Its hard to explain how the braces should be made, but this came up somewhere else and someone posted a patent which pretty much covers all the concepts. I don't have it handy. My design may or may not have infringed on this patent, but its a moot point now.
I have found well damped to be the key. Perhaps not surprising, this is also the best way to make quite cars.
I have found well damped to be the key. Perhaps not surprising, this is also the best way to make quite cars.
Yea, I thought that it was an Enrique Stiles patent. I am working with him right now on another of his ideas. He's quite clever.
Perfect. Thanks. I added that to post 1.
It explains how CLD via bracing is more effective than CLD on panels.
Because the two halves of the internal brace are coupled to two different exterior panels, when either exterior panel flexes or otherwise moves toward or away from the other exterior panel, the internal brace's damping layer is subjected to shearing forces which are almost exactly parallel to the surface of the damping layer. The more normal to this movement the brace can be placed, the more efficient the shearing movement will be.
Another significant improvement results from the fact that the entire body of the damping is subjected to shear. Furthermore, the entire body of the damping material can be subjected to shear-induced displacement which is approximately equal to the maximum distance moved by the respective walls (assuming careful placement of the brace). By way of contrast, the constrained layer damping in the prior art undergoes very little shear, even at the middle of the panels, and essentially zero near the edges.
Another significant improvement results from the fact that the entire body of the damping is subjected to shear. Furthermore, the entire body of the damping material can be subjected to shear-induced displacement which is approximately equal to the maximum distance moved by the respective walls (assuming careful placement of the brace). By way of contrast, the constrained layer damping in the prior art undergoes very little shear, even at the middle of the panels, and essentially zero near the edges.
And, in simple terms why stiffening is not always the answer:
Internal bracing stiffens the cabinet, shifting the panels' resonance to higher frequencies, but does not change the amount of damping of the enclosure. It changes the frequency but not the amplitude of the vibrational resonance.
Is that last sentence right (amplitude is the same)? If you double the pitch and maintain the same amplitude, would that be 6dB louder?
The last quoted sentence is not exactly correct, but your interpretation is not either. What Enrique means is that damping always lowers the resonance peak, but stiffening does not necessarily do that. It might, but not always. A stiffer structure takes more energy to excite it so it may not have as great a response amplitude. The point to take home is that neither mass or stiffness dissipate energy. Only damping does that, so only by damping a system can you ensure that all response is lowered.
This is an idea I've seen expressed before, but I do not understand it.
Scenario:
A normal woofer is in a bare cabinet. The woofer is fed 100watts:
a) ~1 acoustic watt radiates from the front of the cone
b) ~1 acoustic watt radiates from the back of the cone
All of b) has to go somewhere. If it has trouble exciting the structure, where does it go?
Ludwig's example has no external cone, so 100% of his acoustic energy is b)
The first time I played fairly loud music through the speaker inside the otherwise empty box was a real revelation. The large box at this point was sturdily constructed out of 3/4" medium density fiberboard (MDF); it was a complete sieve for the sound. It was just amazing. Of course, this is just physics at work. As noted above, the sound energy had no place to go except to escape the box (actually, the panels do absorb some sound energy as heat, but apparently not a lot).
Although I never said to point the speaker in the box at the back of the driver cone that probably WOULD exaggerate the effect. You might instead can put a mini speaker in a small cube and point it AWAY from the cone, e.g. at the back of the box. I think that is a fair test. The cone leaks way more sound that people realize, especially those lightweight "pro audio driver" cones... I heard this exact problem with a "high scoring DIY loudspeaker" very recently. It used two 8" or 10" pro audio woofers in a large cabinet, possibly vented. Wow, you could hear the internal reflections from the cabinet walls very clearly. I think most of it was coming straight at me right through the woofer cones.
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This is EXACTLY how I planned on doing this exact test. Maybe this next summer. Peerless makes some nice little 2" wideband drivers that would work perfectly. I planned on checking radiation through the "dummy" cone with different lining/stuffing. I would use the SB Acoustics woofers I have in both poly and aluminum to see if they differ. Then capping the dummy woofer hole and seeing just what the cabinet itself radiates, while testing various construction methods.
You are not comparing apples with apples. A panel and a brace provide stiffness and CLD adds damping in an efficient manner. The damping rod in the paper provides negligible stiffness, probably unwanted mass but does add significant stiffness. Rods like this tend to get used more to hammer down problematic modes like in the KEF paper rather than for overall damping like CLD. This is because they can be placed to respond strongly to a particular mode but cannot be placed to respond to all modes.
It is misleading but it's vagueness possibly saves it from being completely wrong. If the cabinet is stiffened then it will deflect less when the same force is applied. However, at a higher frequency it requires less deflection to get the same sound level. So the two effects tend to cancel leaving the sound radiated about the same. The BBC papers have measurements showing this.
Again, it is misleading more than wrong.
Firstly, the work put into vibrating a cabinet comes mainly from the drivers hammering away around their bolts and not from the air pressure within the cabinet. Secondly, the air within the cabinet behaves like a spring over the lower part of the frequency range. This means your 1% sound does not need to be absorbed it simply makes the motion of the cone lead the signal by a bit. At higher frequencies where the wavelength becomes short enough for cabinet modes to be present and the volume of air ceases to behave like a discrete spring acoustic damping material becomes effective. The rule of thumb is quarter of a wavelength for effective acoustic damping which is why the wedges on the boundaries of an anechoic chamber are so long.
So at this point I would make a pool for those who use rubber gaskets beneath the woofer and the cabinet and those who don't.
The forum doesn't want to let me edit this typo. It should of course read: The damping rod in the paper provides negligible stiffness, probably unwanted mass but does add significant damping.
And this should read: So the two effects combined tend to leave the sound radiated about the same.
The lesson to be learned here is not to try poking away with one finger on a phone in a coffee shop particular when the forum is running software that does not adapt to phone use. We live and learn. Hopefully.
What would you use in place of the usual foam rubber (historically/sometimes cork) gasket that comes with most woofers?So at this point I would make a pool for those who use rubber gaskets beneath the woofer and the cabinet and those who don't.
Thin non-corrugated cardboard, 1.5mm or less, such as comes on the back of notepads or composition books.
At LFs the rear sound just acts as a spring and the energy gets absorbed mostly in the speaker itself, but at HFs it gets absorbed inside the cabinet as heat. It is very important to loosely fill the cabinet with absorption to help dissipate this sound because not much sound would be dissipated by the cabinet walls.
Regarding mounting the drivers on soft gaskets etc., I should think this not such a good idea as the drivers frame should be rigidly mounted or you run the risk of adding in resonances. I do believe that the front baffle should be strong/thick and I use CLD on this panel of the cabinet to absorb the mechanical energy imparted by the driver.
A note on good damping materials:
Damping is always done by friction. Friction is created in a material as the particles move against each other. It is always beneficial to add in some particles in the form of a filler. I use 3M micro-spheres which are very small hollow glass beads. Solid ones are also available. These spheres rub against one another and dissipate the energy.
The green glue is OK, but no where near as good as it would be if filled. As a damping compound the binder should not set-up to be solid. The best that I have found is a soft 2-part polyurethane from a place like Innovative Polymers. Then fill it with the spheres into a paste. That stuff works exceptionally well.
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