I've finished recently isobaric subwoofer, now I think, may be it is waste of materials?
http://www.diyaudio.com/forums/showthread.php?postid=1070384#post1070384
When I designed it, I thought about better linearity because of symmetry...
Now I doubt I was right: this way I made 2'nd and other even harmonics less. But for the same sonic pressure if to compare with the same speakers working independently excursion must be higher, as the result odd harmonics may be higher. It means less pleasant distortions.
What do you think?
http://www.diyaudio.com/forums/showthread.php?postid=1070384#post1070384
When I designed it, I thought about better linearity because of symmetry...
Now I doubt I was right: this way I made 2'nd and other even harmonics less. But for the same sonic pressure if to compare with the same speakers working independently excursion must be higher, as the result odd harmonics may be higher. It means less pleasant distortions.
What do you think?
Personally, I like the sound of push pull designs/ isobaric designs, small box deep bass. I tend to clamshell.
Umm the biggest push pull commercial sub I have heard is at Sydney IMAX , 8 JBL 18" = awsome
What problem are you having? doesnt it sound any good? or is it that it wont keep up with the line array?
Umm the biggest push pull commercial sub I have heard is at Sydney IMAX , 8 JBL 18" = awsome
What problem are you having? doesnt it sound any good? or is it that it wont keep up with the line array?
Wavebourn said:
(JPK) See http://www.musicanddesign.com/Isobaric.html, particularly the lower 1/2 where Isobaric results are presented. Isobarics don't reduce even order distortion, nor does push-pull when both drivers are mounted in seprate boxes of a size where air suspension nonlinearity cotributes to nonlinearity.
Also see my post under http://www.diyaudio.com/forums/showthread.php?s=&threadid=90735 for additional discussion of push-pull for non isobaric systems.
long long time ago I built isobaric subwoofer cosisting of four 10" woofers, two and two in specialy designed steel tube facing the same direction (not clamshell), but each pair was mounted in oposite direction one facing out, the other in, in order to reduce distortion, I followed instructions literaly, epecialy internal volume, but constructed box out of concrete!
it was the best sub I ever built/heard (well I have not heard that many subwoofers), still a very successful project
had to leave it behind in europe when I moved into us
it was the best sub I ever built/heard (well I have not heard that many subwoofers), still a very successful project
had to leave it behind in europe when I moved into us
adason said:
I thought I should reduce even order distortions (more symmetrical transfer function), but as soon as more excursion is needed for the same pressure probably I was wrong and got more distortions as the result, especially odd distortions...
The box has huge volume, and speakers are driven by an amp with positive feedback by current, so frequency response is very good, but as I said after finishing it I started thinking that it was probably a mistake...
However, I did not use a concrete, but linoleum tiles glued and stapled to surfaces are heavy enough...
Drivers I use are Alpine SWE-1242.
Wavebourn,
you are absolutely right about the relationship between the woofers excursion and distortion...the purpose of isobaric is to reduce the box volume while keeping the low frequency extension. Assuming you are not abusing your subwoofer, the distortion should not be the issue. However if you use it for high spl, smaller box volume would most likely be better. It would limit low frequency extension, but as well prevent from overexcursion.
Anyway, in my case four woofers in two isobaric pairs in the right volume for home listening were magic. Imagine that I would have to built box four time the size for the same low frequency extension.
Even worse, imagine that I would built one box of the same size with just one woofer. That would mean significanly higher excursion required for the same spl than four woofers.
In any case, the answer your primary isobaric question: to do, or not to do? is...
Yes, no and maybe!
It depends .....
you are absolutely right about the relationship between the woofers excursion and distortion...the purpose of isobaric is to reduce the box volume while keeping the low frequency extension. Assuming you are not abusing your subwoofer, the distortion should not be the issue. However if you use it for high spl, smaller box volume would most likely be better. It would limit low frequency extension, but as well prevent from overexcursion.
Anyway, in my case four woofers in two isobaric pairs in the right volume for home listening were magic. Imagine that I would have to built box four time the size for the same low frequency extension.
Even worse, imagine that I would built one box of the same size with just one woofer. That would mean significanly higher excursion required for the same spl than four woofers.
In any case, the answer your primary isobaric question: to do, or not to do? is...
Yes, no and maybe!
It depends .....
Wavebourn,
looking at the picture, you might just attach front baffle, put two woofers there normal way next to each other, leave the guts as they are for bracing and see if you like the sound better
you might have those woofers way too close to the back side of the box, maybe some distortion originate there
looking at the picture, you might just attach front baffle, put two woofers there normal way next to each other, leave the guts as they are for bracing and see if you like the sound better
you might have those woofers way too close to the back side of the box, maybe some distortion originate there
adason said:
Good idea, thank you!
All surfaces are well strapped by aluminum details and damped by linoleum, foam spray from Home Depot, memory foam from matress topper, and covered by leather imitation. The box is hermetically sealed, I went through all frequencies to find if some air whispers and resonances remained.
So, I am concerned about distortions of drivers, and wonder if I could get less of them using both drivers separated...
I don't understand the picture at all, what you want to achieve, Wavebourne. There's no acoustical lowpass slot to start the horn throat so this is not a bass horn...this looks like an oversized PA midrange horn if you put a 10 or 12 in it. I'm not into isobaric for a horn, except possibly a tapped horn. On a typical bass horn the back volume is not a big problem, and if you want to conserve space you're not going to even consider a horn.
Get yourself a good book on horn design and you could fit a decent folded into that big a box. See what you can find in old speaker builder issues, search for "show horn"...
Get yourself a good book on horn design and you could fit a decent folded into that big a box. See what you can find in old speaker builder issues, search for "show horn"...
The isobaric principle was IMO popularized by LINN AUDIO with their "Isobaric" and "Sara" speakers.
Basically one trades the cost and effort of two drivers against halving the box volume.
What comes along with that design is a specific sonic coloration resulting from this two drivers interacting with each other forming a low beat - much like two slightly de-tuned guitar strings.
Also - as the diaphragm of the outer speaker is virtually transparent and more or less fully transmits the sound of the inner speaker - isobaric arrangement creates a pronounced CMP (Consecutive Min Phase) system with its specific properties.
Especially with wide bandwidth isobaric's (the 2-way "Sara") there also might come some slight BDMD (Back Diaphragm Mirror Distortion) into play as well.
All in all I did not like the mentioned speakers back then - as much as I liked other products from LINN - but thats possibly just me...
Michael
Basically one trades the cost and effort of two drivers against halving the box volume.
What comes along with that design is a specific sonic coloration resulting from this two drivers interacting with each other forming a low beat - much like two slightly de-tuned guitar strings.
Also - as the diaphragm of the outer speaker is virtually transparent and more or less fully transmits the sound of the inner speaker - isobaric arrangement creates a pronounced CMP (Consecutive Min Phase) system with its specific properties.
Especially with wide bandwidth isobaric's (the 2-way "Sara") there also might come some slight BDMD (Back Diaphragm Mirror Distortion) into play as well.
All in all I did not like the mentioned speakers back then - as much as I liked other products from LINN - but thats possibly just me...
Michael
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John K,
I read your paper on Isobaric distortion and as I follow your conclusion, the last plot show where the isobaric chamber is reduced from 4L to 1L (244 cubic-in to 61 cubic-in). When the isobaric chamber is very VERY small compared to the box volume your simulation shows that even order distortion is reduce.
I checked Volume of the diaphragm spec of 2 JBL woofer - 27 cu in (JBL 2225) and 45 cu in (JBL 2235). If I mount these driver between 3/4inch plywood, the total volume would come out to 2 X (Volume of diaphragm) + volume of 3/4 plywood cutout. Mounted this way this setup is small compared to the rear speaker volume.
So can it be said that if you use the minimal volume of the drivers some reduction in even order is suitable. I was wondering, at what ratio of isobaric volume compared to rear volume a reduction is even order benefits or is not suitable. Your paper doesn't mention what the rear chamber volume your simulating or I missed reading it.
Rich J
I read your paper on Isobaric distortion and as I follow your conclusion, the last plot show where the isobaric chamber is reduced from 4L to 1L (244 cubic-in to 61 cubic-in). When the isobaric chamber is very VERY small compared to the box volume your simulation shows that even order distortion is reduce.
I checked Volume of the diaphragm spec of 2 JBL woofer - 27 cu in (JBL 2225) and 45 cu in (JBL 2235). If I mount these driver between 3/4inch plywood, the total volume would come out to 2 X (Volume of diaphragm) + volume of 3/4 plywood cutout. Mounted this way this setup is small compared to the rear speaker volume.
So can it be said that if you use the minimal volume of the drivers some reduction in even order is suitable. I was wondering, at what ratio of isobaric volume compared to rear volume a reduction is even order benefits or is not suitable. Your paper doesn't mention what the rear chamber volume your simulating or I missed reading it.
Rich J
If you have drivers with asymmetric distortion (excursion differs between in and out cone movement), in theory you will get distortion reduction by using a clamshell style isobaric configuration. But for this to work the ratio of the volume betwen the drivers and the volume of the back enclosure needs to be high. If the back enclosure is relatively small, the cone excursions will no longer match and the distortion reduction effect is reduced. This is especially noticeable with ported enclosures, where the enclosure becomes extremely "small" at resonance.
See graph in this post:
http://www.diyaudio.com/forums/multi-way/260382-bass-impact-depends-what.html#post4021523
See graph in this post:
http://www.diyaudio.com/forums/multi-way/260382-bass-impact-depends-what.html#post4021523
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John K paper, uses a 29L (1.0 ft^3) box with a 4L isobaric chamber and from his simulation, there is no reduction in even order harmonics, further more distortion may exceed the standard push-pull setup.
At the end of John's paper, he reduces the isobaric chamber to 1L, a 29 to 1 ratio, of the rear box to isobaric chamber box. Then his simulation plot shows reduction in the even order harmonics but goes on to say the a "1L isobaric chamber is unrealistic". In my mind, 1L is not unrealistic for large cabinets after checking diaphragms volumes (Vd) specs. When I made isobarics, I didn't create a "box" for the isobaric chamber, I did what Don Hills described above as "clam shell" to keep the isobaric volume as small as possible. So take 3/4 inch wood, cut out a baffle diameter, mount your driver face to face on baffle, mount the baffle to the cabinet. According to John's simulation if the ratio of rear volume to isobaric volume is 29 you'll get a reduction is even order distortion. So although isobaric push-pull uses 1/2 the standard box volume, you can get even order distortion canceled if the rear box to iso-chamber is 29. This probably will likely work with woofer's that have high suspension equivalent volume's (Vas). A small living room realistic home sub cabinet is say 87L (3ft^3), I need an isobaric chamber <= 87/29 = 3L (183.1 inch^3) which would be ok to use the woofer I mentioned above depending on your isobaric 1/2Volume T/S alignment.
At the end of John's paper, he reduces the isobaric chamber to 1L, a 29 to 1 ratio, of the rear box to isobaric chamber box. Then his simulation plot shows reduction in the even order harmonics but goes on to say the a "1L isobaric chamber is unrealistic". In my mind, 1L is not unrealistic for large cabinets after checking diaphragms volumes (Vd) specs. When I made isobarics, I didn't create a "box" for the isobaric chamber, I did what Don Hills described above as "clam shell" to keep the isobaric volume as small as possible. So take 3/4 inch wood, cut out a baffle diameter, mount your driver face to face on baffle, mount the baffle to the cabinet. According to John's simulation if the ratio of rear volume to isobaric volume is 29 you'll get a reduction is even order distortion. So although isobaric push-pull uses 1/2 the standard box volume, you can get even order distortion canceled if the rear box to iso-chamber is 29. This probably will likely work with woofer's that have high suspension equivalent volume's (Vas). A small living room realistic home sub cabinet is say 87L (3ft^3), I need an isobaric chamber <= 87/29 = 3L (183.1 inch^3) which would be ok to use the woofer I mentioned above depending on your isobaric 1/2Volume T/S alignment.
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