Stereo Integrity HS-24 - Page 5 - Home Theater Forum and Systems -

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post #41 of 44 Old 12-04-14, 09:26 PM Thread Starter
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Re: Stereo Integrity HS-24

baniels wrote: View Post
Thanks for the response (I also saw the one on AVS). I can imagine the difficulty of trying to weigh such a beast on a scale!

I'm beginning a cabinet design for this driver and I'm trying to determine the adequacy of the cabinet's mass to tame the energy of the driver. With bracing, I'm at 190 lbs. Assuming Ricci's quote of 85lbs is right, that puts the net weight at 275lbs.

Here's a little sneak peak.

15.22ft^3 before the driver.
Let me just start off by saying that your modeling and rendering skills are incredible.

Now, ideally speaking, a subwoofer enclosure should be two things: stiff enough to push the panel resonance above the passband and sufficiently heavy to push the mass resonance below (or at least low enough that most material won't get it rocking except in rare instances).

For the first, you have, well, simply put, an incredible bracing scheme set up. It'll take a lot of CNC time (or inordinate levels of patience) to make it happen and the results will almost be very good. Why almost? I don't wish to be picky here, but you need ribs.

This is the problem I'm facing coming back from a several years long audio hiatus, much of the good old information I knew of to reference to was on webpages that have been lost to time and that were too insignificant to Internet Archive. One of those pages was about ten years back and involved someone who took the time to test several panel designs and measure resonance using an accelerometer. (Needless to say, that was someone who had far more patience than I.) Racking the dustier parts of my memory, I recall several important points from that experiment. Mainly:
1. Doubling thickness ideally cubes stiffness. With wood and regular glue laminates, this overly optimistic, but doubled walls still do a lot and triple anything is really good (but going above is diminishing losses for the material and volume required/lost to manage it)
2. Stick bracing across between the walls helps a little bit, but may or may not be on par with simple double thickness on the panels.
3. Ribs are a significant performance increase, greater with a single thickness panel than a double thickness. The ribs providing a goodly portion of the geometric stiffness increase of the rib's height, but at much less use of material.
4. Ribs with cross bracing are an order of magnitude better than any of the above. (There's a reason wood hulled ships were built this way.)
5. Double thick walls with ribs and cross bracing makes for a bomb proof enclosure, but #4 comes close enough for most applications.

Why is this so? Well, for a lot of reasons beyond my knowledge, that's for certain, but for subwoofers, it is because we're dealing with more than first order panel flex or balloon modes, for want of the right phrase. Stick bracing straight across between panels addresses just that: the driver pushing out and sucking the panels in, the driver pulling in and pushing them out and everything getting set all a-quiver like jello as the mass moving adds its thing. However, things are never so simple, we don't live in an idealized world and wood isn't an ideal material. Other resonance modes get set up in the panels. When we do the knuckle rap test, we're not listening for the two or three hertz resonance of our thunking of the wood, but the higher order behavior that is induced by that energy put into the wood. Higher order, higher frequency resonances that can manifest within the boundaries of panels despite a few cross braces or, indeed, be aided by stick type cross braces should they line up with the nodes. To which, have you ever heard of Chladni Plates and Figures?

Chladni was one of those 18th century figures who stumbled upon what today we'd consider basic physics, but because he was one of the first to ask why something was and create experiments to find an answer, he earned himself the label "father of acoustics". His is perhaps best known for his plates and the figures they created: panels of various materials and shapes with sand dusted atop them that would be excited to resonance by drawing a bow across them where the sand would be driven from where the panels were resonating to the nodes where they were not. The results are informative and beautiful in a way:

They let you see acoustic resonances with your eyes, which certainly beats tapping something with one's knuckles. Now, recall, the sand settles where the panel is not moving, wherever black is seen, the panel is vibrating and creating noise. Put stick braces in those places and they make no difference whatsoever to damp resonances and with many of those plates, such braces would line up perfectly when put halfway across and still quite a few when a third of the way. Stick braces might even exaggerate the movement and extend duration by ensuring the nodes won't be moved by other frequencies reducing any self-damping that might occur from changing input or the inherent nature of wood. Adding boundary conditions with other panels glued at the edges would change the nature of the resonances, of course, but only such that the edges represent a nodal region; the remainder of the panel would still be active, but in different patterns of which there are many.

Taking this into account for a subwoofer box, stick bracing may actually serve more to tune a box like a musical instrument to resonate at particular harmonics more strongly than the lower level, generic vibrations without bracing. Not a good thing even in a midrange-treble box, doubly not good with a box this big where the panels will definitely be excited by the frequency range it is asked to do.

Ribs, however, they are the answer. Add enough ribs and they take a large panel that would act as a single, low frequency resonator, nodes and all, and break it up into many small panels with much higher fundamental and harmonic resonances up where they'll be less likely to be excited by the sub, have less energy to overcome the damping of the wood and will be quieter in general. Plus, as mentioned before, everything's much stiffer. Basically, you'll get a much deader cabinet by knuckle rap and in practice, but only so long as you provide the exceptional cross bracing you've already designed for.

To which, might I suggest:

Please pardon the hack job of your renders, but something like shown in part in the top left (but done on all legs in all three axes) would do a great deal to make the most of your bracing and deaden the cabinet without losing too much volume.

B&W had the right idea with their Matrix series:

They even rounded their corners to distribute forces properly. That may be too minor a detail to matter in a subwoofer box, but you never know.

Last thought on panels, the old rule of thumb I learned from some pretty bright folks years ago: MDF for mid-treble, but hardwood plywood for subwoofers. The former drops resonance below the speaker's passband and internally damps it, the latter raises resonance above the passband and helps the box be stiffer given that subs will require larger boxes. MDF may give mass for a sub, but you lose too much stiffness to make up for it unless going for enough layers to be impractically heavy and expensive. (Although, loads of plywood bracing with double wall MDF on the sides might be a workable hybrid compromise at the cost of being a hassle.)

As for mass, unless you go extreme (concrete enclosure or something bolted down), whatever you do, the HS-24 is too powerful a driver to avoid cabinet shimmies. You can diminish them, yes, but they're not going to go away. Other "best of the best" drivers are like bow tie wearing, kindly old gentlemen beside the heavy metal thrasher a HS-24 at full roar can be. That said except when you have a strong signal at the mass resonance of the cabinet (mine's somewhere between 9 and 14Hz, looks like), you may not have unmanageable amounts of movement. I have so much motion with mine because of two things: first, I put the driver at the top, above the system's center of mass, because I wanted to see this driver straight on and enjoy it for all it was worth and, second, I listen to music that gets it moving like no movie does. That last point is no overstatement. The sub only rocks and shakes a bit with movies, but with my music...well, let's have a look:

The big scene in Olympus Has Fallen that folks like to reference as huge infrasonics:

The infamous opening "system killer" scene from Edge of Tomorrow:

Some of the music I've really come to enjoy thanks to the HS-24:

(Seven minutes of 8-9Hz. The amp hates it, but the HS-24 loves it.)

For when I feel like rearranging the furniture without lifting a finger:

When I talk about my HS-24 shaking and bucking heavy items off the top, these are what I'm playing. I've not had anything fall off the top when watching movies (without the bass excessively hot). That's with an underbuilt, fairly light, built in two days cabinet.

How tame are you trying to make your cabinet? While I wouldn't put a TV or disc player on top, yours looks like it should manage to be fairly stable except during the heaviest bass passages, especially if you're running two. (So jealous.) I assume your space limited and so need the upright position? Else laying your boxes down would do a lot to help reduce rocking. If the movement is too much, perhaps you could make a plywood plinth, fill it with concrete and bolt your boxes to that?

It's easy to forget how heavy these weights are that we're talking about. Get much heavier than you're already planning and you're talking the weight of a large refrigerator or more. Appliance hand trucks are only so handy. Don't go beyond what you can safely manage. My "800lbs" hand truck is rather optimistic in its rating; it gave me almost injurious troubles with my cabinet and driver combo when I took my "new baby" over to visit my friends' place. Just, be careful that you don't end up with a design that you can't get into your listening room without breaking your back.
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post #42 of 44 Old 12-05-14, 10:59 PM
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Re: Stereo Integrity HS-24

Erasmus wrote: View Post
Now, ideally speaking, a subwoofer enclosure should be two things: stiff enough to push the panel resonance above the passband and sufficiently heavy to push the mass resonance below (or at least low enough that most material won't get it rocking except in rare instances).
I'm not going to try to address all of your points at this late hour. A lot of valuable information there and I really appreciate the time you took. High five for that.

Here is the latest version. I moved the center of gravity down significantly, and adjusted the bracing.

Last edited by baniels; 12-06-14 at 11:01 AM.
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post #43 of 44 Old 12-07-14, 06:38 PM Thread Starter
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Re: Stereo Integrity HS-24

Sorry if parts were a bit broken up; I've trouble managing a full five minutes at a time without being called away and my mind can lose its place when that happens.

What you've changed to is looking good. Personally, I'd probably do more cross pieces here and there to break up those long vertical panels, but the way you have it now with the biggest parts braced, that could easily be added onto at a later date with chunks of 1x2s glued in if you felt it necessary.

Interesting baffle choice; I've seen that design on some audiophile monitors a few years back (which despite my trying, I cannot recall the name of right now) and thought it an interesting look. Applying it to a sub is a cool idea and looks good if you can manage a way to carve the pattern out; the baffle size required for a 24" is a lot of real estate that needs something to visually break it up. Are you planning on having the baffle ready before you have the drivers in hand? If not, you may find the drivers inspire you to other ideas; the frame of the 24" has crenellations that look kind of neat sticking out from the baffle, but kind of weird when recessed. Depending upon one's tastes, of course.

Whatever you do, with your attention to detail, you're going to have an awesome pair of subs.
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post #44 of 44 Old 12-07-14, 06:48 PM
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Re: Stereo Integrity HS-24

WOW... this is one insane driver. I want to come listen to it!! I am going to be purchasing a SI but probably only the 18 inch and not the HST either. Maybe in a couple years!
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