Hello all,
To go In the same way than Planet (and to give a little temptation to c2ct)
, I can give you a real life advice:
the enclosed dipole push pull is worth the try in some cases.
I could compare it with the same 15" mounted in 2 H-frames.
First great improvement, all the distorsions in the 20/30 Hz area are gone. And with steel baskets drivers, these distorsions can even be described as aerodynamic noise, it's very annoying.
Second great novelty, as I came from H-frames, no more very big dipole peak. Even properly equalized, a king of harmonics always stayed, here, it's cleaned.
As a trade off, efficiency is 6 dB down, but the power handling increase is more than 6 dB.
Absolutely no pressure in the box, as I removed the dust caps on vented drivers, it's still more obvious.
Of course, as the drivers are out of phase, there is no mechanic cancellation, but with a heavy box it's not a big issue at domestic levels.
I use this with massive EQ ( 20 Hz +15 dB, Q=4 with low shelving 6 dB starting at 50 Hz, no LT here), but the head room is just limited by the rattle of the house.
Since 3 years I use this, it's no way that I could be back to open dipole.
To go In the same way than Planet (and to give a little temptation to c2ct)
, I can give you a real life advice:the enclosed dipole push pull is worth the try in some cases.
I could compare it with the same 15" mounted in 2 H-frames.
First great improvement, all the distorsions in the 20/30 Hz area are gone. And with steel baskets drivers, these distorsions can even be described as aerodynamic noise, it's very annoying.
Second great novelty, as I came from H-frames, no more very big dipole peak. Even properly equalized, a king of harmonics always stayed, here, it's cleaned.
As a trade off, efficiency is 6 dB down, but the power handling increase is more than 6 dB.
Absolutely no pressure in the box, as I removed the dust caps on vented drivers, it's still more obvious.
Of course, as the drivers are out of phase, there is no mechanic cancellation, but with a heavy box it's not a big issue at domestic levels.
I use this with massive EQ ( 20 Hz +15 dB, Q=4 with low shelving 6 dB starting at 50 Hz, no LT here), but the head room is just limited by the rattle of the house.
Since 3 years I use this, it's no way that I could be back to open dipole.
I think it's all about forces, and the forces are created by the bi-directional cone movement acting against the speaker frame. Imagine the cone in the most inward position, and then it starts the movement towards the other extreme. What is it that stops the cone from leaving the box? The attachment with the surround and spider, who in turn is attached to the frame. So, when for example, the forward motion of the cone is stopped by the surround and spider, forces are acting on the frame and propagates to the box. That is why some builders recommend that you use rubber grommets when attaching the driver to the box. That's more or less how a tech paper I have at home explains this phenomenon, at least as I recall it.
Ooops, I'm back on-topic, prior to the introduction of that engaging push-push discussion.
If it is all so obvious and Newtonian why an OB baffle shakes, just why doesn't a sealed box do the same thing since the cone, basket, magnet, and board and assorted pendulums are there? (Sealed boxes have vibrating panels but the box doesn't shake, does it?)
Maybe it has to do with the way the rear wave exits my OB. The giant board is mounted 90 mm from the wall and parallel to it. Perhaps there are no feelable consequences till the frequencies get down to whole-assembly resonance territory and then the whole assembly starts shaking. Or course the area where the sound "exits" the cone is nearly 200 sq inches and increases with distance outwards - so not a high pressure.
If it is all so obvious and Newtonian why an OB baffle shakes, just why doesn't a sealed box do the same thing since the cone, basket, magnet, and board and assorted pendulums are there? (Sealed boxes have vibrating panels but the box doesn't shake, does it?)
Maybe it has to do with the way the rear wave exits my OB. The giant board is mounted 90 mm from the wall and parallel to it. Perhaps there are no feelable consequences till the frequencies get down to whole-assembly resonance territory and then the whole assembly starts shaking. Or course the area where the sound "exits" the cone is nearly 200 sq inches and increases with distance outwards - so not a high pressure.
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Push-push drivers are wired so that they both push at the same time and pull at the same time. Acoustically they act like any other pair of drivers wired in phase. To get no bass at all you'd have to wore them acoustically out of phase -- that would be a wiring error.
dave
At the LF frequencies involved it would be effectively the same as a flat baffle. the entire mass of air in the slot would act as one large radiator
dave
But they do. Panels tend to be smaller and better braced in a sealed box than an OB. So they shake at higher frequencies... you have heard (or seen) subwoofer boxes "walk" haven't you?
dave
No, I have never heard of that - maybe I've been living isolated in carpeted environments - but I do not doubt your word. But a speaker with vibrating panels might walk due to panel vibration, not whole-box vibration which I think is something quite different.
Someone earlier suggested things like that happen with cheap woofers. I thought it was churlish of him to suggest my speakers might be cheap ones and I would never say that to any owner of walking boxes, but what if his theory is right?
Hi Dave,
Okay, around what frequency will the design depart from something like an OB type speaker. What I find attractive is that the out of phase rear wave looks like it's not as important. Since I haven't played with anything even close to this, I'm somewhat at a loss as to it's character.
I do like the idea.
-Chris
Okay, around what frequency will the design depart from something like an OB type speaker. What I find attractive is that the out of phase rear wave looks like it's not as important. Since I haven't played with anything even close to this, I'm somewhat at a loss as to it's character.
I do like the idea.
-Chris
Hi,
It was me, and I was implying variable driver DC offset with bass drive
is more likely the cheaper the cost of the bass drivers. FWIW in boxes
the extra Vas of the box, which can and does in cases dominate driver
Vas variability, therefore linearising the suspension, reducing assymetry,
and therefore reducing any DC offsets could be worth thinking about.
Drivers fine in boxes may have issues used open baffle.
I get the feeling no-ones following what I'm on about, driver offsets.
rgds, sreten.
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But what DC offsets?** Other than a tiny bit of DC at my amp output (50 mv), doesn't everything kind of return to baseline/neutral/spider-happy? What would, say, kind of minimal serious music cone woofers have different that would lead to these offsets - at least within their successful working range?
If you could explain in terms simple enough for me, that would be nice. Thanks.
*I can tell that my 50 year old Stephens 150W in the giant OB is terribly offended.
**On motorcycles, there is a curious phenomenon in the shock absorbers knowns as "jacking" which leads to the shocks pushing the frame further and further from neutral. But that arises because the compression and the rebound circuits are quite different.
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Hi sreten,
Keeping the field linear along the pole piece is not a huge concern for makers of low priced, contract manufacturing. This shouldn't be a shock to anyone. It has to be really bad before you can see it clearly, but with a speaker test jig it shows up very clearly. I tested every single recone job or replacement driver I ever sold to a customer. You would be surprised at how many factory approved recones do not perform properly. It's obvious that many of these people do not check their work.
-Chris
I am, and I agree with you. It just seemed that you had made your point pretty well, so no need to expand on it.
Keeping the field linear along the pole piece is not a huge concern for makers of low priced, contract manufacturing. This shouldn't be a shock to anyone. It has to be really bad before you can see it clearly, but with a speaker test jig it shows up very clearly. I tested every single recone job or replacement driver I ever sold to a customer. You would be surprised at how many factory approved recones do not perform properly. It's obvious that many of these people do not check their work.
-Chris
Hi Ben,
Cost sometimes is no protection from poor workmanship either. That's why I always test them. The DC offset at 50 mV or less will not be a great concern with a woofer, but I'm not sure about a high efficiency tweeter and active crossovers.
-Chris
Same deal with a speaker. The spider and surround (mostly the spider) provides the restoring force for the cone. The surround only needs to keep the cone aligned. Also, the voice coil position on the former, and the pole pieces in the magnet structure may produce an asymmetric drive that is yet another issue. The combination may compensate or exaggerate the problem.
Cost sometimes is no protection from poor workmanship either. That's why I always test them. The DC offset at 50 mV or less will not be a great concern with a woofer, but I'm not sure about a high efficiency tweeter and active crossovers.
-Chris
Let's 1st make sure we are talking about the same concept -- this one?:
This is intended to be a 3-way with MTM mids (tweeter not shown). Mids are mounted OB conventionally (althou depending on actual execution, one might have some u-frame effects). The push-push array of drivers in their integral channel, act as one big OB woofer (mids & woofers were the same surplused PSB/VIFA P17 -- the person i was doing this for had >80 of these drivers).
The curved backwards wings are there for diffraction and to make the big baffle visually smaller. Also aids in keeping the structure self-supporting.
So basic behaviour is a dipole OB up to the frequency at which the basket structure of the mids start interferring with pure dipole. A tweeter + rear-firing tweeter (and the required <2k XO) would tend to keep it dipole all the way up.
dave
Well - as far as the OB bass portion of your design I was thinking along these lines. I would most likely go with some 12 inch drivers tho - and it might need a bit of baffle extension on the top - perhaps a bit of an "H" frame panel on the top horizontal part. Should do some decent bass I think.
If I went with 10 inch or 12 inch drivers I would only use 2 or 4 of 'em.
Attachments
Hi,
Again is has nothing to do with DC drive signal. Its like the shock
absorber. That is assymetrical and all drivers to some degree are
assymetrical, your 50 year old driver, with all respect, is 50 years
old, and I very much doubt the design is particularly advanced.
As described before, heavy bass drive will cause the "zero" position
to dynamically move, usually backwards towards the stronger field.
The more bass drive the further it moves, this can be increased
or reduced by suspension and surround assymetry, and can and
will be reduced by using the driver in a box.
DC displacement ? dynamic offset the voice coil
More info on this site than you would ever need.
So the envelope of the Bass signal can cause low frequency "rocking"
signals to occur in an open baffles, far lower in f than you'd expect.
rgds, sreten.
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This is a simplified plan view
Drivers wired in phase to acoustically act as a larger driver, but mechanically opposite phase to cancel most of the reactional energy. The channel they sit in is acoustically small at frequencies it is operated at, so it acts just like a flat baffle acoustically.
dave
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