You incur more losses in the energy transfer which in this case is a good thing.
Similar or same principle as that behind constraint layer boards.
when stressing that ply piece on the face there is tensile strength in each ply. When stressing the stripey edge, there is perhaps less.
Or I should say that its strength is influenced by the adhesives strength in shear stress, as opposed to tensile/compressive stress. I think I got that right.
Few glues are good at both. For the record im not slating ply for braces, merely stating that I doubt it matters much if the holey brace is MDF, ply or solid wood. I would use what i had in scrap, and save cash. Its not much to lose sleep over
Or I should say that its strength is influenced by the adhesives strength in shear stress, as opposed to tensile/compressive stress. I think I got that right.
Few glues are good at both. For the record im not slating ply for braces, merely stating that I doubt it matters much if the holey brace is MDF, ply or solid wood. I would use what i had in scrap, and save cash. Its not much to lose sleep over
Last edited:
If I remember correctly, I read somewhere, some time ago, Jensen's recommendations for cabinet bracing. To clarify, this was from the days when Jensen made their big coax and triax drivers and had their complicated nomographs to design ports - pre Thiele and Small. Cabinets in those days were plywood, because MDF hadn't been created, and huge. Jensen recommended bracing to run parallel with the long sides, not perpendicular. IOW, divide the long sides into 2 narrow panels to increase resonant frequency, as opposed to 2 more typically 'square' panels.
I would disagree with this. To start a resonance you 1st have to pump sufficient energy into it at just the right frequency to get it going. Fighting this is the natural energy dissipation of the material which is related to the relative dimensional differences between the material thickness and the wavelength in question.
A high Q resonance needs more energy pumped into it and in a narrower bandwidth, making it hard for music to ever deliver that much targeted energy into the panel.
dave
For their own series of LS cabinets, yes.
Here's a quote from Alan Shaw, who now owns Harbeth and is the 'spiritual successor' to Dudley Harwood of the BBC:
"The interesting curves are the blue one, showing the cabinet output alone (ignoring the output of the bass unit) when the box is made from 18mm (3/4 inch) undamped panels. Note again that in the middle frequencies the output of the box is very nearly as strong as from the woofer. That implies that the cabinet is acoustically transparent and the sound normally constrained inside the box is setting the 18mm panels into sympathetic motion and they are acting as a huge radiating surface."
And to answer JRKO, I'd say that the typical 6" driver with a sensitivity of less than 90dB is unlikely to create serious panel resonances when mounted in a 3/4" thick cabinet when following 'standard' bracing and damping practices.
However, those going "pro", that is, using 12" or larger drivers with >95dB sensitivity and higher than 'sub' crossovers, are likely to find that the mass of the moving assembly coupled with requisite box volume does, indeed, create a situation where panel resonances in the midrange need special attention.
And speaking of the BBC papers, there were two notable, and perhaps apropos, discoveries. A box with panels of twice the thickness rang more than a 'normal' box. Also, a box which accidentally had loose battens rang less. IOW, while gluing all the panels together guarantees an air-tight enclosure, the resulting construct rings like a bell.
Good energy is warm and radiant, like sunshine on your face. Bad energy is chaotic and destructive, like an atom bomb or your car going over a cliff.
Quick question guys.
Back to bracing:
I was thinking of using MDF for window bracing on my first DIY project that I am now working on (a pair of Paul Carmody's Amigas - https://sites.google.com/site/undefinition/diy/amiga ).
If I use MDF, or plywood, and create the windowpane braces, then should the holes in the braces be circular? It is my understanding that parallel surfaces within the enclosure are conducive to standing waves. Is this true? Are there any advantages to circular holes in the braces?
Back to bracing:
I was thinking of using MDF for window bracing on my first DIY project that I am now working on (a pair of Paul Carmody's Amigas - https://sites.google.com/site/undefinition/diy/amiga ).
If I use MDF, or plywood, and create the windowpane braces, then should the holes in the braces be circular? It is my understanding that parallel surfaces within the enclosure are conducive to standing waves. Is this true? Are there any advantages to circular holes in the braces?
Last edited:
Wrong. No matter how much energy you put into it, the resonance is always there. And, at resonance, the panel is more likely to receive the energy, because both impedances, mass and stiffness, cancel out and there's only the resistance (the damping) working.
What?
The potential for resonance is always there. But if you never put sufficient energy into it to excite it, it is as if it isn't.
A tuning fork always has the potential to resonant, but if you keep whacking it with a wet noodle it isn't going to make any sound.
dave
That is a band-aid. The resonance is still there. If i have a resonance that is an issue i'll tackle it mechanically
Who cares, i don't listen to dirac impulses, i listen to music. And i don't gohitting my speakers with a hammer while listening.
If of sufficient magnitude a good excitation if you want to find where resonances are
dave
But if you filter it, you reduce the energy transferred into that resonance. According to your argumentation above this would be a good thing. Because it can't resonate any longer.
Did you know that you can represent music as a sequence of Dirac impulses? So, if you can excite a resonance with a Dirac, you can excite it with music.
true, but you'd need to have a steep notch to remove the energy which would screw up the FR and that filter in the signal path all to often does way more harm than good.
Like a CD? You can excire the resonances with a dirac of sufficient magnitude. Doesn't mean that energy exists in the music.
A dirac is like the whack you give a tuning fork to get it ringing. The tuning fork takes the energy available at the frequency of its resonance, and if it is sufficient it starts ringing.The whack has to be large enuff...
As implied above the best way to introduce a dirac into a cabinet is to whack it with a hammer.
You can also decompose music into sins. That hasn't proven very good at predicating the real-world performance of a device.
dave
Just because you 'can' doesn't mean 'it' will.
But you admit that resonance isn't there, as P10 said, unless you hit it with the right frequency, for the right amount of time, at a volume sufficient to overcome the natural properties of the panel in question?
The Albert Bridge crosses the Thames near Chelsea. The sign below was posted many years ago due to a mechanical resonance being created when troops from Chelsea Barracks marched across it. It just took a certain number of men marching at a certain pace. They could however all walk across it at the same time with no ill effect. No doubt that frequency was produced by them walking as well, just not at a volume necessary to overcome the bridges natural structural rigidity.
You do not have to filter out 700hz (for example) just because at high volumes with one or two tracks in you collection you can manage to excite a panel. That would be deleterious to ALL music. You would be better off bracing or better yet not playing those tracks at high volume.
As Keriwena noted most normally braced cabinets of average size (75% of this forum maybe?) will NOT have an issue. Big efficient drivers in big cabinets "create a situation where panel resonances in the midrange need special attention."
Attachments
Sufficient energy would be enuff that every man women & child would not want. And it would need to be clean. We are a ways off...small fusion generators would be a good start.
dave
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.