Hello all, I have been lining up a number of projects at the modelling stage waiting for when I can start setting up the home workshop tools with the final bench stand being setup today. Here is the last one in the set of items of interest that I have acquired most of the recent equipment for. I have two identical 7" sub drivers that will go into this 300x300x300mm sealed cab. Most curious about all the possible wiring configs....... to hear and measure graphs for to learn about such things. Especially interested in when both drivers move together as if connected by an electronic pushrod and very curious about also repeating those tests with a shorter bandwidth on one driver biased low
Predictions? Thoughts? Anything folks would like added to the tests?
Predictions? Thoughts? Anything folks would like added to the tests?
https://www.linkwitzlab.com/models.htm
Reversing the polarity of one of the drivers, making both drivers move together as if connected by an "electronic pushrod" would cause the pair to exhibit a dipole response, but with the displacement of a single driver.
The comparable monopole (both drivers wired with the same polarity) has a flat frequency response (0 dB), whereas the dipole rolls off at -6 dB per octave.
To maintain constant sound pressure level (0 dB) the monopole displaces four times (+12 dB/oct) the air volume for every halving of frequency, but the dipole has to have eight times (+18 dB/oct) the displacement.
I have come across that description before, maybe by yourself. That's what I mean about hearing it for myself. Trying to get my head around these things and the maths that you provided is way over my head. I imagine the rear output of an active driver driving a PR in that same back location and think about what you and others have been saying about compliance and xmax. Sounds like that you folks are saying that a PR needs to efficiently get pushed and pulled by the active. Then the PR Sd adds to the output
So what I don't understand in concept and maths is, if electric drive was added to the equation to help the extra diaphragm to move back and forth cleaner and with less compression of the air inside the cab, why does output go down to one drivers displacement? It seems in a probably flawed misconception types thingy that a motor on the second unit will drive that diaphragm back cleaner than the passive? Let's say power is cheap
So what I don't understand in concept and maths is, if electric drive was added to the equation to help the extra diaphragm to move back and forth cleaner and with less compression of the air inside the cab, why does output go down to one drivers displacement? It seems in a probably flawed misconception types thingy that a motor on the second unit will drive that diaphragm back cleaner than the passive? Let's say power is cheap
A PR is just an exotic vent.
As in a motional feedback system. I have certianly done thot experiments on using a DSP controlled back driver in an isobarik to tailor the rrsponse (ie manipulating the effective box size electronically).
dave
As in a motional feedback system. I have certianly done thot experiments on using a DSP controlled back driver in an isobarik to tailor the rrsponse (ie manipulating the effective box size electronically).
dave
The left side of the illustration below depicts a monopole "clamshell" isobaric loading using two drivers face to face, the exterior driver's polarity is reversed, positive voltage moving the cone forward. Since the two drivers function as one, the displacement is the same as one.
The rear opposite polarity rear output of the driver is enclosed in the box, and does not interfere with the front output.
The right side of the illustration shows a dipole, the box open on the back. Much of the front wave wrapping around the cabinet destructively cancels the opposite polarity rear wave, so it's output is reduced by -6dB compared to the monopole.
Just as in the "clamshell" arrangement, adding a second speaker to the enclosed box with it's polarity reversed is virtually the same as the single driver dipole- there is no more air displaced than a single driver.
A passive radiator is primarily "driven" at Fb, the resonant frequency of the box/PR, and same as in a ported box, the PR Helmholtz resonance is in phase, but one cycle behind the driving force.
A second active driver's output with it's polarity reversed is not "driven" by resonance.
The air in it's sealed box is not compressed, so the polarity reversed drivers won't require as much power to be driven "clean" to full excursion, though full excursion for the pair won't result in as much output as as a single driver in the same sealed cabinet.
Anyway, if you ever get to the tests, you can measure and hear what what you don't understand, hope you post your findings
Thanks for finding the words to describe it, and understanding the gist of the experiment
Thanks for the pics man. Let's take the last one, that represents my screenshot. Do you have a drawing of that second driver being passive and the arrows showing the interaction for that? What do you make of Dave's remarks about messing with the second driver electronically? The other thing, can the second driver be time aligned to that same step behind as the PR
Passive radiator needs weight added to achieve tuning frequency.
And behavior is similar to vented, as far as unloading below tuning.
So often the rule of thumb is 2x passive radiators for more xmax.
Or just use a specifically designed passive radiator that is more stiff
and rated for more xmax to achieve the same goal
Otherwise push push is the same as having 2 bass drivers in a box.
they just face each other instead.
Another solution is 2x drivers / woofers and the second woofer is crossed lower
to dedicate more xmax to bass. And usually used to compensate for baffle step.
Since such a small baffle will have generous amounts of baffle step regardless.
Isobaric loading will half the volume needed but tradeoff is double the power needed
for same SPL
Far as resonance " square " or equal length shaped boxes are usually avoided.
More asymmetrical such as 1.6 to 1.7 ratio
And behavior is similar to vented, as far as unloading below tuning.
So often the rule of thumb is 2x passive radiators for more xmax.
Or just use a specifically designed passive radiator that is more stiff
and rated for more xmax to achieve the same goal
Otherwise push push is the same as having 2 bass drivers in a box.
they just face each other instead.
Another solution is 2x drivers / woofers and the second woofer is crossed lower
to dedicate more xmax to bass. And usually used to compensate for baffle step.
Since such a small baffle will have generous amounts of baffle step regardless.
Isobaric loading will half the volume needed but tradeoff is double the power needed
for same SPL
Far as resonance " square " or equal length shaped boxes are usually avoided.
More asymmetrical such as 1.6 to 1.7 ratio
An active driver will move in the direction of the electrical signal, normally a a positive (+) signal to the (+) terminal driving the cone away from the magnet. If the polarity is reversed (+ to -), a negative signal (-) drives the cone away from the magnet.
A polarity reverse shifts the cone's direction of motion 180 degrees at all frequencies.
A passive radiator has no polarity, it's movement distance (excursion) and direction (phase) varies with frequency relative to the driver.
Arrows showing the interaction by frequency would look like this: ^ (0 degrees), > (90 degrees) v (180 degrees), < (270 degrees),^ (360 degrees).
Driven at the box/PR resonant frequency, the PR is 360 degrees behind the driver, but "in phase"- the arrow points in the same direction.
This video shows the phase/direction of a PR changeing with frequency relative to to the driver:
Dave wrote of a thought experiment using a DSP controlled back driver in an isobarik to manipulate the effective box size electronically, I don't know how the thought turned out.
I don't know what "step behind" you are asking about aligning.
The behind you mentioned
Is this line of discussion upsetting in some way?
360 degrees is the period of one wavelength.
A wavelength's time period is different for every different frequency.
A delay is one specific time period, for instance 20 milliseconds is the time period of one wavelength at 50 Hz.
You may think you are writing fairly clearly, but I'm at a loss as to what you are trying to accomplish with reversing polarity of one of two drivers in the same chamber and then applying delay to it, which would cause the pair's output to vary in and out of phase with frequency.
This is the relaxing part, I have some subs that I am very happy with. Really don't need another sub but.........but it's fun messing just to see what happens. I have never run or heard an opposed setup, and curious about all the different possibilities. I still wonder at and am excited by new experiences and at 49 haven't tired of it... also things like, unbalanced opposed actives. 7" on one side and 10" on the other and things like that. I have spare drivers lying around just for these messings arounds
Bottom line is that;
PR units can enable a smaller volume cab without the issues of BR for powerful low Vas type subs
PR units in a system can cost more than the active driver
Power is cheap
Can a second active driver equal or better the PR system while keeping the PR system's cab volume
I would rather spend a $150 on a second active driver than a drone
This thread is about exploring the possibilities with a second active replacing the usual PR locations
I am also thinking about and wondering the possibilities in exploiting a second active driver of maybe a larger dia as a dynamic, power and frequency driven cab rear wall., Just to see what would happen if the main driver had a box that changed cab volume with music
See why I feel at a loss as to what isn't obvious with the intent here. Dave clicked on and gave a working description to the material in the OP
You're thinking ahead, which is fine but we may not know where you're at or your train of thought. We can't just start talking about a topic without knowing, there's too much.
Anyways, I hope we are all clear on now that this is just another messing around learning types thing. The same 7" driver is hard to beat for musicality in a single woofer ported box. Yet to run it with a PR or any other systems. I have a number of these that would be fun to try in all sorts of configs. On being steered towards the MEH as a possible path to high power point source, I am even curious to use these in such a system
But before any of that. Some different test builds to hear what all these higher spec small subs sound like. What's all these pipe resonance and chuffing stuff that gets spoken about so much, what does it sound like. Learn to hear what the bad stuff is. Try to blow up some stuff. I feel I have a solid PR and port hybrid concept that will be fun seeing it try to work. Also feel, how bad can things like homemade drivers and such be? The making side of things is the easiest part. Coming up with a viable thing to make is the joy
You considered running the two in an unconventional phase arrangement to mimic a passive radiator. Art and I implied that this doesn't make sense for the average user who wants this same result.
This is not disparaging Dave's concept, which I'm curious about myself... but it's not what I think you're looking for today if you want direct results. Many before have run both woofers in phase, and you tailor the response you get to the response you want by using EQ equally to each driver, at least to begin with.
This is not disparaging Dave's concept, which I'm curious about myself... but it's not what I think you're looking for today if you want direct results. Many before have run both woofers in phase, and you tailor the response you get to the response you want by using EQ equally to each driver, at least to begin with.