Hi,
Cheaper drivers with lower Qts ? Cheaper drivers generally have higher Qts.
The implications of the alignment you are suggesting are its near pointless.
Any extra gain from extra depth could be done with a H frame or U frame.
The total free air Fs and thus Qts will drop a little due to the "trapped"
air added mass, but only a little, its a waste of the other driver. Used
with a rear box things are very different, it does allow cheaper drivers
with higher Qts to be used in tandem for effectively lower Qts.
rgds, sreten.
http://www.quarter-wave.com/
http://www.linkwitzlab.com/
http://www.musicanddesign.com/
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Various companies are trying the dual-driver-with-a-space thing lately. Oh well.
But that's not Isobarik, which isn't "useless". Compound loading does all the same things box or not. Suppose you have some woofers with a crappy magnet and a Q well over 1.5. Double them up. Clamshell the suckers and you get 2nd harmonic cancellation.
If you're playing low mids in the biggest drivers of your OB of course the double/spaced driver thing is going to be way different, but I've yet to see a good explanation of how it would help, especially the low bass region whatsoever. Having Celestion do it raises an eyebrow but it looks like the typical way a little soup of the day gets thrown in here and there to bump sales. Anyone feel free to straighten me out.
But that's not Isobarik, which isn't "useless". Compound loading does all the same things box or not. Suppose you have some woofers with a crappy magnet and a Q well over 1.5. Double them up. Clamshell the suckers and you get 2nd harmonic cancellation.
If you're playing low mids in the biggest drivers of your OB of course the double/spaced driver thing is going to be way different, but I've yet to see a good explanation of how it would help, especially the low bass region whatsoever. Having Celestion do it raises an eyebrow but it looks like the typical way a little soup of the day gets thrown in here and there to bump sales. Anyone feel free to straighten me out.
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Oh, you're right sreten. In free air the Qts ia already the driver Qts, therin lies the rub, so no practical change iso'd in OB.
You'd be better off mounting them separate, flipping pairs if you want to get distortion cancellation. (for woofers not playing low mid). Compund would be like throwing drivers in the trash and just slapping a power resistor on your amp.
Edit: My metal transient response is poor 🙂 There should be a change in Q because the same cone area is now driven by two motors. THe mass doubles, the suspension doubles, the motor doubles, but the radiating area does not. You'd have to pay attention to that in order to calculate the new Q. The effect would be enough that you can't say it's a joke, depending on your situation.
Would it actually rise (in the open baffle)?
You'd be better off mounting them separate, flipping pairs if you want to get distortion cancellation. (for woofers not playing low mid). Compund would be like throwing drivers in the trash and just slapping a power resistor on your amp.
Edit: My metal transient response is poor 🙂 There should be a change in Q because the same cone area is now driven by two motors. THe mass doubles, the suspension doubles, the motor doubles, but the radiating area does not. You'd have to pay attention to that in order to calculate the new Q. The effect would be enough that you can't say it's a joke, depending on your situation.
Would it actually rise (in the open baffle)?
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Fooling around in a simulator shows that Q doesn't change for isobaric in an infinite baffle, as expected. But you wind up with double the cone mass and double the motor and the same cone area, so in a theoretical sense you're only losing efficiency. The cone load due to acoustic output is already so low in an open baffle that people often ignore cone distortion in the bass region (even though it's still air loaded, and being driven from the center) and look for the lowest moving mass possible for various reasons, although if you have the power I don't recommend that unless you're looking for mids out of the woofer.
You'll have 6dB less output than if you mounted them conventionally and up to 15dB more even-order distortion at the same excursion than if you mounted one basket forwards and the other cone forwards.
This thing about two drivers and a space is a little confusing. Is the "space" a closed duct or just open separation between the drivers? If it is a closed duct and the drivers are moving in unison then it is basically an H frame with the option to damp the internal resonances. There no gain is SPL if the spacing is the same as the length of the H frame and a 3dB loss in efficiency.
If the space is open then it depends on whether the drivers are moving in unison or opposition.
If in unison it is a compound dipole which at low frequency ends up being pretty similar to an H frame with the correct spacing. In effect, it is two dipole woofers with one in front of the other. It will give 6dB more output at low frequency, but it gets a little messy as the frequency rises pretty fast as the dipoles interfere with each other. It's better to place the two drivers in the same plane (on a flat baffle) or use one driver in an H frame.
If the drivers are moving in opposition then it becomes a 2 nd order gradient system with 12dB/octave roll off which is pretty useless.
But John, I've done both (closed space and open), and if done with an appropriate size space, I've measured a 5dB increase in SPL output for the same terminal voltage (between 40 and 200Hz). What is causing the confusion here?
To stamp your boorish comments with a shortened version of 'regards' is as obnoxious as it is ironic.
Would have to know exact details of the set up. If you look at it a +-+- then the center -+ kind of cancel so you have +(-+)- sort of as if you have +[-inbox+inbox]-. Some strange things happen at low frequency. But it should still be about the same as an H frame, single driver with same correct spacing.
@brisso57
To conribute another strange configuration:
Mount the drivers face to face / face to tail / tail to tail ...
however you like, using a closed duct and a fairly small
and airtight coupling volume.
One driver is connected to the amp, the other is driven
passively by the amp-connected one.
Now you can influence Qts of the compound driver by
having the passive one's VC open, short circuited or
bridged using a resistor.
Adjustable Qts within some range, is this what you
wanted ?
Behaviour towards midrange will be a bit strange,
it is a pure woofer configuration.
To conribute another strange configuration:
Mount the drivers face to face / face to tail / tail to tail ...
however you like, using a closed duct and a fairly small
and airtight coupling volume.
One driver is connected to the amp, the other is driven
passively by the amp-connected one.
Now you can influence Qts of the compound driver by
having the passive one's VC open, short circuited or
bridged using a resistor.
Adjustable Qts within some range, is this what you
wanted ?
Behaviour towards midrange will be a bit strange,
it is a pure woofer configuration.
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I'm having a lot of trouble understanding a 6dB increase for the dual baffle/driver arrangement. Double the current will be drawn from the same voltage source with electrical parallel, but since the drivers are in acoustic parallel it seems like you could only get 3dB with that if cone mass stayed the same. Since cone mass doubles with the motor, and cone area doesn't increase, I'm having a hard time seeing any output increase at all, unless there is some kind of resonance/driver phase going on, which probably wouldn't get you anywhere performance wise. 6dB only happens with doubling cone area and doubling current with same voltage source. Separating the drivers with two open baffles seems to be nothing but a poorly coupled compound loading with possible changes in higher frequency resonant characteristic because of the open enclosure, and then of course there's the H-frame effect due to the distance between the baffles. That much could be important, but the tradeoff is high because for a pair you can never get more acoustic output than one driver diameter and excursion will allow, even if it gets it at higher currents and lower voltages.
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Probably that we still don`t know what you compared exactly. 😉
What I would like to know: Did you have the same driver-to-driver separation in both cases? How did you arrive at the "appropriate" size space?
Rudolf
That doubling of cone ares stuff is just a misunderstanding that comes about by looking at the equation for efficiency. Simply put two drivers radiating the same SPL when connected one at a time will radiate +6db when spaced much less than a wave length if each is given the same power as when connected individually. IE' in parallel. There is a 3 dB increase in power delivered to the drivers over a single driver, and a 6dB increase in SPL. If driver 1 radiates an SPL of 1.0 and driver two also radiates 1.0 then 1+1 = 2 = +6dB. We do this all the time with crossovers LR type are =6dB at the crossover point by the two driver, in phase, sum flat to 0dB, +6dB.
I think I understand what you are saying. My confusion regarding the acoustic output power has to do with a fixed electric parallel arrangement and acoustic seies or parallel changes in driver configuration. What I'm calling "acoustic parallel" is when the drivers are isobaric or compound, the drivers share the same acoustic load. What I'm calling "acoustic series" is when both cones are driving their own displacement, as in a series of any other potential difference, like a battery or power supply. I see the 6dB increase in the "acoustic series" arrangement. I still don't see any change in the OB isobaric. Does excursion increase as a result of the 3dB increased power consumption? Or is this power dissipated by accelerating the doubled mass? Pointing out where this is handled in the equations would be very helpful, I could easily understand that, but I'm too clumsy and lazy to do it myself. It would take me an hour or more just to find all the equations and arrangements needed.
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The thing is that when you take two free standing drivers each acts as a dipole. Now, like any other two sources which are in phase they sum to +6dB. It doesn't matter what the separation is or if one is in front of the other or beside it, as long as the dipole axis of the two are aligned. If they are in phase they sum to +6dB. As the distance between them increase they are only in phase at low frequency. This gives rise to comb filtering at higher frequency just as you get with a monopole source, except superimposed on the dipole response. So if the single driver has an effective separation that places the dipole peak at Fp, placing that driver in an H frame or on a flat baffle or what ever that moved the peak to Fp/2 will have the same low frequency SPL as the two driver system. But the single driver will have a cleaner response as the frequency increases since there will be no comb filtering effects.
I see, the excursion increases. It does this by the aid of the radiated power of the other dipole. 6dB, theoretical total output increase.
There is not increase in excursion because each driver is getting the same power it would if the other were not present. The isobaric thing is another issue. I mean if you have two drives placed at the ends of a 2' tube to form a dipole it is basically no different than a single driver at the center of a H frame 2' long. The H frame with have a 1/4 wave resonance based in 1/2 the H frame length. The other configuration will have a 1/2 wave resonance at the length of the duct. The difference being that it will be an internal resonance. But the dipole behavior will be the same. It is just the wast of a driver unless there is some other reason to do it that way, as, for example, to assure symmetric response from front and rear. At low frequency this isn't an issue.
I think I got all that but no matter what kind of dipole radiator considered you can't have the field on the ends of any stack increase unless it increases at the ends, in the case of a speaker cone, you can't have that unless it moves more.
But then I get confused again because increased excursion causes increased back EMF, which raises impedance and causes trouble with a supposed 3dB power input increase.
I might have to actually calculate this one?
But then I get confused again because increased excursion causes increased back EMF, which raises impedance and causes trouble with a supposed 3dB power input increase.
I might have to actually calculate this one?
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