intuitive - it's easy to visualise a 1 cu ft sub box or the size of a 6.5" driver, etc.
m0tion said:
Motion, perhaps you are overestimating the amount of extra work. If you use a flexible adhesive, the main difficulty is getting the gap between the boxes right - too small and you won't get enough adhesive in, too big and you won't get sufficient contact for the adhesive to work properly. It's not a lot of work to cut a second box, it's just a matter of being more precise.
Hi Paul,
I could explain myself a little better.
A standing wave (room resonance) takes it's time to develop. It doesn't "swing in" instantaneously but builds up energy over at least one period of cone movement. One half for the first half wave to travel to the opposite wall, another half wave to travel back and then add constructively to the next pressure change. Correct me on that if I'm wrong.
Now that's stored energy. Whatever it takes to stop this standing wave from bouncing back and forth between the opposite walls (e.g. damping material), it won't stop in no time. The stored energy has to go somewhere. Damping material is there to absorb this energy (and thus probably prevent the standing wave from occuring in the first place) and convert it into heat.
But let's assume there is no damping at all (an impossible condition in reality). What should the stored energy do, "stand" forever? No, because there is the speaker cone. Model-wise, both the vibrating air of the standing wave and the cone-motor-spider-suspension system can be seen as mass-spring systems. One has stored (kinetic) energy while the other (the speaker) may or may have not.
For simplicity, let's assume the speaker cone is completely stopped while the standing wave is still bouncing. Now those two mass-spring system are directly coupled, as the air is directly touching the speaker. They must interact, except the speaker had infinite damping (e.g. an ideally stiff cone and suspension), which it definitely hasn't, as it is a speaker.
Why should it be possible that the energy in this directly coupled mass-spring system can travel in one direction while it can't in the other? At least, the speaker has built up the energy stored in the standing wave...
In reality, this process is heavily depending on the damping properties of the box and the mechanical and electrical properties of the speaker. The process is extremely frequency dependent and it's likely that you never experience such problems by just building good boxes
No, sorry. That is from (german) textbooks and personal experience (experiments) in class and in DIY. And it could be wrong, don't forget that ;D (everybody jumping in, please)!
But as I said, the difference in "reverberance" between an undamped box and the same box filled with damping material is easily audible and shows up as delayed decay at certain frequency bands (remember, the speaker's properties come into play here, different speakers of the same size would decay differently, time- and freuqency-wise).
puuh, 'nuff.
Sebastian.
I could explain myself a little better.
A standing wave (room resonance) takes it's time to develop. It doesn't "swing in" instantaneously but builds up energy over at least one period of cone movement. One half for the first half wave to travel to the opposite wall, another half wave to travel back and then add constructively to the next pressure change. Correct me on that if I'm wrong.
Now that's stored energy. Whatever it takes to stop this standing wave from bouncing back and forth between the opposite walls (e.g. damping material), it won't stop in no time. The stored energy has to go somewhere. Damping material is there to absorb this energy (and thus probably prevent the standing wave from occuring in the first place) and convert it into heat.
But let's assume there is no damping at all (an impossible condition in reality). What should the stored energy do, "stand" forever? No, because there is the speaker cone. Model-wise, both the vibrating air of the standing wave and the cone-motor-spider-suspension system can be seen as mass-spring systems. One has stored (kinetic) energy while the other (the speaker) may or may have not.
For simplicity, let's assume the speaker cone is completely stopped while the standing wave is still bouncing. Now those two mass-spring system are directly coupled, as the air is directly touching the speaker. They must interact, except the speaker had infinite damping (e.g. an ideally stiff cone and suspension), which it definitely hasn't, as it is a speaker.
Why should it be possible that the energy in this directly coupled mass-spring system can travel in one direction while it can't in the other? At least, the speaker has built up the energy stored in the standing wave...
In reality, this process is heavily depending on the damping properties of the box and the mechanical and electrical properties of the speaker. The process is extremely frequency dependent and it's likely that you never experience such problems by just building good boxes
No, sorry. That is from (german) textbooks and personal experience (experiments) in class and in DIY. And it could be wrong, don't forget that ;D (everybody jumping in, please)!
But as I said, the difference in "reverberance" between an undamped box and the same box filled with damping material is easily audible and shows up as delayed decay at certain frequency bands (remember, the speaker's properties come into play here, different speakers of the same size would decay differently, time- and freuqency-wise).
puuh, 'nuff.
Sebastian.
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