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Old 11th September 2008, 03:17 AM   #11
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OR is that a function of the voice coil?
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Old 11th September 2008, 04:19 AM   #12
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No, you got it right the first time. Understand though that it will raise Qts and lower efficiency, increasing box size. There's various ways to add the mass, from crudely epoxying fishing or wheel weights in the 'valley' of the diaphragm/dustcap or even bonding a flat disc with insulation in the cavity.

From Ron E:

For mass loading:

mass(m) = 21.9453*dia^4(cm)/(Fs^2*Vas(liters))

m' = m + mass added

mass ratio(mr) = m'/m

Fs' = Fs/mr^0.5

Qes' = Qes*mr

Qms' = Qms*mr

then: Qts' = Qes'*Qms'/(Qes'+Qms')

Vas is unchanged

n0 = 9.614*10^-10*Fs'^3*Vas(liters)/Qes'

SPL = 112.02+10*Log(n0)

Anyway, when you 'do the math' you'll see that lowering Fs to ~10 Hz isn't practical.

GM
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Old 11th September 2008, 04:44 AM   #13
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You can add weight. You have to make sure the cone will not flex too much due to the additional weight, and that the motor is strong enough to start and stop the weighted cone. If you use WinISD, there is a place to model the drivers in the desired enclosure with mass added to the cone.
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Old 11th September 2008, 01:48 PM   #14
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Thanks for the replys. I was asking because I was thinking of playing around with a Nestorovic woofer setup as he outlines in his patent. He says that the primary woofer would have a generally higher Fs than the "active" passive radiator which would generally have a lower Fs and a higher mass in its cone. I did some prelim calcs and my box dimensions come out to about 2.5 ft3. I was also going to use some loosely applied damping material in the cabinet as it suggest. Here is the link to the patent if anyone cares to skim over it. It mention the above in section 4, right column, lines 1-9. I know for a fact in all of his Speakerlab projects, his woofer magnets are the same size but the Fs is different.

Maybe I should use a 10" as the Primary and a 12" as the "active" passive????? But I don't have any 10's! UGH!!!

http://www.google.com/patents?hl=en&...m=10&ct=result

I was going to connect the finished subs to an active crossover set at 150Hz so this sub would run from that point down.

I suppose I will experiment without changing the cone mass first but I think without adding some mass to the cone the cone may vibrate too much like a true passive radiator to keep it controlled by the circuit in the Nestorovic system and cause unwanted results.

Let me know if you have any suggestions. I got to get that software too!

Thanks,

Ed
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Old 11th September 2008, 04:42 PM   #15
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FWIW, Tom Danley long ago worked out PR system design and posted some guidelines on the old basslist that have worked well, so probably would be good for your active PR:

Hi
Not all drivers can be used with a passive radiator but the greater the
moving mass the driver has and lower cutoff frequency is for the box
size, the more likely the passive design will be practical.

Take the area of the driven cone times its Xmax to get its displacement,
allow a factor of 1.5 or 2 times that and find a passive with
aproximatly that displacement (Move the suspension from peak to peak and
use like 75% of that displacement). Take the piston area of the passive
and plug it into your math as the port area. Take the port length times
the area to get volume and the density of air to find the port moving
mass. If the mass is less than can be realised in the real world then
use a larger diameter passive and try the port calculations again.
Best regards,

Tom
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Old 13th September 2008, 02:13 AM   #16
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Thanks for the advice.

I am going to download the software tonight.

Do you think I should be using two of the same woofers for this project or should I use a smaller woofer for the primary and one of these 12" woofers for the "Active" passive? By using the two different sizes, that may avoid having to do any weight changes, no?

I was originally calculating volume of the cabinet as a sealed enclosure, but it should be calculated as a passive with my selected woofer area as the port opening from what I understand now?

Thanks!

Ed
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Old 14th September 2008, 08:02 PM   #17
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You're welcome!

As TD noted, with a PR you want it to have quite a bit more displacement, so if the same size driver is used, then it needs 1.5 - 2x the linear excursion or for the same excursion you need 1.5 - 2x more cone area, etc.. I haven't picked apart the patent, but I don't see why it being active would change this as down low it's all about displacement.

WRT to mass, obviously you're going to need a lower Fs driver, so de facto it will be either more massive or more compliant or a combination of them and if you want to keep box size reasonable it will have to have a low Vas, ergo more massive.

Still, you're not going to find a driver with a low enough Fs AFAIK, so mass loading will still be required.

Right, pick a vented alignment and input whatever size driver Sd you want as the vent area and let it calc a length, then find its Vb and air mass and this is how much the diaphragm/VC/DC assembly (Mms) needs to be, so subtract its calculated (or published) Mms to find the amount of added weight required as a first approximation or probably 'close enough' amount.

GM
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Old 17th September 2008, 03:09 PM   #18
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Yet another novice question ...

Xmax: I was looking it up on Wiki on how to calculate.

"In the simplest form, subtract the height of the voice coil winding from the height of the magnetic gap, take the absolute value and divide by 2"

I do not have that information in my specs on the woofer BUT I do have what they refer to as the Voice Coil Overhang given in mm's as 5.1

Would this be it with the measurments/math already done?

Thanks!

Ed
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Old 17th September 2008, 03:27 PM   #19
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Right.

GM
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Old 17th September 2008, 10:57 PM   #20
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I was playing around with WinISP Pro and entered the driver specs for two woofers. One is a 10" and the other is a 12". The cabinet I chose is for a passive design. However, in the Nestorovic system, the 10" is the primary driver and the 12" is the "active" passive, hence it is connected to the amplifier with a magnet, etc., I entered all the parameters in the program and it came up with weird results. It said it could not really calc the box and even gave me a volume of .5 Cu Ft. WAY OFF!

Should I just run numbers by hand for a passive enclosure as if the 12 is a true passive and ignore the fact for now it is an "active" passive?

Should I calculate the cabinet volume as a sealed cabinet instead?

I am kinda new at this part of things, so with a 12" woofer, should the cabinet volume be based on that unit since it will see power allowing for enough volume for it as oppsed to the smaller 10"? Basically the 12" would govern the cabinet volume.

I hope that makes sense.

Thanks,

Ed
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