Hi . . . I'm trying to design a very small subwoofer. Cabinet volume is an important consideration. It is my understanding that if I come up with a design based on a single driver in a cabinet roughly twice my target volume, I can cut that volume almost in half with the addition of the second driver mated to the first in an isobaric configuration. However, it will require more power to achieve the same SPL.
Do I have this right?
If so:
1. if I want to improve the low frequency response of my isobaric design with the use of a passive radiator or port, do I proceed as if I have a single driver (surface area) or a pair?
2. what are the effects on low frequency response and power output if a pair of drivers configured in normal acoustic parallel mode are reconfigured in a cabinet half that size but in an isobaric layout?
Thanks in advance for any information . . .
Do I have this right?
If so:
1. if I want to improve the low frequency response of my isobaric design with the use of a passive radiator or port, do I proceed as if I have a single driver (surface area) or a pair?
2. what are the effects on low frequency response and power output if a pair of drivers configured in normal acoustic parallel mode are reconfigured in a cabinet half that size but in an isobaric layout?
Thanks in advance for any information . . .
The answers of these questions can be found in many places where isobaric subwoofers are explained. WinISD supports isobaric woofer pairs. Today high power, high motor strength, high cone mass woofers are available, so it does not make a lot of sense to go isobaric with new woofers.
Yes. If you go isobaric, cabinet volume halves and input power doubles.
Appreciate the feedback. I'm downloading WinISD now. Thanks.
If small cabinet volume is critical, are you still of the opinion that an isobaric configuration is unnecessary?
Have a look at the bucket sub Bucket Sub as a proven alternative.
Small and quite powerful. I had built 2 and put a modded Behringer iNuke DSP amp (changed the ventilator out to a quite one) to drive them.
Small and quite powerful. I had built 2 and put a modded Behringer iNuke DSP amp (changed the ventilator out to a quite one) to drive them.
Thanks . . . that's an interesting project but WAY too big. I'm looking at a 1 liter box. It's really tiny.
SPL is as yet undetermined. Not overly loud as this is intended to pair with an small portable speaker. LF extension would be hopefully into the mid-50 Hz range (-3 dB). I am expecting the bass to be less than "hifi" (i.e., somewhat boomy) and hope to incorporate a passive radiator to help out a bit. Hence, the question about what is the starting "area" to use -- one cone Sd or two?
EDIT: I'm expecting that I might aim for F3 < Fs and will take whatever bass slop I get (within reason).
EDIT: I'm expecting that I might aim for F3 < Fs and will take whatever bass slop I get (within reason).
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Isobaric subwoofer behaves as one driver (one cone SD), but with half Vas.
With very small volume (1 liter) box maybe it is better to find a driver which has good LF extension in that volume. The whole isobaric construction with two drivers necessarily demands volume occupied by the second driver and the separation between the two - very critical in very small enclosures. Isobaric construction makes sense only in much bigger box volumes.
With very small volume (1 liter) box maybe it is better to find a driver which has good LF extension in that volume. The whole isobaric construction with two drivers necessarily demands volume occupied by the second driver and the separation between the two - very critical in very small enclosures. Isobaric construction makes sense only in much bigger box volumes.
FWIW, I'd stuff as much driver (and PR, keeping 2:1 cone areas) as possible into that volume, and then EQ the result.
Chris
Chris
The low SPL is not a choice. It must be low. This is set by the size limitation and LF requirements.
Tang Band make some drivers that look just right for this. e.g:
Tang Band W3-1876S 3" Mini Subwoofer
Stuff the driver and PR into opposite ends of a mailing / packaging tube, and it'd be a pretty simple build (example pictured).
The 76dB spec is low because of your other requirements.
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Personally, if I was putting effort into a tiny portable build, I'd sacrifice some LF extension and use something more efficient.
If your music source is a device with a decent interface (e.g. a tablet or laptop), you could add equalisation to get more headroom and percieved loudness.
e.g. cut the <50Hz signal and the speaker will be able to play louder without as much distortion.
e.g. add a couple of dB boost 100-200Hz, and the bass will seem a bit fatter.
Attachments
Yes, that Tang Band driver was one I saw on Parts Express, along with 6 different Dayton Audio models (3X distinct models at either 4 and 8 ohms).
I have NO problem with a fairly steep rolloff below 60 Hz. It sounds like you're saying if I force that by high-passing the input signal I'll make things easier on the sub?
Also, a few WinISD calcs seem to indicate one of the other drivers I am looking at would lead to a fairly decent output bump a bit below 200 Hz, which I'm fine with.
Another question: the "mailing tube" configuration more or less forces a single passive radiator at the same diameter as the driver. Would it be unreasonable to use a pair of those passive radiators, one at each end, and have the driver face forward in the middle of the (not tubular) box? I'm thinking something about the size of a brick.
I have NO problem with a fairly steep rolloff below 60 Hz. It sounds like you're saying if I force that by high-passing the input signal I'll make things easier on the sub?
Also, a few WinISD calcs seem to indicate one of the other drivers I am looking at would lead to a fairly decent output bump a bit below 200 Hz, which I'm fine with.
Another question: the "mailing tube" configuration more or less forces a single passive radiator at the same diameter as the driver. Would it be unreasonable to use a pair of those passive radiators, one at each end, and have the driver face forward in the middle of the (not tubular) box? I'm thinking something about the size of a brick.
Exactly.
Have it intrinsic to the build, or add it with eq, doesn't really matter.
You could mount a 3" driver opposite a 4" PR.
That'd work.
For this, it doesn't really matter what shape the enclosure is, or where on the enclosure you put the drivers.
The only exception would be if you used drivers on opposing faces of the box (like a micro Boominator).
The Boominator - another stab at the ultimate party machine
Yep.
I did the maths a little while ago, and you need about 1.8x the cone area for the PRs to match the main driver in SPL.
Since you're happy to EQ things, I stand by my suggestion. Have a look at some portable bluetooth speakers for inspiration - Bose use a down-firing 3" driver with some long thin PRs mounted to the side of the cylinder. Easy to get enough PR area that way, and you could put another driver on the other end of the cylinder.
Chris
Thanks. Without having gotten real deep in the analysis yet, I'm leaning towards a couple of Dayton 3.5" drivers, either ND90 or ND91. I'd prefer the latter, but they have higher Vas and I need to dig in and learn how Vas relates to proper box volume for multiple drivers. Any thoughts on the matter would be appreciated!
I can't do custom PR devices, so unless those long thin versions are readily available, I'd probably go with 3X 4" PR to go with 2X 3.5" drivers, for an area ratio of just under 2.
It may be subject for another thread, but when it comes to great bang for the buck on drivers, is Dayton as good as any? Are there other brands I should look at?
EDIT: now that I look at it, the 8 ohm ND91 has a lower Vas than the 8 ohm ND90. The opposite is true for the 4 ohm versions. Seems like the 8 ohm ND91 might be the best compromise -- lowest Fs of the four models as well as reasonably low Vas.
I can't do custom PR devices, so unless those long thin versions are readily available, I'd probably go with 3X 4" PR to go with 2X 3.5" drivers, for an area ratio of just under 2.
It may be subject for another thread, but when it comes to great bang for the buck on drivers, is Dayton as good as any? Are there other brands I should look at?
EDIT: now that I look at it, the 8 ohm ND91 has a lower Vas than the 8 ohm ND90. The opposite is true for the 4 ohm versions. Seems like the 8 ohm ND91 might be the best compromise -- lowest Fs of the four models as well as reasonably low Vas.
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Hmm, mating a 74 Hz Fs driver to a PR? I didn't look at each one available, but the highest Fs one I found was 38 Hz, still much too low to be of any use.
GM
GM
If you're willing to throw EQ around, the LF response is just a matter of power handling.
The unprocessed response probably won't be great, but I expect it'll be perfectly possible to get a good response from this small cabinet. So long as the amp isn't grossly over-sized, the drivers will probably survive, too.
This seems to be how most portable speakers work these days - take a tiny enclosure and fill it with as much cone area as you can, then EQ the result. Better models use dynamic EQ to avoid amplifier clipping at high levels.
Chris
The Dayton ND91 indeed is the better one of ND90 and ND91: stronger motor, higher power handling and larger Xmax. Pick the impedance version that gets sufficient power from your amplifier.
Vas is a measure for the drivers suspension compliance. It is one parameter to look at to determine what drivers work well in a small enclosure, though other parameters are important too. If you put the driver in an enclosure that is much smaller than Vas, the stiffness of the air spring outweighs the stiffness of the drivers suspension and Vas becomes irrelevant. Then below the resonance frequency (which will be above 100 Hz for these small drivers, so the following is applicable to bass frequencies), the motor strength and stiffness of the air spring determine the efficiency. Note that fs now is irrelevant too. As the air spring stiffness cannot be changed, because the enclosure must be small, the only thing that can be done to increase efficiency is by making the motor stronger. That is why such drivers designed for small enclosures have strong motors and corresponding large magnets. Motor strength can be expressed as BL/sqrt(Re). The units loosely are Newton/sqrt(Watt).
Vas is a measure for the drivers suspension compliance. It is one parameter to look at to determine what drivers work well in a small enclosure, though other parameters are important too. If you put the driver in an enclosure that is much smaller than Vas, the stiffness of the air spring outweighs the stiffness of the drivers suspension and Vas becomes irrelevant. Then below the resonance frequency (which will be above 100 Hz for these small drivers, so the following is applicable to bass frequencies), the motor strength and stiffness of the air spring determine the efficiency. Note that fs now is irrelevant too. As the air spring stiffness cannot be changed, because the enclosure must be small, the only thing that can be done to increase efficiency is by making the motor stronger. That is why such drivers designed for small enclosures have strong motors and corresponding large magnets. Motor strength can be expressed as BL/sqrt(Re). The units loosely are Newton/sqrt(Watt).
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The Dayton Audio ND105-PR is a 4" design with an Fs of 51.5 Hz, so it's getting closer. Its Sd is 1.69X that of the ND91-8 driver, so maybe not "ideal", but perhaps good enough? Its Xmax is about 18% higher than that of the driver as well, which brings volume displacement ratio right into that 2.0 desired target.
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