ISOBARIC sealed enclosure... Any experiences? Any advantages?

The only high frequency sounds that should be in there are distortion sounds from the drivers themselves. The woofers should be crossed over pretty low, but they don't have to be. I don't think it will hurt anything in your speaker because you don't cover the drivers with damping.

Yes. That's what I thought - and also read in the article linked by @Audentid14. Indeed, the inside speaker has not his cone covered with a damping nor blanketed by a curtain.

As you can see on the picture below, I choose to slant the internal speaker baffle, to tame some possible chamber and enclosure resonances :

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I also padded and braced the inside :

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Before fully filling the inside :

1734557825465.png


Sorry, I dis not took pics of the damping in the chamber, but I posted a description on post #220.

T
 
It may not matter but we always tried to make the air chamber between the two drivers as small as possible for better coupling.
This volume may be less important to the last spare inch if it is 100% air tight. The air volumes wheight also lowers Fs. Just keeping it as small as it is conveniant for the drivers should be fine. LINN even made it quite large for my taste and it worked.

Yes. I never found clear or defined rules about dimensioning that intermediary chamber, except than purely mechanical, i.e. : the speakers must not touch themselves in operation, small as possible but not too small, some or no damping...

So here's the choice I empirically made :

xNa6Sb-ISOBARIC-22-07-2024.jpg


I don't think that exhaustive studies on the subject have been published, like the one we find on Sealed or Bass-Reflex speakers. At least, I did not found one, so the article linked by @Audientid14 is a welcome additional knowledge...

T
 
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If you know your physic's book, the Isobaric principle has nothing new to show. It is a problem solver if you got "the wrong" driver, but multiple of it. You can integrate the same result in a single driver just as well, halfing the cabinet volume needed.
I see it as a part of the speaker designers tool case. It may come handy in some cases, interesting but nothing sensational.
 
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Maybe the distortion reduction due to having one driver turned is underestimated.

It looks attractive to me to cancel like this nonlinearities out with isobarik alignment.

If box volume plays no rule then two drivers can be put close together one of them inverted without losing efficiency.

If put on opposite sides of the box reducing vibration (also one aspect) I am not sure if nonlinearities would cancel out as good as the drivers have more space between them.
 
Preparing a pair of cheapo drivers for subwoofer use

marble box will be in a corner and has a downfiring 8cm reflex port 45
cm long in 30 Liter volume.

IMG_20241225_110513.jpg


clamshell alignment.

Spacing of 3mm is enough that the surrounds won't touch during work. Will be sealed with silicone for complete air tightness.

IMG_20241225_110329.jpg
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Although cheapo ones these have four holes in the basket ventilating the area below the yellow spider

IMG_20241225_101436.jpg



nice mellow surround, lightweight paper cone.

no cone treatment here as the combined drivers support each other impressively.

Touching one driver immediately moves the other one.

IMG_20241225_101446.jpg



I will try a 2300uF electrolytic for filtering and linearizing the response. But audition test will decide.
 
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their price in egypt is 7 euro in the shop. I know these drivers in china are produced for 1 to 2 euro.

These are spare parts for bluetooth soundboxes. Its hard to get anything else in the country. So ideal use is here the isobarik alignment.
 
I will try however. After I heard someone abuses small class D chips with one ohm impedance and no failure I have no fear anymore.

see

However there will be 10 meters 0.5mm2 cable for connecting the sub. Will add a bit of resistance. Like 0.35 ohms
 
Such a thin cable of that length will certainly have a bad effect on the sound. 0.35ohm will raise the Qts of your drivers, and reduce the damping factor of the amplifier. Tie them in series, it is not forbidden, in the counter phase of course.
 
Is it wise to connect 6ohm drivers in parallel? The impedance will be somewhere below 3ohm for sure.
yes, if the amp shuts down or gets hot, I would have to rewire in series.

Often 8 ohm drivers have in reality 6 ohms.

now the driver is ready for use, sealed with silicone to get 100% airtightness.

If Q is too high I still can add same size magnets to the drivers which I have in reserve and damp the box partially.

In a similar project a single same type driver did well without extra magnet or damping of the box. I couldn't believe myself. Maybe room resonances play a higher role than the Q of a driver in the bass.

Corner placement seems to make a big difference, too.

IMG_20241225_130428.jpg
 
ISOBARIC sealed enclosure... Any experiences ? Any advantages ?

In a project of a quite large 4-ways sealed enclosure, I wonder if going for the isobaric principle for the woofer setion would offer advantages... 🤔

The goal is not to reduce volume, but improve bass extension in frequency response, transient and step response, distortion, by using a tandem of speakers, like this - but larger about the back volume

View attachment 1323649

According to many expert authors, combining two identical speakers in that way offers an equivalence of :
  • Z / 2 (connected in //)
  • VAS / 2
  • QTS unchanged
  • FS unchanged

If the enclosure volume remains the same (the common chamber is not counted) for one sealed speaker, or two isobaric speakers, as a consequence the result is a lowering of the QTC, so a better transient, earlier bass cutoff start but smoother and longer slope, potentially giving a better bass extrension.

Any advice or experience in that field ?

Thank you Guys !

T
I used to have an incredible system. I had 4 JL Audio subs isobaric. It not only allowed me to use 4 subs in a smaller space but the sound quality was out of this world. I was using precision power amps. Back when they were high quality. Give it a try.
The pressure in the isobaric chamber theoretical varies exactly as the pressure would in a box of twice the size with a single driver. The pressure in the box of the isobaric system is theoretically twice the pressure of the single box system. Thus, the pressure force on the rear driver is (Pbox - Pchhambe). The pressure on the front driver is just Pchamber. Now, with Pchamber being equal to the pressure for a single box system, call it Psb, and with Pbox = 2 x Pchamber = 2 x Psb, P on the rear driver = 2 Psb - Psb = Psb and the pressure on the front driver is just Psb.

The two driver move with slightly different excursions resulting in the pressure variation in the chamber. Gluing, or mechanically coupling the driver would eliminate the isobaric effect.

Note the obvious: If the pressure in the isobaric chamber did not vary the same as the pressure in the single driver box then the response of the isobaric system would not be the same as the single box system with twice the box volume.

https://musicanddesign.speakerdesign.net/Isobaric.html
I used to have an incredible system. I had 4 JL Audio subs isobaric. It not only allowed me to use 4 subs in a smaller space but the sound quality was out of this world. I was using precision power amps. Back when they were high quality. Give it a try. I know that is not very technical, but I was very happy
 
After a while of listening tests between the 375L Bass-Reflex and the 475L Isobaric - with Audiophile and non-Audiophile friends - we can draw the conclusion that between the two models :

1 - the bass extension appears to be a bit deeper on the Isobaric, but we'll say that these are similar.
2 - the efficiencies seems to be slightly higher on the Isobaric, but we'll say that these are similar.
3 - the punch and transients are more underlined and controlled on the Isobaric than on the Bass-Reflex

Taking in count that :

1 - the test are done by instant switch box A/B'ing of the speakers on the same musical programs and amp (Accuphase E-560).
2 - the efficiency specs of the two models of Woofers (Beyma 12BR70 and 12B100R) is very close, both in the 93-94dB/W/m range.
3 - the crossovers are both 1st order serial/parallel configuration.
4 - the QTC of the Isobaric is 0.69-0.70 (critical is QTC=0.71), while the n on the Bass-Reflex is 5.4-5.5 (critical is n=5.7).

T
 
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I can give subjective feedback on the sound of the clamshell isobaric of post 255 in this thread as I finished a 2.1 amplifier system with it.

Sub like all my corner placed low tuned reflex boxes reaches the lowest octave and gives a decent result for such cheap drivers.

Heard making of "Wish you were here" and "Dark side of the moon" from Floyd "Which one is Pink?"

Articulate enough to keep it going as is.

 
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