Isobaric Designs

I am looking for any insight on building an isobaric monitor speaker. I do not have the space for a floorstander or a subwoofer, but would like as full range of a speaker as possible. Something similar to the totem mani 2 would work for me.

I am interested in isobaric as it presents more of a challenge in construction, and I would like to build something more complicated than a ported, single woofer design.

Any insight would be helpful, and if no schematics exist, anyone who has done this and would tell me about it would be greatly appreciated.
 

ocool_15

Member
2004-11-26 3:15 am
sk
If you look at a 3 way design it would work. I don't think you will find any 2 way design like the totem mani 2 indicated using an isobaric configuration. You could try isobaric woofer running to a full range 2" or 3" driver.
As the distance between the isobaric cones becomes significant you will have alot of interference and a poor frequency response.
The only improvment is at low frequencies.
 
ocool_15 said:
I don't think you will find any 2 way design like the totem mani 2 indicated using an isobaric configuration. You could try isobaric woofer running to a full range 2" or 3" driver.
As the distance between the isobaric cones becomes significant you will have alot of interference and a poor frequency response.
The only improvment is at low frequencies.

Exactly what i was thinking. The isobarik is only effective if the size of the wavelength is significantly larger than the size of the chamber. We have been playing with a wonderful 3", and have been giving lots of thot to 2-ways with an XO 200-350 Hz. Our favoured woofer thou is really happy in a really small box, so an isobarik looses its advantages... todays thot experiment was an MTM in a 14 litre aperiodic box... with a bit of EQ on the bass amp, i'd guess flat to 40. The claimed response of the mid-tweeter goes to 32k...

dave
 
probably a 2 way. a three way would require more cabinet volume, and a more complicated crossover. It would allow for the 2 woofers to be used only for bass, but I think I will probably do a 2 way, with the woofers either magnet to magnet, or magnet to cone.

And a TL isobaric design could be done I believe, most easily using the cone to cone arrangement I would think.
 

marchel

Member
2007-11-18 4:51 am
IMO, you will gain nothing by making isobaric speaker, While the box volume is reduced, the effeciency is also reduced, Just imagine that the speaker is eating up twice the power for the same output.


The distortion cancellation with isobaric speaker is limited only to the low freq. And You cannot use isobaric to do midrange, cause the sound coming from the back of the speaker is nowhere near as good as the front, You then add the noise of the speaker, which usually originate from the back , to make things work, I mean worst.


You're better off with conventional design if you want good sounding speaker.
 
Hi,

The simple fact is for a 2-way a correctly chosen single driver for the
box volume will outperform 2 drivers in isobaric configuration. Bass
might not be quite as clean but you will have no midrange problems.

As stated isobaric loses efficiency, 3dB its this that halves the volume.
So simply get a low efficiency driver with good bass. e.g. (or similar) :

http://www.rjbaudio.com/Extremis/extremis.html

:)/sreten.
 
Adire audio is out of business, so that driver will be hard to find. I may try a 3 way with isobaric woofers, a midrange and a tweeter. I am surprised at the midrange problems of the configuration, as the totem mani 2 has a very high crossover point and still sounds very good. It may be the best speaker they make.
 
Hi,

http://www.stereophile.com/standloudspeakers/462/

I suppose the back to back magnets, some judicious stuffing and
rolling off the rear driver early avoids most of the midrange problems.
I do wonder though what % of internal volume the iso arrangement
wastes, seems to me a single drive box would not be much bigger.

Zaph seems to like the HiVi D6.8 http://www.zaphaudio.com/6.5test/ :
Comments: This Hi-Vi poly cone driver has a 3" voice coil with an inset
magnet similar to Morel and Dynaudio woofers. The difference is that,
while they perform similarly, the Hi-Vi outperforms them in the harmonic
distortion department. If I were going to use this style of inset magnet
woofer, I would choose the Hi-Vi over Morel or Dynaudio. I hope you like
screwing, because the Hi-Vi has 12 mounting holes which is a bit excessive.

A driver (or similar) not ignored by Totem either ......

3-way iso ? Again 1 good driver is better than 2 not so good, and
distortion issues in drivers are not as simple as they may first
appear. Push pull cancels 2nd harmonic but does not do anything
about 3rd, some might prefer keeping the 2nd harmonic warmth.

http://www.klippel.de/download/Nonlin/klippel, Loudspeaker nonlinearities - causes and symptoms.pdf

Is a very interesting read if you can follow it. It should be obvious
that minimising distortion my maximising Vd (volume displacement)
is more important than trying to cancel distortion near overload.



:)/sreten.
 
while I am not fixated on building an isobaric design, I would like to try it. I really just want to build a large, standmounted speaker with enough bass to need no sub.

Also, how much harder is a 3 way crossover construction than a two way if I decide to do an isobaric design with a midrange and tweeter?
 
sreten said:
CSS-SDX7

Is a replacement for 600Hz downwards.

Overall i think the SDX7 is a better woofer than the Extremis. I never felt that the Extremis was any good when used above 1kHz. I am working the SDX7 now to make it an even better driver -- i particularily want to reduce noise as well as smooth the top... we'll be using these as woofers XOed 80-350 Hx depending on ap. These will wotk in 7-21 litre sealed. We also have a TL & BIB in the works.

dave
 
what recommendations would anyone have for plans on building a more or less full range 2 way (either MTM or standard TM) that is full range is used on a stand mount? I probably would think about taking a plan for an MT design and adapting it for 2 woofers by doubling the cabinet volume. What crossover changes would that entail? Also what design software and books do you guys recommend as I am new to this?
 

tinitus

diyAudio Moderator R.I.P.
2005-11-24 1:47 am
A 2.5way(double midwoofers) would be more efficient

Wonder if anyone has tried to mount a passive slave behind a midwoofer, like in isobaric/compound :D

Or the isobaric might be interesting with 2.5way xo, with a bigger champer between the two midwoofers, so that the inner woofers acts like in a compound
 

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tinitus said:
A 2.5way(double midwoofers) would be more efficient

Wonder if anyone has tried to mount a passive slave behind a midwoofer, like in isobaric/compound :D

Or the isobaric might be interesting with 2.5way xo, with a bigger champer between the two midwoofers, so that the inner woofers acts like in a compound



Hi,

Not quite a classic 2.5way c/o because the external unit still needs BSC.

But the principle is sound, take an MT and add a 0.5 way unit.


:)/sreten.
 

jdlech

Member
2007-12-25 7:51 pm
I'm also toying with my first project and it also involves a isobaric chamber.

I chose my bass drivers for their low Fs and am building an (unreasonably?) huge box for them (331.6 liters minuse some bracing) tuned to 24Hz. Between all that, my F1 should be about 20.5Hz in a speaker that should get an average of 92db SPL. It should be better but I'm still quite the novice. For what little I know, it's acceptable to me as a first try.

Your space requirments are completely different from mine, though. So your considerations will be quite divergent. I'm going with a twin iso chambers - 4 woofers per box. I seriously doubt you will want to try that with a bookshelf speaker.

Forgive my ignorance, Tinitus, but am I misunderstanding your musings? An isobaric mid-bass driver? I suppose if one wanted to halve the size of an inner chamber, that would work well enough. But in my ignorance, I see no other reason. Can you? Would you ever want to extend the response of the mid bass driver? Seems to me, it would just be better to get a different driver even if it costs nearly twice as much. Besides, as I undertand it, the mass of the air in the isobaric chamber changes some of the T/S parameters. Lot's of air in a large camber --> lots of mass to move --> higher Vas. I'm probably wrong, though.

I've also mused about quick temperature changes inside the chamber (heated air expands) reducing a woofers Xmax. But I've been assured that, even in a sealed chamber, woofers could never hold a pressure behind them for very long. So I guess such concerns are without merit.

Here's something you might find of more than casual disinterest. Although I'm sure it's far from being a complete list.

Even though he thinks I'm a complete ditz, here's the guy who inspired my design

As always, you milage may vary.
Perhaps you could look for a small Vas driver with a low Fs suitable for a large bookshelf design. Just don't expect to "crank it up" as it will likely be a low spl driver.