Over the years I read here and there about a old speaker type called a boffle.
It consists of a open backed box with many felt curtains with different sized holes behind the driver. Here are some images to illustrate what it looks like
The idea is to "eat away the back wave" to emulate a infinite baffle.
My question is this, wouldn't it be better to use CLD material (constrained layer damping) in stead of felt curtains......or CLD material covered in felt?
In my mind CLD dividers would suck more energy out of the back wave, especially at low frequencies. I could make sheets from bitumen sandwiched between thin metal plates covered with thin automotive felt....
Is it worth a try?
http://p10hifi.net/tlinespeakers/forum/boffle-RadioElectronics.pdf
It consists of a open backed box with many felt curtains with different sized holes behind the driver. Here are some images to illustrate what it looks like
The idea is to "eat away the back wave" to emulate a infinite baffle.
My question is this, wouldn't it be better to use CLD material (constrained layer damping) in stead of felt curtains......or CLD material covered in felt?
In my mind CLD dividers would suck more energy out of the back wave, especially at low frequencies. I could make sheets from bitumen sandwiched between thin metal plates covered with thin automotive felt....
Is it worth a try?
http://p10hifi.net/tlinespeakers/forum/boffle-RadioElectronics.pdf
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Felt is the only material to be used for its acoustic properties. It is dense but the reflection and refraction of sound ( diffraction can be corrected through geometrical shapes) compared to a rigid material is totally different.
Here is the text from the original article:
Electrical filters are made up of in- ductors, capacitors, and resistors. The components of acoustic filters are masses, springs, and friction. In this design, the air between the screens forms the masses, the stretched screens themselves are the springs, and the ab- sorbency of the screens represents the friction. The holes in the screens, tapering in size, control the slope of the filter; Fig. 2 shows a two-stage filter. The action is somewhat as fol- lows: The enclosed air is broken up into small sections, each of which has a rather high resonant frequency. The energy of the moving air in each sec- tion is absorbed by the screen immedi-
ately behind it. The first section gets
the greatest impact from the moving
air, so the first screen has a rather
large hole to allow most of the moving air to escape into the second section; the smaller hole in the second screen allows the same proportion of air to escape into the third section, and so on, to the end of that stage in the filter. The process can be repeated sev- eral times if the cabinet is so deep as to form a real tunnel
So if: the stretched screens themselves are the springs..... Wouldn't CLD drain a lot of energy out of the back wave? Imagine a dozen CLD dividers flexing back and forth......
Felt doesn't do much at low frequencies, that is why subwoofer boxes only use bracing and no lining on the walls.
Yes, well actually none can be 'corrected', but all 3 can be optimised using geometrical shapes.
You're right...I was thinking to myself that the usual flat felt panels ( like the one that is star shaped around a tweeter, in recent times...) don't offer much in comparison to a sculpured one. I mean, the star shape offers angles ( and cavities ) but
it's still primitive, so the correctness is related to the efficiency. Now don't bring the AR9 as another example please!
it's still primitive, so the correctness is related to the efficiency. Now don't bring the AR9 as another example please!
Nope. I asked about a month ago if anybody was interested in seeing some pics of some 'felt work' around the tweeter ( I didn't mention the back of the midrange or in sight of a woofer) but nobody replied.
The attached text also talks about box resonance and enclosed air column resonance which are lower frequency in nature. I suggested CLD covered with felt where the felt is for higher frequencies and the CLD to absorb low frequency energy.
Yes you can make cells that behaves each as a Helmholtz resonator, but the key word is absorption.
I hadn't read the full text, but think as your bold text (in bold) about springs: sound travels through air as a spring, the air is a spring, waveforms adapt to the 'media' they 're in.
The box pictured just makes resistance to the airflow. If the back was closed, it would behave like a pneumatic (?!) suspension box.
But you don't want that, as the other key word is 'annihilate the back wave'
What you say, instead, doesn't take the natural decay of sound in account, which is what the felt accomplishes (to). Right?
I hadn't read the full text, but think as your bold text (in bold) about springs: sound travels through air as a spring, the air is a spring, waveforms adapt to the 'media' they 're in.
The box pictured just makes resistance to the airflow. If the back was closed, it would behave like a pneumatic (?!) suspension box.
But you don't want that, as the other key word is 'annihilate the back wave'
What you say, instead, doesn't take the natural decay of sound in account, which is what the felt accomplishes (to). Right?
When the back wave travels through a hole and into the next chamber pressure builds which pushes on the CLD screens. This pressure will cause them to bend. CLD doesn't act like a spring but more like a shock absorber converting the sound wave energy to friction heat losses in the CLD damping layer.
At least this is what I hypothesize. I hope someone with a better grasp of physics can chime in.
Did I make a bunch of logic errors or did I just invent the bass equivalent of the Kef tweeter meta material?
At least this is what I hypothesize. I hope someone with a better grasp of physics can chime in.
Did I make a bunch of logic errors or did I just invent the bass equivalent of the Kef tweeter meta material?
Should that imply you need not answer my question? I for one missed that post of yours and am really interested in your findings.
This is reasonable. The question may be what in the box does it affect.. In other words is it linear by pressure, or balanced by frequency? It could be argued that simple resistance is similar to aperiodic or stuffed sealed, but could this be more effective? Could it be better at reducing the modal contribution (this is the touted benefit I recall from reading it in one of the better books from the '70s).
Sure, stuffing works best in the center of the box where velocity is highest, but the rest is a bit of 50's wishful patent-applied-for thinking...
Felt? I have always seen thick insulation. FIberglass but things like UltraTouch would work. The idea is to try to make an open backed box act more like an IB when the enclosure size wiulkd otherwise be prohibitive,. Bigun’s (@Bigun) Boffle for his AN 15” is a good example, Cal (@Cal Weldon) also used it for his Altec 604 midTweeter
Here is my origibal scan + the patent (same as attachment in post #1 replaced with URL). I also fixed the spelling in the title.
http://p10hifi.net/tlinespeakers/forum/boffle-RadioElectronics.pdf
http://p10hifi.net/tlinespeakers/forum/Boffle-patent.pdf
CLD is for box wall damping not acoustic airapace damping. It would be pretty much useless as the damping in a boffle. The idea is to make the cabinet effectively deeper without the U-Frame resonaces.
dave
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Sorry, bad choice of words. English is not my first language. In the dutch language screen=scherm and it can refer to solid sheets.