Is an open baffle isobarik alignment possible or useful?

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I'm embarking on building the MJK open baffle with Eminence 15A Alpha bass drivers.

I started to wonder what the effect would be if the baffle were thickened sufficently for two (2) 15A drivers to be mounted nose-to-tail (with sufficient clearance for excursion!) in "isobarik" fashion.

The front driver would experience isobarik loading when the cone moved backward and the rear driver would experience isobarik loading when it cone moves forward. I'm thinking the net "work" done by each driver per cycle would be reduced, compared to the effort of a single cone.

What would the net effect be?

1. Would the cones be better controlled near resonance? If so, could we cadge a few more Hz lower than the standard MJK system?
2. Also, would the choice of upper freq limit be affected? ie could we XO higher?
3. If there is any real and useful effect, what is the implication for driver selection? (The useful characteristic of the 15 Alpha is the high Qts. Would "faux isobarik" loading mean a rethink on the best Qts?)

cheers

Doug
 
Hi there - I've tried just what your talking about, and I've seen someone do it elsewhere too.

You end up with some funny results. If you are doing it the way you're talking about (with drivers close together), for some reason, you get a fairly unusable result. But, if you give the drivers enough volume between them (>5" between cones?), you actually get the predicted increase in output. I don't know why one way works, and another doesn't, but I've tried these arrangements a few times, and this seems to be the way it is...
 
1. Would the cones be better controlled near resonance? If so, could we cadge a few more Hz lower than the standard MJK system?
2. Also, would the choice of upper freq limit be affected? ie could we XO higher?
3. If there is any real and useful effect, what is the implication for driver selection? (The useful characteristic of the 15 Alpha is the high Qts. Would "faux isobarik" loading mean a rethink on the best Qts?)

ad 1.
If the parameters of both drivers are sufficiently close (which they should), they simply work as one. Isobarik mounting will increase the depth of the dipole, increasing the dipole length. This will give some gain - exactly the same as when mounting one driver in a H frame of the same depth.

ad 2.
On the contrary. The volume between the two drivers will set a new upper frequency limit, appr. at 2 kHz. :rolleyes:

ad 3.
There is no useful effect in case of a woofer.

Rudolf
 
Hi,

Adding a further two drivers isobaric style is pointless you get very liitle
in return and halve the load impedance, doubling them up on the baffle
is very different and possibly more worthwhile for maximum bass SPL
rather the senstivity increase it will cause.

rgds, sreten.
 
I'm embarking on building the MJK open baffle with Eminence 15A Alpha bass drivers.

I started to wonder what the effect would be if the baffle were thickened sufficently for two (2) 15A drivers to be mounted nose-to-tail (with sufficient clearance for excursion!) in "isobarik" fashion.

The front driver would experience isobarik loading when the cone moved backward and the rear driver would experience isobarik loading when it cone moves forward. I'm thinking the net "work" done by each driver per cycle would be reduced, compared to the effort of a single cone.

What would the net effect be?

1. Would the cones be better controlled near resonance? If so, could we cadge a few more Hz lower than the standard MJK system?
2. Also, would the choice of upper freq limit be affected? ie could we XO higher?
3. If there is any real and useful effect, what is the implication for driver selection? (The useful characteristic of the 15 Alpha is the high Qts. Would "faux isobarik" loading mean a rethink on the best Qts?)

cheers

Doug


Hi,

some time ago I read a " relatively" same thought in a patent applied for,design.

The designer uses two indentical low frequency drivers ,mounted in tandem, with the front driver in an open baffle.The second driver is mounted in a smaller area baffle with an acoustic suspension round enclosure.The whole thing (front open baffle and back closed box) is separated by rods of a calculated distance.The claimed response is of a controlled radiation in front and the back is a combined response of the back wave of the front woofer and the front wave of the second closed back woofer.Claimed also is the smaller size. Kind like your isobaric thinking,and interesting to boot.
Sorry I am not able to guide you directly to the patent,but a little google search on open baffle patents will take you there.


B.L
 
What makes it pointless? When done properly, I've been able to get 4-5dB
greater output (in the bass), for only a small increase in total space used.
For OB designs, that seems like a good thing.


Hi, its pointless because your numbers are wrong, output is not greater, rgds, sreten.

FWIW face to face works as well as any other arrangement, here its pointless.
 
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I'm embarking on building the MJK open baffle with Eminence 15A Alpha bass drivers.

I started to wonder what the effect would be if the baffle were thickened sufficently for two (2) 15A drivers to be mounted nose-to-tail (with sufficient clearance for excursion!) in "isobarik" fashion.

The front driver would experience isobarik loading when the cone moved backward and the rear driver would experience isobarik loading when it cone moves forward. I'm thinking the net "work" done by each driver per cycle would be reduced, compared to the effort of a single cone.

What would the net effect be?

1. Would the cones be better controlled near resonance? If so, could we cadge a few more Hz lower than the standard MJK system?
2. Also, would the choice of upper freq limit be affected? ie could we XO higher?
3. If there is any real and useful effect, what is the implication for driver selection? (The useful characteristic of the 15 Alpha is the high Qts. Would "faux isobarik" loading mean a rethink on the best Qts?)

cheers

Doug

Power handling will double, but efficiency is reduced. So it's a wash.

As for changing QTS, there are much easier/affordable ways to do it. (Raise the Re via a resistor, or the QMS via added mass on the cone. Or both.)
 
Hi, its pointless because your numbers are wrong, output is not greater, rgds, sreten.

FWIW face to face works as well as any other arrangement, here its pointless.

You know, that is rude of you to outright denounce my numbers as being 'wrong', as you don't know much about how I did my measurements.

With a 2.83Vrms terminal voltage, I measured one driver at a specific distance (1m or more). I then built the box housing both drivers, and connected the second driver in parallel. Using the same terminal voltage and mic distance, I then measured the new arrangement. Between 40 and 100Hz, there was a 4-5dB increase in SPL. I probably have the graph saved somewhere on another computer. Drivers are Goldwood GW-210, available from PE. If you don't believe my numbers, well, try it yourself. I've done this sort of trick a number of times, and the results are consistent, if done properly. Who else here has actually tried this?
 
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Here are some measurements, of one of the setups I've done. Notice the difference between 40 and 200Hz.
 

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Hi,

some time ago I read a " relatively" same thought in a patent applied for,design.

The designer uses two indentical low frequency drivers ,mounted in tandem, with the front driver in an open baffle.The second driver is mounted in a smaller area baffle with an acoustic suspension round enclosure.

B.L

I'm aware of the Tekton (Sp?) design aligment, but I want to keep the discussion focussed to two drivers (mounted FACE-TO-BACK, sealed) on a suitably thick baffle.

cheers

Doug
 
As for changing QTS, there are much easier/affordable ways to do it. (Raise the Re via a resistor, or the QMS via added mass on the cone. Or both.)

I'm not concerned about power or efficiency.
My question goes to whether such an arrangement, properly executed, will permit cheaper drivers with lower Qts to be effectively used on an open baffle. I acknowledge that simply having 2 drivers one above the other may be a low-tech solution, but I was curious to explore the implications of the alignment I suggested. It occurs to me also that having one behind the other will also not require the FR driver to be moved to a higher position on the baffle.

cheers

Doug
 
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