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Old 30th March 2006, 09:40 AM   #1
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Default Phase alignment of the isobaric design

Hi all,
Anyone know the voice coil phase alignment of the mid/high frequency unit with the isobaric woofer? Align with the front or the rear unit?

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Somebody claim the isobaric design make the output double(3db gain) but I find a website said:

" The volume of the enclosure wil be approximately half the space of the above enclosure with the same results, and the output will be the same."

Is it right?

Rgs

Jimmy
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Old 30th March 2006, 09:40 AM   #2
lndm is offline lndm  Australia
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Default Re: Phase alignment of the isobaric design

The output is a combination of the two units so at bass frequencies I would think the acoustic centre is half way between the two voice coils. At higher frequencies I think the coupling between the drivers would fall apart so I think the front driver would dominate.

Quote:
Originally posted by Jimmy DIY
Somebody claim the isobaric design make the output double(3db gain) but I find a website said:

" The volume of the enclosure wil be approximately half the space of the above enclosure with the same results, and the output will be the same."

Is it right?
If you have two 8 ohm drivers in parallel, you have 4 ohms so your amp should be pumping 3dB more into them for the same volume control setting.
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Old 30th March 2006, 10:01 AM   #3
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Default Re: Re: Phase alignment of the isobaric design

Quote:
Originally posted by lndm
The output is a combination of the two units so at bass frequencies I would think the acoustic centre is half way between the two voice coils. At higher frequencies I think the coupling between the drivers would fall apart so I think the front driver would dominate.
I agree with that. The front cone will be the one emitting sound as the rear one serves to simply make the front cone seem like it's twice as heavy. The acoustic centre is also NOT the magnet which is so commonly misthought. It is where the sound radiates from, which is usually just down by the area around the dust cap.

Quote:
Originally posted by lndm
If you have two 8 ohm drivers in parallel, you have 4 ohms so your amp should be pumping 3dB more into them for the same volume control setting.
Again I agree. The SPL will be the same but the power requirements double, which is the price you pay for half the box size.
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Old 30th March 2006, 10:03 AM   #4
Eva is offline Eva  Spain
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Isobaric configuration is mostly a waste of resources. It draws double the amplifier power and produces the same output as a single driver.

If you have two drivers, use them together both radiating, this will also draw double the amplifier power but will produce 6dB more output than a single driver or a isobaric system.
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Old 30th March 2006, 11:14 AM   #5
SY is offline SY  United States
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Quote:
Originally posted by Eva
Isobaric configuration is mostly a waste of resources. It draws double the amplifier power and produces the same output as a single driver.

If you have two drivers, use them together both radiating, this will also draw double the amplifier power but will produce 6dB more output than a single driver or a isobaric system.
The engineering choice isn't that simple. With high Vas drivers, the isobaric can usefully allow significant size reduction.

Acoustic center for crossover to the midrange needs to be determined experimentally on a case by case basis. Rules of thumb almost never are accurate enough to prevent severe crossover problems.

One nice trick is to add a choke in series with the inner driver.
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Old 31st March 2006, 04:56 AM   #6
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Quote:
Originally posted by SY


The engineering choice isn't that simple. With high Vas drivers, the isobaric can usefully allow significant size reduction.

Acoustic center for crossover to the midrange needs to be determined experimentally on a case by case basis. Rules of thumb almost never are accurate enough to prevent severe crossover problems.

One nice trick is to add a choke in series with the inner driver.
It's seem the claim of isobaric design has double output is not true, but as SY said it's worth to apply for significant size reduction in high Vas drivers.

About the phase alignment, how to measure it accurately if Rules of thumb can't help? Are there any reference or books for the diyer? Is the "Loudspeaker design cook book" has the solution? The "Speaker Workshop" freeware can hlep or not?

And what is the purpose to add a choke in series with the inner driver? How to practical it?

Rgs

Jimmy
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Old 31st March 2006, 06:11 AM   #7
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As we have already said isobaric halves your box size and output is identical. Power input requirements double.
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Old 31st March 2006, 06:15 AM   #8
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Default Re: Phase alignment of the isobaric design

Quote:
Originally posted by Jimmy DIY
Somebody claim the isobaric design make the output double(3db gain) but I find a website said:

" The volume of the enclosure wil be approximately half the space of the above enclosure with the same results, and the output will be the same."

Is it right?

Rgs

Jimmy
With two identical drivers, in terms of *voltage sensitivity* (dBSPL/V) this is correct. Voltage sensitivity will stay unchanged. But since the second driver will halve the overall impedance and thus both drivers now draw double the current for a given voltage (and SPL). the *power sensitivity* changes accordingly.

You may now go ahead and do the "power efficiency calculus" (dBSPL@1W/1m) by impedance over frequency...

regards
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Old 31st March 2006, 09:52 AM   #9
sreten is offline sreten  United Kingdom
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Hi,

if its not yet obvious you should not use an isobaric configuration
up to frequencies where phase alignment or acoustic centres are
an issue. You will get severe cancellation at certain frequencies.

One way of avoiding this is the series inductor for the rear unit.

However the small coupling space between drivers is not good
for preventing rereflection through the front cone, but can work.

/sreten.
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Old 31st March 2006, 12:04 PM   #10
SY is offline SY  United States
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sreten's got it- the choke rolls off the rear driver at frequencies it's not needed to prevent interference notches in the midrange.

Jimmy, phase the two so that the cones both move in the same direction.
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