Isobaric Cab design question

[Twincam]

In an isobaric design (Cone to magnet), to port the rear do you just measure the area as normal and tune the port length as standard?

[Bill Fitzmaurice]

The tuning is the same as with a standard cab. The modeling software will take into account that it's an isobaric alignment.

[Twincam]

[quote name='Bill Fitzmaurice' timestamp='1471787531' post='3115836']
The tuning is the same as with a standard cab. The modeling software will take into account that it's an isobaric alignment.
[/quote]

Unfortunately I don't have that option. I'm using maths and a very simple port tuning program. It doesn't have to be perfect but "right".

[Bill Fitzmaurice]

[quote name='Twincam' timestamp='1471788894' post='3115857'] I'm using maths and a very simple port tuning program.
[/quote]Why? I could see it to some extent if modeling software was a major investment, but as it's free I can't comprehend not using it.

[Twincam]

[quote name='Bill Fitzmaurice' timestamp='1471797620' post='3115963']
Why? I could see it to some extent if modeling software was a major investment, but as it's free I can't comprehend not using it.
[/quote]

I don't have a computer.

Its ok however I think I've got it sorted.

Edited August 21, 2016 by Twincam

[MoonBassAlpha]

I may have asked this before, but I don't remember any replies.
Could you run an isobaric cabinet with the speakers magnet to magnet, one facing forwards, one facing backwards, both emitting sound? Speakers wired in antiphase to retain isobaric operation. I suppose there wouldn't be any acoustic suspension to control the cone overshoot...
Would the sound phase issues be any different to using a rear port?

[HowieBass]

[quote name='MoonBassAlpha' timestamp='1471871112' post='3116532']
I may have asked this before, but I don't remember any replies.
Could you run an isobaric cabinet with the speakers magnet to magnet, one facing forwards, one facing backwards, both emitting sound? Speakers wired in antiphase to retain isobaric operation. I suppose there wouldn't be any acoustic suspension to control the cone overshoot...
Would the sound phase issues be any different to using a rear port?
[/quote]

Yes you could but you have potential distortion issues according to this article: http://en.wikipedia.org/wiki/Isobaric_loudspeaker

[Bill Fitzmaurice]

[quote name='MoonBassAlpha' timestamp='1471871112' post='3116532']
Could you run an isobaric cabinet with the speakers magnet to magnet, one facing forwards, one facing backwards, both emitting sound?
[/quote]Technically they don't both emit sound. The area of only one cone is exposed to the air, so the maximum output with two drivers isobaric is the same as with one in a standard configuration. The only advantage is a halving of the Vas, but due to the space taken up by the second driver and the connecting plenum chamber the net cabinet size is not halved, while cost is greatly increased by using two drivers. That's why manufacturers in general have not jumped onto this particular band wagon.

[Twincam]

Indeed it does seem a pointless exercise. I got a homemade isobaric 2x10 cab in a trade.
The port was all wrong! At higher volumes it would make all sorts of noise I've never felt that much velocity of air come out such a small cab.
I've altered the port well 2 ports now. Ive done this a few times, it seems still too high a velocity, although calculations are saying things should be good. Although I've solved some (most) of the port noise issue. I've lost some low end, the overall responce has improved maybe.
I just don't think it was a well designed cab in the first place tbh.

[alexclaber]

One of the problems with isobaric cabs is that the smaller main enclosure volume means you need a much greater port length to get the right tuning frequency with a sufficiently large cross-sectional-area port, or you have to reduce the port area to get the length shorter - and too little port area results in chuffing noises, loss of output, distortion, compression etc.

They made quite a lot of sense decades ago for hi-fi when the available speakers needed big sealed enclosures but not anymore...

[spacey]

They are an old idea re-heated to get the "vintage" sound... LOL
Modern drivers and higher power rating of drivers have made such ideas obsolete, similar with horn loaded cabs.

[bassmachine2112]

Just copy a 212 Orange cab,there are 2 models-SP212 and OBC212.
I use a SP212 ,the old one for 3 1/2 years now and it sounds great.

[taunton-hobbit]

I owned a couple of Celestion SR bass bins in the eighties - I think these were supposed to be a type of isobaric? - I know they were this side of useless.

[Twincam]

Taken the rear speaker out and I've replaced the front driver with a better one also.
Will test it at volume tomorrow. Although already it sounds better.

[MoonBassAlpha]

[quote name='Bill Fitzmaurice' timestamp='1471876703' post='3116607']
Technically they don't both emit sound. The area of only one cone is exposed to the air, so the maximum output with two drivers isobaric is the same as with one in a standard configuration. The only advantage is a halving of the Vas, but due to the space taken up by the second driver and the connecting plenum chamber the net cabinet size is not halved, while cost is greatly increased by using two drivers. That's why manufacturers in general have not jumped onto this particular band wagon.
[/quote]
That's not quite what I meant Bill, I meant the rear-facing speaker is also open to the air, just facing backwards, or even upwards or downwards.

[Bill Fitzmaurice]

[quote name='MoonBassAlpha' timestamp='1472042537' post='3117929']
That's not quite what I meant Bill, I meant the rear-facing speaker is also open to the air, just facing backwards, or even upwards or downwards.
[/quote]Without a picture I can't be sure of what you mean, but on the face of it that's not an isobaric alignment.

[alexclaber]

[quote name='MoonBassAlpha' timestamp='1472042537' post='3117929']
That's not quite what I meant Bill, I meant the rear-facing speaker is also open to the air, just facing backwards, or even upwards or downwards.
[/quote]

A speaker emits sounds from both sides of the cone. With a sealed cab you get output from the front of the cone. With a ported cab you get output from the front of the cone and further bass output from the port which is driven by the back of the cone.

With a sealed isobaric cab you get output from the front of the front speaker, the back of the front speaker and the front of the back speaker do nothing except move in unison on either side of the isobaric chamber, and the back of the back speaker produces no sonic output either. With a ported isobaric cab the back of the back speaker drives the port to produce additional bass output.

This is not an isobaric speaker:

[Dan Dare]

Have to agree with Alex that isobaric arrangements are not necessary with modem driver and cabinet design. The considerable extra weight and cost does not translate into a proportionate increase in performance.