Isobaric for subs only?

[speekergeek]

I was looking into buying some stryke audio sa071 7" drivers recently because they are pretty ******* cheap if you buy 8 or more, they are only 10 US bucks. I was gonna buy 8 and make 2 MTM speakers, with the 7" drivers in an isobaric config with a tubular enclosure connecting them magnet to magnet to act sort of like a bucking magnet, plus it should cancel out any magnetic irregularities caused by the position of the voice coil vs. magnet stuff. is this feasable? or is it only for subs? winisd told me that if i have 2 pairs of speakers in an isobaric config i could have an F3 of 29hz in a 2 cu ft box! which surprised me quite a bit. that would be awesome. i also trid to do calculations on fitting 4 drivers in a ported box and it was almost 8 cu ft for the same F3! of course i would have to account for the cylindrical enclosure for the sealed iso enclosure. Does anybody understand what i am talking about? I mean come on! $80 for 8 7" woven cone, apparently very nice drivers, its a steal!

 

[sreten]

Back to back you will waste enclosure volume that simply not
needed for Isobaric coupling.

I'd go for a 2.5 way with the lower bass units reversed for
distortion cancellation.

The basic principle of twin isobaric compared to a single unit is
you get the same bass response with half the enclosure volume
but this costs you halving the drive impedance.

with 4 units - isobarically paired - in series parallel - you get
the same impedance as one drive unit - the same bass response
in the same volume - but twice the bass power level and 6db
more sensitivity.

Obviously compared to 4 normally connected units the bass power
level is halved - and you need 4 times the enclosure volume.

/sreten.

 

[speekergeek]

I am not quite sure i understand that post.... i didn't understand this part.

"I'd go for a 2.5 way with the lower bass units reversed for
distortion cancellation."

 

[7V]

I don't completely understand the design that you're talking about here but all the main isobaric points have been covered - half the volume (except for wastage in connecting the two drivers), -3dB efficiency, double the power handling.

Don't forget though that the isobaric layout will only work for the bass frequencies. If you go near the mids it will sound horrid.

 

[sreten]

I am not quite sure i understand that post.... i didn't understand this part.

"I'd go for a 2.5 way with the lower bass units reversed for
distortion cancellation."

Thinking about it not a helpful statement, sorry.

You are correct that 'magnet to magnet' will significantly reduce
the stray magnetic field. It will also tend to cancel distortion in
the bass units caused by assymetry in the magnetic field for the
voice coils.

Note that you don't need to link them by a tubular structure -
a simple box will be fine. If magnetic cancellation is not such a
concern the volume of this box can be reduced significantly
by offsetting the drivers such that they are next to each other.
If your speaker is vertically narrow the box becomes a sub-baffle.

Have to agree with 7V regarding the midrange having thought
it about some more - so a 3 way would be a much better choice
than a MTM (2 way) or TMM (2.5 way).

hope this is more helpful, /sreten.

 

[speekergeek]

thanks much for your help, i understand now. so isobaric is for subs only, muddies up the midrange and all the good stuff. THX!

 

[RHosch]

Don't forget though that the isobaric layout will only work for the bass frequencies. If you go near the mids it will sound horrid.

That's true only for face/face configurations. Isobaric mounting of magnet/face or magnet/magnet using a coupling cylinder shouldn't have any negative impacts on the mids, other than perhaps a pipe resonance in the coupling cylinder that could be in the desired bandwidth of the drivers. Magnet/magnet would offer the best assymetrical distortion cancelling properties, but would require a bit more volume in the cylinder than magnet/face.

 

[sreten]

Isobaric bass midrange has been / is being done, but requires
a lot more care and thought than just using it for bass.

Its certainly a lot easier for the amateur just to use it for bass.

/sreten.

 

[sardonx]

I would say all that is partly true and partly untrue.

I'm going to do an isobaric mid/bass design and have given it some thought. Magnet/magnet config doesn't necesserally have to take up more room... in fact in some cases it takes up LESS room. This is the config i'd go with too. The reason being that when you're mounting magnet/face, if you go with the magnet 'into' or too close to the rear speaker, when the XmaX gets going... the rear cone might travel far enough to hit the front magnet. Which would lead me to distance the drivers ever further for safety reasons with a magnet/face config.

So not only is magnet/magnet configuration safer, it also cancels out magnetic irregularities and seems a better way to go. Of course i am just talking out of my *** cuz i don't have experience with isobaric, but i think everythign out to the last detail before i make it.

Oh and those of you who say that isobaric is only for sub applications... did you forget about the Totem Mani-2?