Earl, I won't argue that the differences between conventional PR and your AL. Yes, they are there, but mechanically it is still an mass/spring resonator coupled to the driver by a spring. At low frequency it will follow the driver, at resonace it will peak and at high freuqency the response will fall off.
If you draw the mechanical mobility schematic for a BP PR and a BP AL aren't they are identical except for the additon of a compliance element between the moving mass element (the PR or double disk) and ground in the case of the AL? I think so. Differences come in the values of the parameters. It is precisely the difference in area of the disks which leads to different volume velocities which creates the additional compliance element as it leads to compression/expamsion of the air in the common chamber. Make the disks equal in area and this compliance element goes away.
If you draw the mechanical mobility schematic for a BP PR and a BP AL aren't they are identical except for the additon of a compliance element between the moving mass element (the PR or double disk) and ground in the case of the AL? I think so. Differences come in the values of the parameters. It is precisely the difference in area of the disks which leads to different volume velocities which creates the additional compliance element as it leads to compression/expamsion of the air in the common chamber. Make the disks equal in area and this compliance element goes away.
Am I missing something or is the lever component simply a solid rod connection between the PRs? If so, many of today's PRs have a rigid coupling on the back for adding weight that is usually a metric thread into a solid disc. All one need do is mount them back-to-back with the correct length threaded rod. Those made by Peerless or Seas are pretty solid units. The area ratios are limited, but what's available off-the-shelf may still be sufficient.
Dave
Hi John
Yes, its a bandpass system, nothing new there, and yes if you make the two pistons the same size then its just a bandpass system with a PR. But the two disks AREN'T the same size, thats the point. Up to 6 dB more efficiency is nothing to scoff at. Try to double efficiency any other way.
It doesn't double the efficiency, maybe 2-3 dB tops. And the driver then costs more. For the cost of a simple modfied PR you get the same out as if you had TWO drivers. This was the test that we did and showed that it does work. One driver and a lever had precisely the same output as two drivers - and the same cone excursion, BUT 1/2 THE POWER WAS REQUIRED.
One driver and a PR is a LOT cheaper than two drivers and twice the amp power.
I can fully appreciate how it works and don't dispute there would be a gain in efficiency but at the cost of bandwidth (like you've said) - low frequency cut off is the big one. We could live (actually welcome) the high frequency limit in a sub but the result looks like a one note wonder.
I think I'd gladly trade higher power / price for lower extension.
Interesting idea though.
I think I'd gladly trade higher power / price for lower extension.
Interesting idea though.
What tunings are reasonable? That is, the latest PRs tend to be tuned very, very low to give extra extension with targeted woofers and are frequently tuned below the lowest musical notes, so the highpass system F3 is usably low, especially for music only. Would the gain allow for more output at lower tunings or is it not suitable for very low tunings? Considering that two PRs are required, it does seem that the requirements for each one may limit the options.
Dave
Dave, we can compare an AL to a PR. Given a PR with area of the driven, smaller disk, mass of the double disk structure, and suspension compliance equal to the total compliance of the double disk suspension and common chamber, the motion of the AL and PR will be identical if the other factors (driver, rear box, coupling box) remain unchanged, so long as we can ignore effect of the radiation impedance on the motion. Given the same motion, the pass band will be the same and the only difference will be due to the increase in radiating area of the outer disk compared to the inner disk resulting is some gain in SPL.
The AL is a novel/clever idea, from an academic point of view, but I would think the need for the additional compliance element in the form of a (large) , sealed box makes it impractical. It seems to come down to the same old thing. If you want higher efficiency at low frequency, you need a bigger box.
Still no way around Hoffman's Iron Law without a Linkwitz Transform, eh?The AL is a novel/clever idea, from an academic point of view, but I would think the need for the additional compliance element in the form of a (large) , sealed box makes it impractical. It seems to come down to the same old thing. If you want higher efficiency at low frequency, you need a bigger box.
Dave
First there aren't two PRs, there is one lever. In practice there would only be a single unit, not two connected by a rod as discussed (this would be very innefficient). I don't think that I want to discuss the details of how I would make the actual lever in practice except with someone who is actually serious about making and selling them - in cooperation with me.
The lever has all the same limitations and advantages of a standard Bandpass (BP) design, except more efficiency. You can trade off efficiency for bandwidth, but about two octaves is going to be the practical limit - just as it is for bandpass subs. Of course this does limit a lever to use as a sub. The lever can be tuned as low as you like, but then the driver needs to be tuned low as well, etc. etc. Think of a lever as simply a BP with more output and everything else falls in line.
This IS the single biggest limitation, that is correct - at least on paper. But in practice we found ways arround this resulting in more actual output for a fixed volume of enclosure. That aspect is a trade secret however that I have to reserve for a time when I might make a lever sub. I've always wanted to and I intend to, I just need to financial resources to have the lever made. Gluing two PRs together is not an effective way to go in any real production sense.
In fact, our studies showed that for a given driver, and a fixed total volume no other design could produce as much volume velocity (thats what we are really talking about here for a subwoofer) as the lever - not even a horn. In fact in terms of efficiency for a given volume a horn is terrible. As far as I am concerned, in a small room where I am going to EQ the low end with multiple subs, total linear volume velocity is really all I care about. LF extension and bandwidth are all just a matter of EQ (as long as the "power handling" of the driver is sufficient). But total volume velocity - thats what the sub has to do because electronics cannot help.
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The trick of efficiency
Active driver is coupled to a "small" PR
We know a small driver moves more than a big, fore same output etc
Fore a small PR the ressonance would move up
Here it doesnt because its coupled to a bigger PR, thus get quite heavy, and ressonace get low
The smaller PR couples to a bigger PR, that normally would move less, but moves more because its coupled to the smaller PR that moves more, and efficiency goes up
Is that how it works
Active driver is coupled to a "small" PR
We know a small driver moves more than a big, fore same output etc
Fore a small PR the ressonance would move up
Here it doesnt because its coupled to a bigger PR, thus get quite heavy, and ressonace get low
The smaller PR couples to a bigger PR, that normally would move less, but moves more because its coupled to the smaller PR that moves more, and efficiency goes up
Is that how it works
Actually you are spot on in the first assumption. The acoustic lever is just a bandpass box where the port is replaced by a passive radiator. Nothing more, nothing less. It's really nothing new. The technique has been used in several commercial and DIY designs in the last 20-30 years.
The caveat of the design always comes back to that if tuned imperfectly, or due to changes in the drivers used over time, ie. loss in magnet strength and gain in driver compliance, the design goes from combining the best aspects of a BP box and a PR to combining the worst aspects of the same.
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Roughly - yes. If we didn't have to worry about the space in between the two pistons then it would be trivial - twice the area is twice the volume displacement for a given PR displacement. The inside surface displaces the same volume as the driver, but the outside surface displaces twice that of the driver. Masses, volumes and all that is just tuning. But the space between the two pistons is an issue that has to be dealt with.
Of course we could always just evacuate this space and then it goes away acoustically, but that adds another problem. Oh well.
Make an equivalent schematic for the same values, and you'll find that it comes out as exactly the same thing.
There are losses - coupling the driver to the rear piston through air, suspension losses. The air space between inner and outer pistons would need to be vented to avoid low pressure (larger piston displaces more air than the smaller piston, creating a low pressure situation if the space is sealed).
The outer piston becomes the sole radiating surface and is a flat disc the ideal shape?
Seems to me a better approach would be to directly couple the driver to the outer piston - or, in essence, use the drivers motor to directly power the outer piston, creating what would be a larger driver. Efficiency would be greater since some of the losses have been eliminated.
The outer piston becomes the sole radiating surface and is a flat disc the ideal shape?
Seems to me a better approach would be to directly couple the driver to the outer piston - or, in essence, use the drivers motor to directly power the outer piston, creating what would be a larger driver. Efficiency would be greater since some of the losses have been eliminated.
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