Woofer technology - isn't it surprising...

[Gruuve]

Originally posted by C. Alexander Claber:

I don't quite get how isobaric designs work - obviously the name means constant internal pressure, which the second woofer provides - but surely the external wave from the second woofer moving out of phase cancels out the omnidirectional bass from the front wave of the first woofer. Hmmm...

 

Alex

Alex...the cones always move together, which is why you only switch the phase on one driver if they are facing each other. If they're in a tube (ie. facing the same direction), they are both hooked up in phase. In a nutshell, you get the equivalent of a single driver that has a suspension twice as stiff (plus a few other summation characteristics). Plug that into WinISD or something and you'll see that the internal cab volume for the same frequency response is cut by about half, but the sensitivity stays about the same. Now, if you can find a Neo driver that weighs about half of what it's non-Neo cousin does, the cab will be about half the size and the same weigh (or a little less, since there'll be less wood).

 

Given the lighter-weight Neo drivers and today's potent power amps, it sure seems like this has some practical application. Even if it's a little heavier, isn't heavier and half as bulky a little easier to manage than lighter but twice as bulky? Two drivers will add expense, but hey...there's plenty of boutique cabs that cost $2K or so, and people buy those. I guess it depends on what's most important to each of us!

 

Dave

[lug]

Originally posted by Dave Sisk:

Originally posted by C. Alexander Claber:

I don't quite get how isobaric designs work - obviously the name means constant internal pressure, which the second woofer provides - but surely the external wave from the second woofer moving out of phase cancels out the omnidirectional bass from the front wave of the first woofer. Hmmm...

 

Alex

Alex...the cones always move together, which is why you only switch the phase on one driver if they are facing each other. If they're in a tube (ie. facing the same direction), they are both hooked up in phase. In a nutshell, you get the equivalent of a single driver that has a suspension twice as stiff (plus a few other summation characteristics). Plug that into WinISD or something and you'll see that the internal cab volume for the same frequency response is cut by about half, but the sensitivity stays about the same. Now, if you can find a Neo driver that weighs about half of what it's non-Neo cousin does, the cab will be about half the size and the same weigh (or a little less, since there'll be less wood).

 

Given the lighter-weight Neo drivers and today's potent power amps, it sure seems like this has some practical application. Even if it's a little heavier, isn't heavier and half as bulky a little easier to manage than lighter but twice as bulky? Two drivers will add expense, but hey...there's plenty of boutique cabs that cost $2K or so, and people buy those. I guess it depends on what's most important to each of us!

 

Dave

I would have stated this more eloquently but I lack the knowledge, desire, education, and motivation to do so. To me the kool application would be in small portable amps utilizing 8" or 10" speakers. You could make a tiny decent sounding practice amp, it just wouldn't be cheap. I made one iso cab utilizing two 8" speakers and it sounded great for it's size.

[alexclaber]

But I still don't get what happens to the out of phase waves from the external sides of the two drivers. Why do they not cancel each other out when they meet?!

 

Alex

[TimR]

Won't you only get full wave cancellation when the unit is facing you and the distance from the rear wall is near to 1/2 a wavelength of the particular frequency you are looking at. I think with 8" speakers and a short tube, the whole device may start to behave more like a point source. Especially if it is side on. I still think these are OK in near field situations but won't work in the far field. Arn't they designed for use inside car boots so the whole boot becomes the resonating chamber?

[Gruuve]

Originally posted by C. Alexander Claber:

But I still don't get what happens to the out of phase waves from the external sides of the two drivers. Why do they not cancel each other out when they meet?!

 

Alex

I'm not sure I understand what you're asking (and that's a reflection on my lack of knowledge rather than your question!). The cones will move together, so I wouldn't think cancellation from any incidental waves would be any different that say a 210 cab? I'm probably not understanding the question though...

 

Dave

[alexclaber]

Originally posted by Dave Sisk:

Originally posted by C. Alexander Claber:

But I still don't get what happens to the out of phase waves from the external sides of the two drivers. Why do they not cancel each other out when they meet?!

 

Alex

I'm not sure I understand what you're asking (and that's a reflection on my lack of knowledge rather than your question!). The cones will move together, so I wouldn't think cancellation from any incidental waves would be any different that say a 210 cab? I'm probably not understanding the question though...

 

Dave

My understanding is that as one cone moves in, the other moves out. That's what increases the apparent internal volume and allows you to put them in 1/4 of the volume. If you wired them with opposite phase then it would simply be a dipole sealed cab and would have a severe shortage of bass due to being in a far smaller cab than ideal.

 

I need to do some more thinking about this - gb tells me its analogous to reflex cabs in the way the front and back wave work together...

 

Alex

[Gruuve]

Alex...nope, both of the cones move together. If they are in a tube with both drivers facing the same direction, then they are wired in-phase...when one cone moves forward, the other cone moves forward...the air between them is essentially trapped and does not compress or rarify (ideally). If they mounted "clam-shell" with both drivers facing each other, then they are wired out-of-phase so that when one cone moves what it thinks is forward, the other cone moves what it thinks is backward. Again, the air between them trapped and is simply pushed back and forth, not alternatively compressed and rarified.

 

You essentially taking two drivers and making them function as one, so it's modeled for enclosure design almost exactly the same way you would with a single driver...you can mount this fixture into a sealed cab or a ported cab, depending on the drivers you've chosen.

 

Did that answer the question?

 

Dave

[TimR]

That makes more sense. What wasn't clear was that the unit was mounted inside another enclosure. I was thinking that both speakers were on the outside.

 

Esentially if the latter was the case the wave coming out of one end would bounce off the nearest wall and merge with waves produced from the other end, under certain conditions these would be out of phase and cancel each other out.

 

Hence my gut feeling that these were ok in the boot of the car as this acts as a sealed enclosure.

[alexclaber]

Found some diagrams - this one shows it well:

 

http://www.mtxaudio.com/caraudio/education/enclosureDesign.cfm

 

So the second woofer is mounted on a sealed chamber, it then faces into a small chamber which the first woofer also faces into, and then the other side of the first woofer faces out into the room. So in effect you get a double thickness diaphragm and twice the suspension stiffness and power handling, thus halving the total Vas.

 

And the back wave from the second woofer is contained and doesn't cancel out the front wave from the first woofer!

 

Alex

[Big Red 67]

Originally posted by C. Alexander Claber:

 

And the back wave from the second woofer is contained and doesn't cancel out the front wave from the first woofer!

 

Alex

Almost, that is the same thing a sealed cab does.

 

The cones are both moving in the same direction at the same time. So there isn't a conflicting wave. Unless of course I missunedrstood the first question which is very possible.

 

Have you ever put two speakers that are 180 out of phase face to face? The bass goes away. It is kinda cool. Plus a great way to make sure your speaks aren't out of phase. It can come in handy if you don't want to track a long cable run that isn't marked.

[alexclaber]

Originally posted by Big Red 67:

The cones are both moving in the same direction at the same time. So there isn't a conflicting wave. Unless of course I missunedrstood the first question which is very possible.

I originally thought that both woofers were on the outside of the enclosure - see why I was confused?

 

Alex

[Gruuve]

Alex...you've got it now. Designing that small cab yet?

 

The clamshell approach is kinda ugly, IMHO. The other approach is to mount one driver to the back of a short tube the same diameter as the driver, then mount the second driver to the front of the same tube. Then this whole assembly mounts into the enclosure, which can of course be sealed or ported, depending on the driver. Here's some better pics...actually there's a third way, which is back to back.

 

http://www.caraudiohelp.com/custom_car_audio_boxes/custom_car_audio_boxes.htm

 

The disadvantage of mounting the drivers in a tube facing the same direction is that one driver can disapate heat more easily, so in theory they aren't "exactly" the same after they've run for a while since one is hotter than the other.

 

This design has it's disadvantages, but it is a pretty elegant approach to get a very small cab that still has extended LF response.

 

Dave

[Gruuve]

Here's an out-of-the-box idea...use an infinite baffle or "open air" design for your bass rig. Simply cut holes in a wood door, and mount a bunch of drivers to the door, and bring the door with you to your next gig. Attach the door into an open doorway at the venue (so that the backs of the woofers are either in another room or outside the building. Plug up your amp and rock...the best LF response you can get!

Dave

[jeremy c]

Has anyone mentioned servo-controlled woofers yet?

[Texian]

Two things I haven't seen mentioned in regards to a lack of new woofer/speaker technology (although I could have missed since I read through all the post quickly) are:

1. Cost and reputation - how many of you guys have gone out and tried the Bose cylindrical radiator speaker systems? (o.k. so there's an external woofer to buy for low freq., but you get my point) Possible reason: about $2k to get into and Bose is not a name that comes to mind when you are shopping bass amps (or other musical instrument amps for that matter).

2. There are companies that are making speaker arrays using very high frequency drivers that utilize 'beat' frequencies to produce audio range sounds (like when you tune using harmonics). Basically arrays of high freq. drivers that constantly put out something like 200 KHz or so. A computer/DSP controlled processor modulates a certain set of the drivers X Hz above 200 KHz in order to produce the beat frequency. In theory, you not only can produce a very broad frequency band, but can also get very directional sound (even bass frequencies) IF desired. I've read about the technology for the last 10 years or so, but can't claim to have experienced it in real life to know if it works or is just theory.

Again, as posted before, it's about being able to move enough air and I'm curious if that's possible with the above theory.

[Big Red 67]

Originally posted by Dave Sisk:

Here's an out-of-the-box idea...use an infinite baffle or "open air" design for your bass rig. Simply cut holes in a wood door, and mount a bunch of drivers to the door, and bring the door with you to your next gig. Attach the door into an open doorway at the venue (so that the backs of the woofers are either in another room or outside the building. Plug up your amp and rock...the best LF response you can get!

Dave

That would be cool and funny!

 

Here is another, I had an amp and a speaker. No box.

I found out that the speaker was the same size as my shop vac. SOOO, wired up the speaker, dropped it into the shop vac canister and, bam speaker cab!!!

[TimR]

If the magnet is large and heavy partly to dissipate the heat, what about using some of the new liquid cooling technology being used in PC CPU cooling. Would the mass added from the cooling unit cancel out the mass lost from the magnet?

[Rocky McDougall]

The very small "Bose" radio uses, I think, the "Folder Horn" theory. Also, 40 years ago "Klipsh" was the leader in home hi-fi, they also used this approach on their "Corner Cabinets". Any thoughts on this design for Bass amp cabinets?

[Gruuve]

Rocky...check the Bill Fitzmaurice site that Alex mentioned above...he has build-it-yourself designs for folded horn cabs that are reasonably small but have huge sensitivity (ie. efficiency). Also, Phil Jones Bass has one folded horn can, but it's a huge heavy monster!

 

Dave

[moot]

I have a set of folded horn cabs and while really loud, they are big and had trouble with lower freqs so I pulled the drivers out - but hey, that's just mine.

 

They are scheduled for demolition. Anyone wanna buy em?

[Rocky McDougall]

quote:

---

Originally posted by Big Red 67:

 

 

Here is another, I had an amp and a speaker. No box.

I found out that the speaker was the same size as my shop vac. SOOO, wired up the speaker, dropped it into the shop vac canister and, bam speaker cab!!!

Rocky

[Big Red 67]

I didn't have the hose on it. It would have mess up the porting. The hole was just about the right size port too. I wonder if port size and the size of the hole a vacuum has are related as far as the physics.

[Gruuve]

Red, that is just too funny. And yes, the hose hole would function as the port as far as the enclosure design was concerned. How did it actually sound?

 

I just can't get that picture out of my head...I'm going to have to go buy a driver just so I can mount it in my shop vac...

 

Dave

[rdepelteau]

Dave Sisk: "wouldn't it be possible to use a second coil winding on a driver rather than a permanent magnet? Replace the permanent magnet with another electromagnet (like the voice coil) plus a circuit to rectify the signal (make the electromagnet always have the same pole direction rather than switching back and forth like the voice-coil does)

 

I like this idea. To me, this seems to be the most practical way to improve the weight/volume of bass speakers. I would not use an electromagnet, but have three coils With one floating in between the two, that is so that their fields act to pull and push together, either ++-, or +--, so that the energy required to drive the speaker is minimalised. That would be easy to do, a stereo amp, and a phase inverter to drive the secondary coils. I would also drive the speaker from the outside rim, since the bigger diameter would require less energy to drive the cone. Speaking of cones, since this speaker is driven from the outside, I would change the shape from a cone to a shallow spherical section.

 

 

Or what if you have one winding in one direction, and the other winding in the opposite direction, that would cause the the fields to push against each other, or pull each other, so that a stereo amp could drive the speaker directly, by having one channel on one winding, and the other channel on the other winding.

 

 

 

 

That gives lesser weight, and lesser depth to the cabinet. Is this possible? I can see that floating the cone might be a problem.

 

(I obviously need to sharpen my electromagnetic skills...)

[Gruuve]

Hmmm...moving the voice coil to the outside edge of the speaker cone rather than the back of it is an interesting idea. I wonder if it would be practical to actually spread the windings for both coils out over the entire cone? That should help with cooling I would think...

 

Dave

[rdepelteau]

The gap between the coils has to be constant.

 

What if the cone was pushed from one point in the back? I would think a big coil around a powerful little magnet that is connected to the center of the cone, instead of a coil around the magnet. Like a bell ringer.

[TimR]

Attaching a magnet to the cone would increase its mass. Making it more difficult to move and requiring more power.

[TimR]

Remember increasing the number of coils means reducing the current flowing in each so the power reduces and the magnetic force will as well. You don't get something for nothing.

[Gruuve]

It still seems to me that just getting rid of the permanent magnet altogether and replacing it with an electromagnet coil holds the most weight benefit...the permanent magnet is the heaviest part of the driver isn't it?

 

Dave

[TimR]

Yes, but electromagnets get HOT. Remember up to 90% of a speakers efficiency can be lost due to the heat. Just take a look at how the bars of an electric fire work. So you'd have to have an efficient way of dissipating the heat.

Also there is time lag for the magnetic flux to grow, and there will also be phase shift due to the induction and capacitance of the windings, I don't know if this would affect things too much assuming all your coils were the same size.

The present idea is to use lighter more powerful magnets. I don't know how important the magnet is as a heat sink but it would seem to me that some sort of aluminium heat-sink like they use in computers or amplifiers could be of use.

 

Imagine a 500 watt amplifier. Up to 450 watts of heat?