[audiochef]

Wow Rory B,
you remind me of back in the days of experimenting with LSD. We'd be    flying and each one of us trying to describe in words what was going on in our minds. We were going in circles of course when someone would say "stop, don't  try and explain , your ruining my buzz."

It was beautiful then , and it's beautiful now.
Who cares!
 Don,t try to explain just sit back and enjoy the" VMPS tripp."

[Redbone]

Again I learned something new about speakers:  On the 40s, if one manually moves either of the woofers by hand, the other one moves in perfect inverse unison.  I always thought that this was because of air pressure in the sealed cabinet.  Completely freakin wrong !  It is electrical induction, yikes.  Hooked a voltmeter up to one of the mega-woofers and actuated it by hand, it generates 4 volts easy.  This is what the damping factor of amps is for.  I've read that crossovers can play havoc with this induction.  

In any case it is not the sealed cabinet that dampens the upper mid-woofer, as I had previously thought.  I am playing the 40s right now without the PRs and the mid-woof is heavily damped.  Having it on the same circuit as the mega-woofer is what causes it to sound so different from native.

Scraped all of the putty off the PRs and am going to weigh it.  Am going to coat the paper cones with Elmer's and let them dry a day or two, then reinstall.  I think that I've got a lot better feel for tuning the PRs now, we'll see.

Oh yeah, the bass is a bit muddy and boomy, but not as bad as you might think.  

Here are some pics of the XOs

[doug s.]

Again I learned something new about speakers:  On the 40s, if one manually moves either of the woofers by hand, the other one moves in perfect inverse unison.  I always thought that this was because of air pressure in the sealed cabinet.  Completely freakin wrong !  It is electrical induction, yikes.  Hooked a voltmeter up to one of the mega-woofers and actuated it by hand, it generates 4 volts easy....

i think your first supposition was correct.  i suspect yure wrong now - the movement of the megawoofer will generate a woltage reading yust by the relationship of the coil/magnet/etc....  you will find that if you move the passive radiator by hand, both megawoofers will also move, & they should move in unison to each other, & inversely to the passive....

doug s.

[Redbone]

i think your first supposition was correct.  i suspect yure wrong now - the movement of the megawoofer will generate a woltage reading yust by the relationship of the coil/magnet/etc....  you will find that if you move the passive radiator by hand, both megawoofers will also move, & they should move in unison to each other, & inversely to the passive....

doug s.

Nope, if you're reading carefully I'm running these speakers with the PRs completely removed and still get the exact same effect.  Maybe someone else who has the PRs still in would like to test your hypothesis.

[Rory B.]

But keep in mind that when reproducing the audio signal, the passive radiator does not move inversely to the woofer. At those frequencies, it is doing its own thing, and that is what contributes to the bass. At the passive radiator's resonant frequency, it will indeed be moving inversely to the woofers, causing an effect which manifests itself in a very deep, narrow dip in the frequency response of the driven system. However, this is generally quite low, like 10 or even 4 Hz. In the case of the passive radiators I am using in a DIY subwoofer I am working on, the resonant frequency (with 1400g loaded to each 15" diaphragm) is about 4.5 Hz. The only concern is to keep the resonant frequency of the passive radiator(s) below that of the operating range of the speaker. If you want the low frequency cutoff to be 16 Hz, you just need the resonant frequency of the passive radiator to be less than 16 Hz.

Unrelated: I wonder what happens if you have a passive radiator with a relatively high resonant frequency (like 60 Hz) and then you put in a reflex port tuned to 60 Hz?

[Rory B.]

From MAD Magazine Special, Number Fifteen (this is an ancient MAD magazine my dad found in his stiff from the 60's or something), in the section entitled "The MAD Blow-Your-Mind Drug Primer":

"See the other young man.
His name is Elwood.
Elwood loves to travel by air.
Can he afford expensive airline tickets?
Of course not...but who needs planes?
Elwood just sucks on a sugar cube of LSD...and takes off!
Some day, when Elwood is on LSD
He will zoom right off the roof.
Crash, crash, crash.
Bye-bye, Elwood.
LSD is swell for flying.
The trouble is, it's not much good for landing."

[ctviggen]

I've been thinking about the slot loading.  If there was no slot loading, the PR would be able to move freely.  With slot loading, however, the PR now will have to "fight" to get air through the slot.  This has the basic effect (at least to first order effects) of the putty.  Additionally, this may be why the Elmer's glue works, as it allows the PR to push air through the slot in a better manner.  Without the glue, the PR deforms and therefore does not act to push air as well.  

Just a thought, after a few beers while rearranging my system and adding new wires.  I'm off to see how my newly bi-amped center channel sounds!

[doug s.]

i think your first supposition was correct. i suspect yure wrong now - the movement of the megawoofer will generate a woltage reading yust by the relationship of the coil/magnet/etc.... you will find that if you move the passive radiator by hand, both megawoofers will also move, & they should move in unison to each other, & inversely to the passive....

doug s.


Nope, if you're reading carefully I'm running these speakers with the PRs completely removed and still get the exact same effect. Maybe someone else who has the PRs still in would like to test your hypothesis.

oh, i wasn't aware that yer li'l test was done w/o the passives in place.  perhaps ya can check the difference yourself, when ya reinstall them...

in any event, what i wrote is what happens w/my larger subs.  perhaps having a x-over inwolved makes a difference - the larger subs are passive...

regards,

doug s.

[Rory B.]

On another forum, I started a topic to explore the effects of slot-loading on a diaphragm. We speculated that slot loading is the equivalent of extra mass-loading, and the floor-boundary coupling is just icing on the cake. The net result of mass-loading a woofer with a slot is that you lower the woofer's Fs without actually adding any mass to the woofer. Theoretically, if you mass-load a woofer, you make it harder to drive. Essentially, you lower its sensitivity. This would also account for the fact that the passive radiator is not overdriven even when there are two high-displacement drivers loaded to it. That way, the excursion of the passive radiator remains manageable. I do not know how the slot loading modifies the passive radiator's behavior. Ordinarily, it would perform as a very close approximation of a helmholtz resonator.

One thing that I am looking at is the possibility that the passive radiator behaves similarly to something called a Decoupled Antiresonant Line, a variation on Transmission Line theory that instead harnesses a line's "antiresonance" to damp a woofer's motion and boost a low bass signal. I have not built a Daline (though someday I hope to) so I do not know how transient response is with a Daline. The idea is that you design a sealed enclosure, but then you add a tunnel to the enclosure at a point where the line's antiresonance is of sufficient intensity to add to the signal in such a way that when the output from the terminus of the line and the output of the drivers are summed, the result is a relatively flat frequency response but it can be extended lower than the woofer's Fs. I do not know how such an approach affects transient response, and that is why I do not know whether it is appropriate to say that the slot loaded passive radiator interacts with the enclosure's air mass, and ultimately the drivers, in a similar way to the Daline. I still think, though, that the slot loading has the net effect of broadening the effective operating band of the passive radiator as a helmholts resonator while mass-loading the diaphragm to lower tuning and sensitivity. Though I am still in doubt as to whether sensitivity is actually lowered, and then boosted again by the slot or what is going on with that. One thing's for sure - What Brian invented back in the 70's is still on the cutting edge of bass reproduction today. That says something about the engineering talent of a speaker designer.

[Redbone]

oh, i wasn't aware that yer li'l test was done w/o the passives in place.  perhaps ya can check the difference yourself, when ya reinstall them...

in any event, what i wrote is what happens w/my larger subs.  perhaps having a x-over inwolved makes a difference - the larger subs are passive...

regards,

doug s.

I reintalled the PRs last night.  The elmer's worked quite well and really stiffened the paper cones.  With the PR installed, pushing on the PR resulted in opposite motion of the two active woofers.  The effect was slightly to somewhat larger than what I saw from the active woofers when the PR was removed.  So there are two effects on the active woofers, one is air pressure and one is electrical.  

The PRs are clearly custom built, with hot glue binding the cones to the spring mech (don't know what is called).  By tapping on them you can hear their resonant frequency, which is quite low.  I couldn't tell if adding putty changed their resonant frequency or not, the sound is very low pitched, but clearly audible.  I will do a first tune tonight.