Isobaric subwoofer design, many questions!

[daryl]

Jeff! (How you been man!)

You know as well as I that Grant said that that vent length would be longer because the enclosure would be smaller (for the same alignment).

Yes vent length is related to enclosure size and tuning frequency as you said but Grant is keenly aware that the enclosure will be half the effective volume in an isobaric setup while you and I are only suspecting it.

If you halve the enclosure volume then in order to maintain the same tuning frequency you must double port mass which would require a vent more than double the length since it will still only have two ends which also contribute to effective vent mass.

[daryl]

As for Grants benefit #2 and disadvantage #2........

Doubling input power for the same output is not an advantage.

Power sensitivity (efficiency) is -3db.

2.83 Volt sensitivity is the same with a parallel connection or -6db with a series connection of the drivers.

Keep in mind that power sensitivity is -3db because the effective enclosure volume is half and not necessarily because two drivers are being used.

Meaning that if you used a single driver with Double Mms and a bigger motor in an enclosure half the size you would still lose 3db.

So sensitivity remains as it should be for the EFFECTIVE enclosure volume even with isobaric arrangements but you do have the volume of the extra driver to contend with.

[daryl]

I would point out that the isobaric configuration made more sense in the old days when all that we had were drivers with very light moving mass (Mms) and small motors (low BL).

Back then it was common to see enclosures bigger than 10ft^3 for drivers with only a small displacement capability and reducing enclosure volume by half was very appealing.

Enclosure volume is halved because Mms and BL are doubled

Nowadays however most bass drivers already have considerably more Mms and BL than bass drivers of the old days.

Often more than double!

So the 'Solobaric' concept marketed by the folks at 'Kicker' where a single driver would have double the Mms and BL instead of using two bass drivers to accomplish the same thing has become the norm.

Nowadays you know the 'Solobaric' line as the square subwoofers from 'Kicker' but when they were first introduced they were round and they were meant to use the same enclosure volume as an isobaric pair while using only a single driver (for those who aren't that old or have just forgotten).

The smaller enclosures used by bass drivers nowadays means that an isobaric arrangement is not as beneficial if at all.

The extra volume required to house the second driver and larger vent (if a vented alignment is to be used) will likely take away much if not all of the volume savings the isobaric arrangement was meant to give in the first place.

I might add that BL isn't truly 'doubled'.

With a series connection of the drivers BL really is doubled but then a parallel connection of the drivers would have the same BL.

If you could rewind the voice coils of the two drivers so that when they were connected together the impedance would be the same as for one of the original drivers and still have the exact same characteristics except for the impedance change then BL for the pair would be multiplied 1.414 (square root of two).

So effectively BL is multiplied by 1.414 in and Isobaric arrangement.

[bill poster]

As for Grants benefit #2 and disadvantage #2........

Doubling input power for the same output is not an advantage.

Power sensitivity (efficiency) is -3db.

2.83 Volt sensitivity is the same with a parallel connection or -6db with a series connection of the drivers.

Keep in mind that power sensitivity is -3db because the effective enclosure volume is half and not necessarily because two drivers are being used.

Meaning that if you used a single driver with Double Mms and a bigger motor in an enclosure half the size you would still lose 3db.

So sensitivity remains as it should be for the EFFECTIVE enclosure volume even with isobaric arrangements but you do have the volume of the extra driver to contend with.

Just to be 100% clear, is a parallel isobaric 3 or 6db down on a regular parallel two woofer design?

[Jeff B.]

Voltage sensitivity is -6 dB

[bill poster]

power and voltage sensitivity are not the same I take it

[billfitzmaurice]

Voltage sensitivity is -6 dB

+1, not that it matters with power being as cheap as it is today. What's significant is that since the area of only one cone is utilized in creating output the maximum SPL of two drivers isobaric is -6dB compared to the same two drivers in a conventional alignment.

[DanP]

Just to be 100% clear, is a parallel isobaric 3 or 6db down on a regular parallel two woofer design?

In either case you gain 3dB over a single woofer for the parallel electrical connection, then in the normal double woofer configuration you'd gain another 3dB for doubling the enclosure volume, while in the isobaric configuration, you'd lose 3dB for halving it - leaving a net 0dB for isobaric and +6dB for normal. Again, this is for voltage sensitivity compared to a single driver. Efficiency compared to a single driver will be -3dB for isobaric and +3dB for normal dual woofer.

Dan

[daryl]

Just to be 100% clear, is a parallel isobaric 3 or 6db down on a regular parallel two woofer design?

Parallel isobaric has the same voltage sensitivity as a single driver.

Series isobaric has a voltage sensitivity that is -6db compared to a single driver.

Either isobaric connection has a power efficiency of -3db compared to the single driver.

[daryl]

Just for extra clarity....

Say you have a bass driver with these specs.

8ohm Nominal Impedance

90db spl @ 2.83V Sensitivity

90db spl @ 1W Efficiency

2.83V into an 8ohm load results in 1W power consumption so power sensitivity is the same as for 2.83V for 8 ohm drivers.

Now if you put two of these same drivers into an enclosure with half the effective volume in an isobaric arrangement the results are as follows......

Parallel connection results in half the electrical impedance compared to the single driver setup meaning that twice the power will be consumed compared to the single driver setup.

Each driver is receiving the same power as before in the single driver setup (same voltage, same current).

Sound is radiating from only one of the drivers (the outside one) so spl is the same as for the single driver setup.

Parallel isobaric specs (using the individual driver outlined above).....

4ohm Nominal Impedance

90db spl @ 2.83V Sensitivity

87db spl @ 1W Efficiency

You see you have 90db for 2W input so when you reduce that 2W by 3db to the 1W reference level you have 87db.

Series connection results in double the electrical impedance compared to the single driver arrangement so only half the power will be consumed compared to the single driver setup.

Each driver is receiving 1/4 the power as before in the single driver setup (half voltage, half current).

Sound is radiating from only only the outside driver and each driver is being driven with 1/4 the power (-6db) so the output is -6db for the series connection.

Series isobaric specs (using the individual driver outlined above).....

16ohm Nominal Impedance

84db @ 2.83V Sensitivity

87db @ 1W Efficiency

That 84db is for 0.5W input, now add 3db to get the output for 1W and you have 87db.

[bill poster]

So with parallel isobaric you'll get the same output as a single 8ohm driver but(as it's now a 4ohm load)the amp is having to work a lot harder..

[daryl]

So with parallel isobaric you'll get the same output as a single 8ohm driver but(as it's now a 4ohm load)the amp is having to work a lot harder..

Yup!

[aaronm]

I think the isobaric idea really only works if you just happen to have a pair of woofers on your shelf that require something like a 10cuft box each in a standard arrangement and you could bring the box size down to an almost acceptable size with an isobaric setup.
Nobody I know would do such a thing on purpose though...

[Paul K.]

I can remember ads for Isobaric woofers stating "handles twice the power" as for a single woofer, when in reality it takes twice the power to generate the same SPL as for a single woofer. Isn't "marketing speak" wonderful!
Paul

So with parallel isobaric you'll get the same output as a single 8ohm driver but(as it's now a 4ohm load)the amp is having to work a lot harder..

[badman]

As aaron said, isobaric is most useful when you already have woofers, but they want too big a box. For scratch design, they have a few instances where they can be advantageous. "Clamshell" style isobaric mounting can reduce some even-order distortion (at the cost of a low-pass filter, or vent/frame noise in poorly-designed woofers working hard, and looking at the motor rather than the cone). If you have a stock of woofers but they want too big a box, you can fit 2 clamshells onto opposite sides of a box sized for a single driver, with equivalent efficiency and voltage sensitivity, but 4x the power handling, 2x the output, and drastically lower distortion. For a given SPL you benefit from half the excursion demand on the driver, distortion help from the clamshell even-order cancellation, and 1/4 the power into the coil (more power, like more displacement, makes things worse). That arrangement also has force cancellation, equal but opposite forces on the box reducing wall flexure and "box walking". It's what I did when I had a lot of reasonably priced, good quality woofers, but couldn't afford the space that 24 cubic feet would take- I made 2 dual clamshell subs, and got away with 6 cu ft in 2 enclosures. Decent drivers still matter, but if you find decent higher-vas subwoofers on clearance (some drivers labeled as "woofers" will work too), it can be a pretty good way to go.

For when a bigger box is okay, PPSL style manifolds give even better distortion performance but the 2 drivers need >2x the box (due to extra manifold displacement)