'isobaric' schooling help please.

[gafhenderson]

i have a pair of 12" subs that i picked up a while ago and never got around to doing anything with them. now i've started sorted out other parts of my audio set up i need to get these doing something.

when using the parameters i have for them.
fs: 37.4hz
qms: 5.28
qes: 0.88
qts: 0.76
Vas: 69

its suggested that the driver will be best suited to a sealed box or open baffle type designs.

using the calculators at mh-audio.nl

they gave me basic sizes for enclosures. unsure if these are 'correct' it was just the numbers presented to me.

PORTED

Vdr - 4.25
460.25 litres
77.45cm X 125.31cm X 47.86cm
fb : 20.38
peak : 0.078
f-3 : 14.48
port area : 63.67cm2
port length : 3.43cm

SEALED

460ltr
82.69hz peak db
0.247db peak level @ fb
f-3 : 35.69

i had been interesting in playing with transmission lines/TQWT but my drivers are not well suited after some reading? after using the 'plot frequency response' function the sealed box is smooth, but the ported box gives a lot of output very low down which is something i'd consider desirable but my experience with ports have been boomy, harsh and port noise.

i would like low frequency response that is smooth. no nasty peaking at certain audible frequencies. if the port can be tuned to 20hz so its basically out of audible range then that would make a ported design more desirable.

during this reading i came across isobaric and their ability to reduce the total enclosure volumes? but i'm not sure i full understand all terminology given.

given the examples i quickly drew, for both ported and sealed boxes, what do the following setups do to the box sizes, output etc? if possible can it be explained in the very simplest of terms as there's a lot of stuff to try and assimilate at once with regards to these things.

thanks for your help.

[nigelwright7557]

I would go for the sealed box, less chance of surprises at the output.

[ODougbo]

I've tried building a few Iso boxes and found an inherent problem with the tunnel design (which are fun to make and look better). The front woofer is in a small - round space, not a good combination, you may end up with a ringing sound.

I found it starts up around 80hz; however, if you crossover say,,,60hz to 70hz I don't think you will hear it.

All said....I've been toying with an idea that may help the ringing problem, cut the rings in different sizes, make the tunnel an hour glass shape, vs. a straight shot; e.g. ><

Also may be a good place to spay car undercoater, but let that dry for about a week.

Here's a how-to pic to glue a tunnel up.

[bjorno]

gafhenderson:

...i had been interesting in playing with transmission lines/TQWT but my drivers are not well suited after some reading? after using the 'plot frequency response' function the sealed box is smooth, but the ported box gives a lot of output very low down which is something i'd consider desirable but my experience with ports have been boomy, harsh and port noise...i would like low frequency response that is smooth. no nasty peaking at certain audible frequencies. if the port can be tuned to 20hz so its basically out of audible range then that would make a ported design more desirable...during this reading i came across isobaric and their ability to reduce the total enclosure volumes? but i'm not sure i full understand all terminology given...

Hi,
Here is a suggestion of an OD-TL,IMO better than a ported design or a closed one:

b

PS: I didn't use your provided data but I suspected that in the given sparse data a crap Car driver was hidden that really IMO don't belong to this forum but should be posted to the 'Car Audio' forum in the first place.

[TheBaronGroog]

Clamshell mounting and a sealed box will give the smallest enclosure and easiest build.

Sealed isobaric 34.68l F3 47Hz
Vented isobaric 125.1l F3 19Hz

Below are the models for sealed, vented, iso-sealed, iso-vented, sealed double and vented double. You should be able to read the box volume and see the outputs relative to one another off 100w.

[gafhenderson]

Originally Posted by TheBaronGroog

Clamshell mounting and a sealed box will give the smallest enclosure and easiest build.

Sealed isobaric 34.68l F3 47Hz
Vented isobaric 125.1l F3 19Hz

Below are the models for sealed, vented, iso-sealed, iso-vented, sealed double and vented double. You should be able to read the box volume and see the outputs relative to one another off 100w.

that was incredibly helpful, thank you for your time. givem the suggestion that they're probably fairly crappy driver units i'm drawn more towards the space saving that the isobaric seems to provide.

Originally Posted by bjorno

Hi,
Here is a suggestion of an OD-TL,IMO better than a ported design or a closed one:

b

PS: I didn't use your provided data but I suspected that in the given sparse data a crap Car driver was hidden that really IMO don't belong to this forum but should be posted to the 'Car Audio' forum in the first place.

OD-TL? i'm not very familiar with all terminology used on this forum, very very beginner.

i suspect you are correct that they are car audio drivers. they have that look. are dual voice coils more common in car subs?

tbf its a subwoofer. any correctly assembled is going to improve my current low end reproduction in movies no? my floor standing speakers seem to run out of useful noise below 50hz. so any extra grunt in the 20-80/100 range is welcome.

what would this set up be called and does it do anything/work? its not the passive radiator type, but both drivers active. is it still isobaric?

[TheBaronGroog]

Bjorno's box is a transmission line of some discription-not sure myself what the OD stands for!! I could do with a breakdown of all horns/TL/TQWT designs so it'd be easier to spot the difference! The SPL response of his design is very odd-not like the normal stuff I've seen him post-but I'd guess this is down to the drivers.

The picture you post above is a push pull design, but not isobaric. The benefit of the above design isn't smaller box volume, but reduced distortion from the two drivers opposing motion.

If the subs are the Audiobahn Bjorno's specs came form I'd love to know how he spotted it-he must be a speaker lexicon!

[gafhenderson]

Originally Posted by TheBaronGroog

Bjorno's box is a transmission line of some discription-not sure myself what the OD stands for!! I could do with a breakdown of all horns/TL/TQWT designs so it'd be easier to spot the difference! The SPL response of his design is very odd-not like the normal stuff I've seen him post-but I'd guess this is down to the drivers.

The picture you post above is a push pull design, but not isobaric. The benefit of the above design isn't smaller box volume, but reduced distortion from the two drivers opposing motion.

If the subs are the Audiobahn Bjorno's specs came form I'd love to know how he spotted it-he must be a speaker lexicon!

reduced distortion sounds good. what would be a suitable search string for 'push pull design' to further read about them. the lack of familiarity with terminology and charts outside of basic frequency response are hindering the speed at which i am learning.

so far i am siding with the isobaric designs, slightly towards the sealed box but the frequency responses posted for the vented design didn't have any particularly nasty peaks so that has also captured my attention due to how low it could go. if its a crappy driver, then i won't waste a lot of space accommodating its use and i'm sure it will do fine as a beginner play thing till i better understand the concepts involved.

with regard to 'audiobahn' i just had a look on their website and the driver does look quite similar to some on there, so i am fairly impressed also at his investigational skills.

[18Hurts]

I built a clam shell style face-to-face isobaric with a passive radiator on the back.

The face-to-face is the best way to do it for several reasons. The tunnel thing has too much air space between the drivers to couple accurately, the air space is very small for one driver while the other one operates in a much larger space. Basically, one woofer will run hotter than the other one when driving them hard.

The face-to-face also is a push-pull loading scheme which cuts distortion to improve your sound quality. One of the drivers is sticking out in the open air for much better cooling and the other one is inside the box air space so it runs cool.

My isobaric-passive radiator sub was an entertaining and simple build, very simple to change the tuning on a passive to get it smooth in the actual room. Have fun with your spare parts laying around and hope your results surprise you.

[gafhenderson]

Originally Posted by bjorno

Hi,
Here is a suggestion of an OD-TL,IMO better than a ported design or a closed one:

b

PS: I didn't use your provided data but I suspected that in the given sparse data a crap Car driver was hidden that really IMO don't belong to this forum but should be posted to the 'Car Audio' forum in the first place.

oh, OFFSET driver - transmission line? sorry for being blind. that post has so much information on it.

the part about Xmax of the driver, is that not going to kill the woofer given the specs you provided for the possible identity of the driver gives Xmax at 7mm?

if i'm honest most of your schematics completely elude my level of knowledge and intelligence. whatever you have created it seems quite loud. but fairly unusable after 60hz due to the irregularity of sound levels?