Seem to be tuned to about 29Hz... so looks like they should be ok.
Again, may be a case of get those mids sealed and spend some more time with them before I make any rash decisions!
If I called a vote, who would say TL and who would say keep the bass reflex?
Should point out that I will probably have to sell them at some point, so if they're sold as 'PMC clones' maybe that would get more than 'some diy speakers'???!!
Again, may be a case of get those mids sealed and spend some more time with them before I make any rash decisions!
If I called a vote, who would say TL and who would say keep the bass reflex?
Should point out that I will probably have to sell them at some point, so if they're sold as 'PMC clones' maybe that would get more than 'some diy speakers'???!!
Totally agree.
Tho will add that IF you look at this as only a sub (for the time being) finding what will match makes it easier. If done active DSP that is. The normal starting point is how big of space and listening requirements (type of and level of). Once this has been established then we select a midrange, match a tweeter to that and then, lastly address the low end. When done in reverse the compromises made will greatly affect the end result, rarely matching your expectations.
To answer BR or TL, I'd swing TL as I'm testing one currently. Properly designed the lower impedance peak is greatly reduced and without the sharp phase swing tht occurs in all BR designs below port frequency. The reason for this is that the length of a TL tunes the enclosure with very minor influence by the port whereas BR is tuned via Helmholtz Resonator.
An example would be the MLTL sub experiment I'm working on. Can change the port from none, which is a port e.g. 2.9" ID x 1.5" L (baffle thickness) to 2.5" ID x 4" to 2.5"ID x 6" and the Fb changed 1Hz lower, that's it. Actually the only thing that did change was the Q of the port, longer port higher Q, up to a point. For giggles slapped a 2.5"ID tube 42" long and took a few measurements, even stuffed some polyfill to see the effects it had. Hey you'll never truly know without cuttn some wood, build and measure.
Dampening the line also affects tuning, getting this part right is very tricky and is integral to the design. The end result can be very impressive, tho not done by a long shot in my testing can say that being able to hit 18Hz with a little 6.5" is rather impressive without needing EQ. In fact the driver is so well dampened on the low end have decided that no low freq filter will be required to prevent over excursion. This is where a BR falls flat on it's face.
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Sorry, I should bring in that this is the same speakers that I bought of fleabay for £200, there's a lot of work to do on them - see my other thread
http://www.diyaudio.com/forums/mult...-3-ways-flea-bay-sound-awful-help-please.html
All said though, I think they were pretty good VFM... If my wife would let me they'd be in my main system now, unfortunately there size says they go or I go....
http://www.diyaudio.com/forums/mult...-3-ways-flea-bay-sound-awful-help-please.html
All said though, I think they were pretty good VFM... If my wife would let me they'd be in my main system now, unfortunately there size says they go or I go....
Think you intended to say the red line is the port and the black line is the driver.
Bass reflex and "transmission line" are really one and the same, just at opposite ends of a continuum in terms of port dimensions. A conventional BR can be designed and the port can be progressively lengthened and area expanded (to keep the same tuning). Small signal performance will hardly vary (or can be held constant with Bl changes). As the port gets longer/wider the large signal performance improves, so freedom from port noise is achieved but the port length resonances drop in frequency and become more problematic.
The fundamental resonance of a TL is determined by the mass of air in the port and not tied to length. All the upper resonances are length determined.
Finally, determining TL length to match woofer resonance, i.e. 1/4 wave tuning, is no more proper than tuning a vented box to match woofer resonance (the discredited method used before Thiele wrote his paper). Like a BR design a TL design must have the proper box volume and tuning to match the parameters of the woofer.
David
David respect your word, but tend to disagree and not blindly so. If what your saying then explain why the Fb did not change when ports (four configurations in all) were tested? By your way eg TL= BR then why this discrepancy? BR eg helmholtz resonators very much do change with the changes made above. But after testing the nearfield response of both the driver and of the port at near maximum power, with before and after of both changing of the port tuning and then went on to measure the effect of various dampenings and it's effects have I have come to this conclusion. I'm still messing around with a IKEA pillow stuffing suggestion bjorno made the otherday. Works well, time consuming to fluff it out properly and pain to place correctly due to the narrow line.
Do have to say I would very much like to discuss this topic. When running power vs distortion tests I see an odd effect, but do to enclosure or room construction (odd vibrations, rattling, etc <room not enclosure
cannot isolate the cause of or it's bandwidth product to anything particular other than what's been mentioned. Can say that it doesn't appear relative to tunings. Hit a certain level and it increases into audibility, which I'd say is >3% to my ear. Need ground plane measurements, which I was hoping to complete today, but with a bad storm last night and windy today didn't happen. I do note a change in character at upper power levels, mainly attributed to the distortion products of a 6.5" at Xmax (6mm) driving such low frequency signals. I start the sweeps at 15Hz in almost every case.
Hmmm, a sweep test at various power levels or would an integrated solution be better mapping out each frequency in power increase before moving to the next higher frequency or better C weighted pink noise like broad band power pulse tests? Or another test where instead of testing from some low frequency running up start from the top and move down product a difference statistically? Basically looking at pro level solution than anything marketed to joe consumer, likely.
Now if you can suggest a test, a jig, a widget, more theory... that would assist me answer these variables, I can and would build/do, $$ withstanding. I don't do programming, Did a very long time ago, nothing in 20+ years. Not that I'm not handy around a PC, was one of the original overclockers starting back in the '80's and a old commodore pet owner. Have been on the hardware side of things exclusively since my last personal speaker build, an MMTMMMWW built back in '91-92 Even then I used shading to control the mids and took polar response into consideration. What a bloody difficult crossover. Used an old Ivey IB30 spectrum analyzer to rough it out along with a Crown PZM6R for other various measurments
Now if you can suggest a test, a jig, a widget, more theory... that would assist me answer these variables, I can and would build/do, $$ withstanding. I don't do programming, Did a very long time ago, nothing in 20+ years. Not that I'm not handy around a PC, was one of the original overclockers starting back in the '80's and a old commodore pet owner. Have been on the hardware side of things exclusively since my last personal speaker build, an MMTMMMWW built back in '91-92 Even then I used shading to control the mids and took polar response into consideration. What a bloody difficult crossover. Used an old Ivey IB30 spectrum analyzer to rough it out along with a Crown PZM6R for other various measurments
I have always asked myself why there was not more manufacturers
that would produce rather TL than BR boxes.
My reasoning tells me that if TL by its nature would be something
really special and so much better than common BR, everybody
would have done it, no matter the more cost and complexity involved.
TL line has one bad feature, especially applicable to 2 way designs
and that is it's output goes way to high in frequencies and this sound
can't be of any good to the direct sound coming from the cone of the
driver. No matter how one tries to kill the standing waves with wadding
they still live there and if overdamped then the output suffers.
BR has got much favorable feature not doing what TL does, it appears
to be low passed.
There is this theory people believe in, TL is exotic and allegedlly sounds
much better. I can't say I agree with it. IMO
that would produce rather TL than BR boxes.
My reasoning tells me that if TL by its nature would be something
really special and so much better than common BR, everybody
would have done it, no matter the more cost and complexity involved.
TL line has one bad feature, especially applicable to 2 way designs
and that is it's output goes way to high in frequencies and this sound
can't be of any good to the direct sound coming from the cone of the
driver. No matter how one tries to kill the standing waves with wadding
they still live there and if overdamped then the output suffers.
BR has got much favorable feature not doing what TL does, it appears
to be low passed.
There is this theory people believe in, TL is exotic and allegedlly sounds
much better. I can't say I agree with it. IMO
The spreadsheets MJK has produced over many years of refinement have been used in the past without commercial consent. So today we must pay for the privilege. Commercial use it's much much more expensive. It is not done normally due to R&D costs involved and complexity to just get right. Doesn't have that easy cheap factor added to it that manufactures are looking for.
BR have their own issues and many of those interested in tl designs understand the diffrences and make the leap. My reasoning is more toward the balance that can be struck if done properly. The improved dampening characteristics at low frequency trumps BR leaving you with a blend of good of both sealed (displacement control) and LF output of a BR without the sharp phase reversal below tuning. The problem issue is in the upper harmonics being correctly damped.
This is the signature quality of RF transmission lines. They operate from a given low frequency and up eg a high pass filter. Whereas an RF coax operates like a BR or AS eg a low pass filter.
Transmission lines do not automatically offer 'better' or 'worse' performance than anything else. They have their own set of characteristics and compromises. One of those is that they can be slightly more complicated to manufacture (MLTL types et al aside) and that = more expensive. Then you hit the second point which is that, at least since Thiele, BR cabinets have been very straightforward to design (although many still manage to screw up by not using an appropriate alignment). TLs were more complex, not helped by the fact that Bailey, Bradbury et al, for all their best intentions, led people right up the garden path in design terms. The basic principals of pipe resonance and the volume requirements of a driver seemed to be forgotten.
Either way, TLs (of all types referred to under that umbrella term) can be very effective types of cabinet load if designed properly. Alas, they often aren't. George Augspurger and particularly Martin King have done much in terms of tools, papers &c. to aid designing them.
Either way, TLs (of all types referred to under that umbrella term) can be very effective types of cabinet load if designed properly. Alas, they often aren't. George Augspurger and particularly Martin King have done much in terms of tools, papers &c. to aid designing them.
I get different results for the same type of changes
I was intrigued by your experiments in changing a port's length and wanted to see if I would have similar results. Modeling with a 10" woofer (Seas CA26RFX), I set up an ML-TL for reference. It's 60" long with the driver at the 1/3 mark and the port at 10" from the other end of the line. With a total line volume of 5.35 ft3 and 3"-diameter port 0.75" long (i.e. wall thickness), the system tuning frequency was 30 Hz, f3 was 28 Hz and the overall response is flat from just above the knee in the curve to 1 kHz, with a mild depression from 100 to 300 Hz. After increasing the port's length to 1.5", with no other changes, the system tuning frequency became 28 Hz, f3 was 27 Hz, and as would be expected, the overall response is starting to shelve down. Last, increasing the port's length to 3" shifted the system tuning frequency to 23 Hz, f3 to 33 Hz, and the response now looks much like an EBS alignment. Oh, I modeled with the first half of the line stuffed at a density of 0.75 lb/ft3.
This is entirely a modeling exercise with Martin King's worksheets, no actual building or measuring, and it may be somewhat of an apples and oranges comparison, but it does cause one to wonder. Not necessarily doubting your results, but I don't understand how what you describe could occur. Maybe I'm not understanding completely what you did and what you stated?
Paul
I was intrigued by your experiments in changing a port's length and wanted to see if I would have similar results. Modeling with a 10" woofer (Seas CA26RFX), I set up an ML-TL for reference. It's 60" long with the driver at the 1/3 mark and the port at 10" from the other end of the line. With a total line volume of 5.35 ft3 and 3"-diameter port 0.75" long (i.e. wall thickness), the system tuning frequency was 30 Hz, f3 was 28 Hz and the overall response is flat from just above the knee in the curve to 1 kHz, with a mild depression from 100 to 300 Hz. After increasing the port's length to 1.5", with no other changes, the system tuning frequency became 28 Hz, f3 was 27 Hz, and as would be expected, the overall response is starting to shelve down. Last, increasing the port's length to 3" shifted the system tuning frequency to 23 Hz, f3 to 33 Hz, and the response now looks much like an EBS alignment. Oh, I modeled with the first half of the line stuffed at a density of 0.75 lb/ft3.
This is entirely a modeling exercise with Martin King's worksheets, no actual building or measuring, and it may be somewhat of an apples and oranges comparison, but it does cause one to wonder. Not necessarily doubting your results, but I don't understand how what you describe could occur. Maybe I'm not understanding completely what you did and what you stated?
Paul
Totally agree.
Tho will add that IF you look at this as only a sub (for the time being) finding what will match makes it easier. If done active DSP that is. The normal starting point is how big of space and listening requirements (type of and level of). Once this has been established then we select a midrange, match a tweeter to that and then, lastly address the low end. When done in reverse the compromises made will greatly affect the end result, rarely matching your expectations.
To answer BR or TL, I'd swing TL as I'm testing one currently. Properly designed the lower impedance peak is greatly reduced and without the sharp phase swing tht occurs in all BR designs below port frequency. The reason for this is that the length of a TL tunes the enclosure with very minor influence by the port whereas BR is tuned via Helmholtz Resonator.
An example would be the MLTL sub experiment I'm working on. Can change the port from none, which is a port e.g. 2.9" ID x 1.5" L (baffle thickness) to 2.5" ID x 4" to 2.5"ID x 6" and the Fb changed 1Hz lower, that's it. Actually the only thing that did change was the Q of the port, longer port higher Q, up to a point. For giggles slapped a 2.5"ID tube 42" long and took a few measurements, even stuffed some polyfill to see the effects it had. Hey you'll never truly know without cuttn some wood, build and measure.
Dampening the line also affects tuning, getting this part right is very tricky and is integral to the design. The end result can be very impressive, tho not done by a long shot in my testing can say that being able to hit 18Hz with a little 6.5" is rather impressive without needing EQ. In fact the driver is so well dampened on the low end have decided that no low freq filter will be required to prevent over excursion. This is where a BR falls flat on it's face.
Heya Paul, that's the odd thing, models do show a change but measurements show no difference. I intentionally designed this shelved eg instead of Fb at Fs of 40Hz, set this to 35. Am trying to explain to myself the differences between reality and what Leonard Audio Transmission Line software. For one LATL shows no change in tuning regardless of dampening. This is an obvious error. With that said, upon corresponding with Peter showing the results vs model due to a mistuning. This lack of dampening correction lead to an Fb of 27Hz. Retested with less dampening, Fb shifted higher, finally reading through bjorno's posts used Ikea pillow stuffing and followed his suggestion. This resulted in an Fb of ~32. Much closer to the model.
The original design used a slot port with an area of ~25cm² with a 3.8:1 aspect ratio (was actually 3.8" x1") and a depth of 3.25". This caused a very high port noise level and was suggested by Peter to use a 3"ID port. Seemed a bit overkill for a 6mm Xmax 6.5" so went with a 2.5"ID port x 3.75" and added a 3" precision port as the port is not terminated to a baffle ie external and felt a little end correction wouldn't hurt. Same result, might have changed 1 Hz but within measurement error. Then decided to change port again as I made it be easily swapped out. This time to a 6"length w/ same ID, no change. Lastly I removed the port and left only the hole in the baffle. This is 2.9" ID x 1.5" (baffle thickness), again no change.
Once that was all done went on to test various dampenings, all of which showed a marked change in tuning. Far more so than expected quite honestly. Will say the microfiber towels are extremely effective without flow loss. Sadly far to much for my needs, am storing that away for future reference
The highest Fb to date is with bjorno's method. Unstuffed it's ~34.5Hz and almost spot on to the model, albiet the model shows it stuffed like in the first test Fb27 and other oddities that are simply not there. Need more tests for sure, another test enclosure is on the drawing table. Perhaps this weekend early next week. Weather looks good
http://www.diyaudio.com/forums/subwoofers/250902-interpretation-results.html
What I've found about dampening, which I assume you mean the effects of stuffing, if the stuffing is confined to no more than the first half of the line, there is less effect on system tuning and f3 over a moderate range of stuffing density. But, stuffing added beyond the halfway point has significantly greater effect--a whiff of stuffing in a terminus, for instance, will really kill f3. For stuffing, I've used Acousta Stuf, Polyfil and bonded Dacron from Meniscus Audio. When weighed or cut correctly and installed correctly, I find the actual system tuning agrees well with that modeled, but frankly, if they differ by a couple of Hz, I probably wouldn't mind. I prefer to use round ports for an ML-TL, but have no problem using a rectangular terminus for a tapered TL as long as the aspect ratio is not more than 5:1.
Paul
Paul
Heya Paul, that's the odd thing, models do show a change but measurements show no difference. I intentionally designed this shelved eg instead of Fb at Fs of 40Hz, set this to 35. Am trying to explain to myself the differences between reality and what Leonard Audio Transmission Line software. For one LATL shows no change in tuning regardless of dampening. This is an obvious error. With that said, upon corresponding with Peter showing the results vs model due to a mistuning. This lack of dampening correction lead to an Fb of 27Hz. Retested with less dampening, Fb shifted higher, finally reading through bjorno's posts used Ikea pillow stuffing and followed his suggestion. This resulted in an Fb of ~32. Much closer to the model.
The original design used a slot port with an area of ~25cm² with a 3.8:1 aspect ratio (was actually 3.8" x1") and a depth of 3.25". This caused a very high port noise level and was suggested by Peter to use a 3"ID port. Seemed a bit overkill for a 6mm Xmax 6.5" so went with a 2.5"ID port x 3.75" and added a 3" precision port as the port is not terminated to a baffle ie external and felt a little end correction wouldn't hurt. Same result, might have changed 1 Hz but within measurement error. Then decided to change port again as I made it be easily swapped out. This time to a 6"length w/ same ID, no change. Lastly I removed the port and left only the hole in the baffle. This is 2.9" ID x 1.5" (baffle thickness), again no change.
Once that was all done went on to test various dampenings, all of which showed a marked change in tuning. Far more so than expected quite honestly. Will say the microfiber towels are extremely effective without flow loss. Sadly far to much for my needs, am storing that away for future reference
The highest Fb to date is with bjorno's method. Unstuffed it's ~34.5Hz and almost spot on to the model, albiet the model shows it stuffed like in the first test Fb27 and other oddities that are simply not there. Need more tests for sure, another test enclosure is on the drawing table. Perhaps this weekend early next week. Weather looks good
http://www.diyaudio.com/forums/subwoofers/250902-interpretation-results.html
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