Good point about the description and clarity.
I don't think however that a "cross" shape is required. Flexing (..or moving "to and fro"), should be exceedingly minimal because there will be virtually no force placed on the long dimension of the dowel/rod/brace and of course because the dowel/rod/brace shouldn't be particularly flexible.
Hi CrazyHub, the measurement was nearfield (mic about 3mm from the cone) so I doubt it had anything to do with new resonances. The resonant freq of the cabinets is probably in the low Khz already without the brace.
These woofers have a large vent in the magnet which is probably around 20-30mm in diameter, and a 3" voice coil, with the brace directly behind this vent (perhaps only a few cm away) and roughly the same size as the diameter of the vent then I would think that it could provide a reflective surface big enough to show the anomaly. Have a look at this post for an example of how nasty the FR is without damping and how smooth it is with damping.
Tony.
Bracing and internal reflections are two different things.
One is stopping the cabinet walls from resonating and projecting sound out into the room.
The other is absorption of the sound energy. It looks like this design is a sealed design. So, you want maximum absorption of the rear wave so that it does not return and affect the driver cone. If the wave returns and strikes the driver's cone it will easily pass through the cone and also cause the cone to sympathetically vibrate. Extra stuffing on the back wall facing the driver is a good idea.
Stuffing is a great way to convert the internal sound wave to heat. Bracing reduces the cabinet walls from sympathetically resonating.
The cross braces will be strong with respect to reducing antinodes on the panels. However, they are still prone to resonating themselves (like a string) and causing the walls they are attached to vibrate. I would think that making shelves that attach to all three walls with large cutouts for air passage would be more effective than simple side to side braces.
The shelves not only stiffen the panel, but effectively divide the panel into two or more panels so as to drive panel resonances higher in frequency. Shelves should be placed so as to asymmetrically divide the wall panels.
There are other methods to reduce panel resonances. You can add layers of roofing felt or other materials inside (good) and you can also build the cabinet walls with a constrained layer (better yet).
If you want to quantifiably test what bracing does, buy an old acoustic guitar pickup and stick it to one of the cabinet walls with wax or hot melt glue.
Use the guitar pickup like a microphone. It will pick up wall vibrations like an accelerometer. Run this into a program like HolmImpulse and you can do before and after plots.
The guitar pickups that use piezo elements are good choices. Alternatively, you can buy accelerometers from Newark and Digikey relatively cheaply.
Use the guitar pickup like a microphone. It will pick up wall vibrations like an accelerometer. Run this into a program like HolmImpulse and you can do before and after plots.
The guitar pickups that use piezo elements are good choices. Alternatively, you can buy accelerometers from Newark and Digikey relatively cheaply.
Your bracing scheme could actually be a bit more efficient. The individual braces should also brace each other. Each of the braces span the entire width and will be lacking stiffness as Earl mentioned. It would be better to arrange them so that braces meet in the middle and mutually brace each other. If those cross braces interect then the "spans" are cut down and the stiffness is increased. So that means you can make them more slender for a given stiffness.
Regarding reflections, also consider that the driver basket itself is a source of reflections, and in truth reflections can't be avoided. With no bracing you will still have reflections from the internal surfaces, the diffence is perhaps less chance to intercept with filling.
Regarding reflections, also consider that the driver basket itself is a source of reflections, and in truth reflections can't be avoided. With no bracing you will still have reflections from the internal surfaces, the diffence is perhaps less chance to intercept with filling.
Hi Paul
You have this exactly correct. A "star" made of three mutually perpendicular "across" braces all rigidly joined in the center is the most effective use of a given amount of material. Of course if one wants the "ultimate" then fill the whole cabinet with cement - nothing will move then. The point is to get maximum outside panel reduction with the miminmum of internal structure.
If the "star" is not effective enough then use CLD on the remaining unrestrained porions of the panels. I did this for years, then deleted the CLD becaues I could not find that it made any difference, but added a lot of cost and weight.
As to the back wave impinging back on the cone, this is what the enclosure does! Its whats causes the driver resonance to shift upwards.
Frankly I think there is always a psychological aspect of these things. As a diyer you know what you've put into the box. That in itself makes things more critical even if just for that reason, especially if you are the type to wish you had done more, thinking that perhaps you could extract just that little bit of extra performance. When you sit back with a feeling that you really did go into overkill, I'd guess you probably get a few happy chemicals buzzing around that might create a sense of them sounding better. It's probably similar to when someone spends thousands on a power cable. As one engineer said "the weird thing is, it actually works - you will hear a difference ... just don't expect you friends to!"
Reading this thread it seems everybody agrees that it is obvious you should brace a cabinet to the brim. But what is the purpose of bracing really? Yes, it increases stiffness of the cabinet walls. Increasing stiffness means the fundamental frequency of the panels goes up. Is that a good thing per se?
My first DIY speakers were MDF boxes into which I crammed as many crossbraces as I possibly could. Well, guess what? At certain specific frequencies the boxes rang like a bell. It was not in the khz range, but between about 400 and 1000 hz.
The purpose of the box is to 1- keep the pressure generated by the drivers from short-circuiting (funny I should say that, as a dipoles-man), and 2- keep all the rear-radiation inside the box and kill it and thus not let it interfere with the sound generated at the front.
The latter is in my believe impossible to realize completely. Therefore I've changed the objective to making the sound radiated by the box and re-radiated through the cone inaudible, or at least as unobtrusive as possible.
In my experience this can be done by making the box so very stiff, so that the fundamental frequency of the box lies above the operating range of the driver. For the midrange, I think the usually used MDF isn't the best material. Stiffer materials are needed, coupled with a lot of bracing and preferably a small box.
The other option is to make very flexible walls that are very well damped. This way the the fundamental resonance of the panels can be lower than the range in which the driver operates. It can be done with thin wooden walls with dampening material and maybe CLD. Braces across a panel can also help to dampen the resonance of the panel, but also increase its stiffness/fundamental frequency. The Q of the resonance of the flexible walls will be low, so it will probably be slightly excited, but the lower the frequency of the resonance, the less it will be audible as you get into the modal range of the room where there already are many other resonances.
For the bass extensive bracing works best, is my experience. With enough bracing, the fundamental resonance of the box can be pushed high enough.
My first DIY speakers were MDF boxes into which I crammed as many crossbraces as I possibly could. Well, guess what? At certain specific frequencies the boxes rang like a bell. It was not in the khz range, but between about 400 and 1000 hz.
The purpose of the box is to 1- keep the pressure generated by the drivers from short-circuiting (funny I should say that, as a dipoles-man), and 2- keep all the rear-radiation inside the box and kill it and thus not let it interfere with the sound generated at the front.
The latter is in my believe impossible to realize completely. Therefore I've changed the objective to making the sound radiated by the box and re-radiated through the cone inaudible, or at least as unobtrusive as possible.
In my experience this can be done by making the box so very stiff, so that the fundamental frequency of the box lies above the operating range of the driver. For the midrange, I think the usually used MDF isn't the best material. Stiffer materials are needed, coupled with a lot of bracing and preferably a small box.
The other option is to make very flexible walls that are very well damped. This way the the fundamental resonance of the panels can be lower than the range in which the driver operates. It can be done with thin wooden walls with dampening material and maybe CLD. Braces across a panel can also help to dampen the resonance of the panel, but also increase its stiffness/fundamental frequency. The Q of the resonance of the flexible walls will be low, so it will probably be slightly excited, but the lower the frequency of the resonance, the less it will be audible as you get into the modal range of the room where there already are many other resonances.
For the bass extensive bracing works best, is my experience. With enough bracing, the fundamental resonance of the box can be pushed high enough.
That is a great post
btw, will you have a thread for your TD12M/Beyma CP380M build? I would like to know more about the "& Rythmik bass horns", also Augerpro over on HTguide has a build with those drivers (I think) and the QSC HPR-152 waveguide so the XO might be pretty similar.
I'm in agreement. To my way of thinking bracing creates more problems than it solves. Higher frequencies are more noticeable to the ear, so my theory is that pushing the resonant frequency of the panels up will mean that any of those higher frequency residual resonances that emanate from the panel's exterior will be more noticeable and objectionable, and any of those higher frequency residual interior resonances that go back out through the driver's cone will be more noticeable and objectionable.
It would also seem logical that the more braces there are in an enclosure, the more undamped surface area there is for higher frequency waves to bounce off of, and the more those undamped higher frequency waves are going to come back out through the driver's cone. And braces are located closer to the driver(s) than the interior walls are, which exacerbates the problem. I suppose making the braces round in shape will lessen the effect somewhat, but I still think that no braces are a better option.
For me 1 1/2" panel walls, covered on the interior by two layers of sound deadening damping sheets, with some 1 1/2" egg-crate foam over the top of the damping sheets seems to offer the best compromise. Your mileage may vary.
So according to this , Would you think this design is better and more rigid ?
Also , I'm thinking about using birch plywood 18mm instead of thick ( 27-35mm ) MDF , What do you think ?
McCormack I think you are mistaken.
Also :
-The driver holes will be chamfered from the inside.
-All bracing done will be rounded.
-The cabinet will be stuffed with 250g acoustuff foam. I'm thinking about using some foam on the walls as well , But I dont think it is vital as long as I use the acoustafill foam scattered inside.
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I've built a few enclosures and have found that more bracing is a good thing, regardless of how it is braced. An 18mm MDF box of 30L internal volume sounded extremely coloured when compared to a 25mm thick panel with 5 internal braces.
With the 18mm brace-less box, the range of resonances were of low Q peaks, ranging from 275-700Hz. Even at low volumes, the panels were still excited and you easily hear how coloured they were.
With the 25mm heavily braced box, there is quite a serious peak at 450Hz, but with a high Q. Although it is a serious peak, it rarely resonates because the music that is playing cannot get it excited for long enough. For example, it might take 0.3 seconds of a continuos tone before the resonance gets fired up. A guitar note in a passage of music might last for 0.1 seconds, thus there is not enough time to get the panel excited.
We tend to identify a broad range of resonances easier than we do for a high Q, single point resonance - as in the case of the 25mm thick panel/braces.
What is the trick? We need to identify at what single frequency our ear is least sensitive to. Then add braces till you reach that frequency. Even better if you can get that special "frequency point" to fool our ears into thinking it is "musical" sounding.
With the 18mm brace-less box, the range of resonances were of low Q peaks, ranging from 275-700Hz. Even at low volumes, the panels were still excited and you easily hear how coloured they were.
With the 25mm heavily braced box, there is quite a serious peak at 450Hz, but with a high Q. Although it is a serious peak, it rarely resonates because the music that is playing cannot get it excited for long enough. For example, it might take 0.3 seconds of a continuos tone before the resonance gets fired up. A guitar note in a passage of music might last for 0.1 seconds, thus there is not enough time to get the panel excited.
We tend to identify a broad range of resonances easier than we do for a high Q, single point resonance - as in the case of the 25mm thick panel/braces.
What is the trick? We need to identify at what single frequency our ear is least sensitive to. Then add braces till you reach that frequency. Even better if you can get that special "frequency point" to fool our ears into thinking it is "musical" sounding.
Dave, do you have amplitude measurements for this? I don't see how it can be meaningful without them. Of course, I might be wrong, as I don't make cabinets from .02" steel, YMMV. Since it's taken form the AES Journal, is it safe to assume they weren't using sound waves for excitation?
Keri
Higher frequencies are easier to dampen than lower frequencies. So, yes, it is better to raise the resonance of the panel.
The reason that higher frequencies are easier to dampen is that for the same acoustic power, the higher frequency panel movement is physically less. In essence, you have less mechanical work to do dampening a higher vibration than you do a lower frequency vibration. The second reason is if you push the panel resonance up high enough and get above the crossover point, the woofer's response level is much lower (by virtue of the crossover) and contributes less energy into the panel walls.
As I posted earlier, it is very easy to quantifiably prove this with a makeshift accelerometer from an acoustic guitar pickup or any piezo element affixed to the cabinet wall. Additionally, getting accurate cabinet wall vibration results is far, far easier than trying to measure a loudspeaker's frequency response, which we all know is compounded with room effects and other artifacts.
Measuring can help you dial in the exact amount of bracing you need without guessing, which is how most of us do it.
I'm not sure how to interpret 'mechanical work' from your post. In physics it is the amount of energy you get when you multiply force and the distance traveled.
If the driver has a flat frequency response, the amount of energy transferred to the cabinet is essentially the same for all frequencies. Remember that the movement of the panels may become smaller at higher frequencies, but there is also less movement necessary to make the sound.
chlorofille said:
Agreed. Yet with a wide-band music signal it is a lot more obtrusive on my ears than a low Q resonance at a much lower frequency as you get with well-dampened unbraced panels.
If you want to push resonances out of the working range of a driver, in my experience you have to make the enclosure extremely rigid - doing it halfheartedly can do more harm than it does good.
I don't understand.... have you built a cabinet with flat response?
I think you have it backwards - higher resonances are more likely a problem simply because it takes less energy to excite them. You also get better acoustic coupling to the room, and the Fletcher Munson curve is working against you.
Does anyone know what is the ratio of the resonances caused by the speaker's backwave to the resonances caused by the coupling of the speaker to the cabinet?
My gut feeling (read as: not based on measurements “yet”) tells me that ultra stiff is not the way to go. As stated before, bracing shifts the resonance frequency upwards. A big problem with a resonance is that it needs very little energy to “resonate”. I believe an “ideal” resonance needs to energy at all.
Remembering that the same amount of energy radiated in the room is also radiated inside of the enclosure, I like to have some energy consumers inside the box (and damping material is not that terribly efficient). Damped panels (like the constraint layer damping Earl was speaking of) seem like good energy consumers. Because they are damped, if there is no new energy supplied, they resonances rapidly fade out.
Stiff undamped panels behave more like and ideal resonator (I think). Once a resonace is excited in these, it takes a long time to fade out, and I think that is what is clearly heard.
A great idea for bracing combined with damping is described in the following patent:
Loudspeaker enclosure with damping ... - Google Patent Search
It seems like the best of both worlds (CLD and bracing), with only the minimum of materials needed…
Remembering that the same amount of energy radiated in the room is also radiated inside of the enclosure, I like to have some energy consumers inside the box (and damping material is not that terribly efficient). Damped panels (like the constraint layer damping Earl was speaking of) seem like good energy consumers. Because they are damped, if there is no new energy supplied, they resonances rapidly fade out.
Stiff undamped panels behave more like and ideal resonator (I think). Once a resonace is excited in these, it takes a long time to fade out, and I think that is what is clearly heard.
A great idea for bracing combined with damping is described in the following patent:
Loudspeaker enclosure with damping ... - Google Patent Search
It seems like the best of both worlds (CLD and bracing), with only the minimum of materials needed…
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