In reply of the last question "why rigid" ?
Because a surface when vibrating produces sound, and we want only the membrane of the loudspeaker to produce sound. The purpose of the cabinet itself is to eliminate the contribution of the rear radiation of a speaker, and to do this you don't want any extra sound generated. When mounted on a baffle only (i.e. dipole configuration) you don't have the extra stiffness that a solid ( should) gives due to the incident planes -mechanical stability; in other words, the vibrations of the basket transfer to the baffle which flexes, thus producing "returning mechanical currents"
which again return to the basket. It's all about vibrational analysis and how to drain them substantially.
Then you can ask yourself: if the membrane/diaphram is so light and thin...is there the possibility that the sound generated by it will go across it again ?
So we return to the term trasparency
This, we cannot cope with that, given the physical world...as we need a stiff light membrane which has to produce frequencies varying from 20 to 20kHz so wavelenghts from 17 m to 2 cm which evolve and then decay; knowing the fact that low frequencies "don't see" any obstacle and are omnidirectional ...
Because a surface when vibrating produces sound, and we want only the membrane of the loudspeaker to produce sound. The purpose of the cabinet itself is to eliminate the contribution of the rear radiation of a speaker, and to do this you don't want any extra sound generated. When mounted on a baffle only (i.e. dipole configuration) you don't have the extra stiffness that a solid ( should) gives due to the incident planes -mechanical stability; in other words, the vibrations of the basket transfer to the baffle which flexes, thus producing "returning mechanical currents"
which again return to the basket. It's all about vibrational analysis and how to drain them substantially.Then you can ask yourself: if the membrane/diaphram is so light and thin...is there the possibility that the sound generated by it will go across it again ?
So we return to the term trasparency
This, we cannot cope with that, given the physical world...as we need a stiff light membrane which has to produce frequencies varying from 20 to 20kHz so wavelenghts from 17 m to 2 cm which evolve and then decay; knowing the fact that low frequencies "don't see" any obstacle and are omnidirectional ...
Isolation between the front sound and the back sound. We want high isolation and also want the isolation properties to be smooth.
People who deal with noise isolation between rooms are very good at measuring and predicting TL: transmission loss. They know that the best physical property is membrane mass. If you have a flexible membrane then isolation is totally defined by mass per unit area. The flexible membrane is known to give a rising 6dB per Octave isolation curve. Doubling mass per unit area raises the whole curve 6dB.
As soon as you add rigidity to the mix then you will have a resonance where the wall is totally transparent (unless significant damping is added to the system). Acousticians call this the coincidence frequency. Theory and measurements show this to be true.
So you could make an excellent cabinet with heavy rubber sheet or any material with high mass and low stiffness. Stiffness doesn't help as its most likely contribution is to push cabinet resonances to frequencies where they are just more audible.
The Harwood approach is to reduce the wall stiffness with thinner plywood and increase mass and importantly, damping, with a thick damping layer.
David
Thanks for that David. I have retained above what I believe are the key points. I understand the "low stiffness" but not the "high mass", why is the high mass important? Is it simply to absorb all the rear radiation without restricting the movement of the cone?
The basic intention with any cabinet is to contain the energy coming off the back of the cone. It is out of phase with the front and would tend to cancel front radiation. Worse than that, if energy leaks through at panel resonances, then it will ring on in time and be much more audible than its level would suggest.
That is why all the comparisons with room acoustics and wall losses. The back side radiation is the noisy neighbors that you don't want to hear.
David
Hi Dave,
Did KEF use vibration isolation mounts on midranges as well, or only the low frequency drivers? I'm wondering about the case of a 3 way with the midrange crossed over somewhere above 300 or 400 Hz. Is the output of the midrange high enough in frequency that its contribution to wall vibration is small enough to make isolation mounts redundant?
Dan
Hi Dan,
I don't recall them using isolated mids but my recollection is hazy on this. I know the R101 small 2-way used a B110 in the stamped chassis and was definitely isolated. It used the simple isolation of rubber grommets and foam tape (the B300 used more sophisticated isolation with commercial motor mounts and foam tape).
Since the 105 and 107 used the same chassis B110, it is possible that they were isolation mounted but I just don't remember that being the case. For the most part, we were concerned about mid range cabinet resonances in the 200 to 500 Hz range and that was generally a woofer issue. (Also, Kef made a lot more 2-ways than 3-ways.)
You might ask over at the Speaker Talk forum as that has some other ex Kef people that might remember.
Regards,
David Smith
Dave and Dave,
Thanks for the responses. It sounds like it's probably worth a try then if I cross over in the 300-400Hz range. Speaker Dave, here's another question for you. You've mentioned using Noise Killer Yellow from a Swedish source for constrained layer damping. It looks like it is still for sale from Newfoam.com in New York. Was it used as glue to hold the two stiff layers together as well as provide damping? Or was it just to provide damping with some other mechanical means such as screws to hold the panel together?
Dan
Thanks for the responses. It sounds like it's probably worth a try then if I cross over in the 300-400Hz range. Speaker Dave, here's another question for you. You've mentioned using Noise Killer Yellow from a Swedish source for constrained layer damping. It looks like it is still for sale from Newfoam.com in New York. Was it used as glue to hold the two stiff layers together as well as provide damping? Or was it just to provide damping with some other mechanical means such as screws to hold the panel together?
Dan
The Noise Killer product was not a great adhesive, it always stays a bit gooey to achieve the desired damping so you might be able to peel it apart. It will probably survive moderate treatment, though.
Now, the beauty of constrained layer damping as opposed to having a soft isolation layer between two hard surfaces (say a springy foam layer) is that screws across the layers had little impact on performance. When we tried compliant layers the effect was strongly impacted by screws.
The same applies to woofer compliant mounting. Unless you can find a way to fasten the woofer to the cabinet without "shorting" the suspension, there won't be any benefit to foam gaskets, etc.
Probably the 105mkII. The earlier version had clunky wooden head structures and a less stylish bass cabinet. In fact, the whole compliant mounting came about because there were audible issues with the first cabinet style. As the big HP Fourier analyzer was just coming on line they used its capability to do modal analysis of the cabinets and found there was a big bounce mode to the woofer where the whole baffle was going in and out. That is where the impetus for the compliant woofer mounts came from.
Regards,
David
Based on my experience making cabinets, damping is more important than stiffness, but the best is a very stiff well damped cabinet. Stiff move the resonances higher in frequency where damping is more effective.
I use CLD on the two largest panels and custom designed high damping internal cross braces. My 20 years in automotive noise control came in handy doing this. The materials are the critical thing - high damping panels and high damping adhesives. I use no screws as they cannot be made high damping. I make my own high damping adhesive and nothing that I have seen works as well.
I use CLD on the two largest panels and custom designed high damping internal cross braces. My 20 years in automotive noise control came in handy doing this. The materials are the critical thing - high damping panels and high damping adhesives. I use no screws as they cannot be made high damping. I make my own high damping adhesive and nothing that I have seen works as well.
Hello,
Did you ever build these? Most interested to find out if you did and how they sounded?
Regards
Simon
Extremes of engineering sound great on paper and tickle the ear , BUT a skilled balance of the real world limitations has in my experience put much of "high tech" "advancement" to shame.
In my own experience nearly every good design practice I have stumbled on came from working on a limited budget.
In my own experience nearly every good design practice I have stumbled on came from working on a limited budget.
In 60 years of knocking my knuckles on every cab I met, I can't recall seeing measurements on the subject of enclosure vibration. Perhaps just my memory.
I believe all the heavy-duty opinions in this thread should await somebody showing the evidence one way or the other. Seems like a trivial experiment to measure a cab without bracing and then add the bracing.
The evidence must be out there, eh. Can't anybody post links?
BTW, used quarter-inch plywood on my 17-foot 12 Hz labyrinth. The cab does vibrate. So do the walls in my room.
B.
I believe all the heavy-duty opinions in this thread should await somebody showing the evidence one way or the other. Seems like a trivial experiment to measure a cab without bracing and then add the bracing.
The evidence must be out there, eh. Can't anybody post links?
BTW, used quarter-inch plywood on my 17-foot 12 Hz labyrinth. The cab does vibrate. So do the walls in my room.
B.
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