I am thinking of a small project that includes 2 (a pair of) cheapo polycone (6.5 inch) woofer drivers.
I plan to build a sat/sub setup with these - using these as the sub drivers.
I am wondering... would it be more efficient/practical to use the two of these in an isobarik + vented setup? Or would it be overall more efficient to design a single (conventional) vented box that basically places both drivers in the same cavity?
My thought was: using both in isobarik will half the size requirements (very desireable in this particular application).
Here are the T/S deets for these drivers:
If I could tune these to dig down into the low 30's, that would be great. High-20's would be even better. I am NOT trying to win any SPL contests here - and sound quality is more important than overall output.
I imagine I could also just place them in dual sealed enclosures, but will loose a lot of low-end output.
This "sub" will be passive. So having 2 drivers doubles as an additional benefit in that it provides an easy way to "sum" the two amplifier channels (each channel wired to a seperate driver). This brings up another concern: should I include any "summing" circuitry in the crossover that combines the L+R channels equally to both drivers? My initial design though was to simply create a basic 2-way crossover in the sub cabinet that sends the high-freq to the sats and low frequencies to the two woofers (electrically isolated L+R).
-Dean
I plan to build a sat/sub setup with these - using these as the sub drivers.
I am wondering... would it be more efficient/practical to use the two of these in an isobarik + vented setup? Or would it be overall more efficient to design a single (conventional) vented box that basically places both drivers in the same cavity?
My thought was: using both in isobarik will half the size requirements (very desireable in this particular application).
Here are the T/S deets for these drivers:
| Resonant Frequency (Fs) | 52Hz |
| DC Resistance (Re) | 3.37Ω |
| Voice Coil Inductance (Le) | 0.48mH |
| Mechanical Q (Qms) | 2.99 |
| Electromagnetic Q (Qes) | 1.4 |
| Total Q (Qts) | 0.95 |
| Compliance Equivalent Volume (Vas) | 0.65ft³ |
| Maximum Linear Excursion (Xmax) | 3.5mm |
| Sealed Volume | 0.4ft³ |
| Sealed F3 | 61.4Hz |
| Vented Volume | 0.63ft³ |
| Vented F3 | 47.2Hz |
If I could tune these to dig down into the low 30's, that would be great. High-20's would be even better. I am NOT trying to win any SPL contests here - and sound quality is more important than overall output.
I imagine I could also just place them in dual sealed enclosures, but will loose a lot of low-end output.
This "sub" will be passive. So having 2 drivers doubles as an additional benefit in that it provides an easy way to "sum" the two amplifier channels (each channel wired to a seperate driver). This brings up another concern: should I include any "summing" circuitry in the crossover that combines the L+R channels equally to both drivers? My initial design though was to simply create a basic 2-way crossover in the sub cabinet that sends the high-freq to the sats and low frequencies to the two woofers (electrically isolated L+R).
-Dean
Hofmann's Iron Law: low, efficient, small, pick two.
Isobaric loading allows a smaller box. A smaller box is less efficient.
Isobaric loading would allow the two drivers to equal the output of one driver in a similar volume, but using twice the power.
The displacement of the isobaric pair is the same as a single driver, and a 6.5" driver with 3.5mm Xmax does not displace much.
Tuning an octave below Fs will reduce efficiency and create a stepped low frequency response even using a larger box size.
If you don't use equalization to compensate for the dropping low frequency, it will sound "thin" compared to a box tuned around FS.
No, that would short circuit the two amp channels, the output impedance of an amplifier output is very low, like 1/10th an ohm- the outputs will be feeding current into each other instead of the speakers, and you'll likely release the amplifier's "magic smoke".
Each sat/sub should have an independent passive crossover.
It would be best to use separate chambers for the two drivers- an isobaric pair driven with L/R inputs may fight each other.
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In general, what is the safe low-tuning limit for any given driver when going with a vented enclosure?
From what I understand, QTS lower than .4
But the thing I do not understand is this: go back just 10-15 years and MOST ALL woofer drivers in vented enclosures were loose as a... well.. hot dog thrown down the hallway sideways. They did NOT have stiff suspension and lot's of damping and it was not a stretch to find even the crap speakers at Sears and other department stores that could crank out 40hz bass. So I assume QTS lower than .4 is ONE requirement, but there may be another way?
Hell, I had an old set of Polk Audio bookshelf speakers from early 2000's that I re-foamed years ago... They were rather small with 6 inch drivers and a VERY loose suspension as well as a small box and THEY did 38-42hz. Again, those drivers were nothing special.
So what was the secret sauce to getting acceptable sound from small-to-mid-sized woofers using run-of-the-mill enclosures and drivers?
From what I understand, QTS lower than .4
But the thing I do not understand is this: go back just 10-15 years and MOST ALL woofer drivers in vented enclosures were loose as a... well.. hot dog thrown down the hallway sideways. They did NOT have stiff suspension and lot's of damping and it was not a stretch to find even the crap speakers at Sears and other department stores that could crank out 40hz bass. So I assume QTS lower than .4 is ONE requirement, but there may be another way?
Hell, I had an old set of Polk Audio bookshelf speakers from early 2000's that I re-foamed years ago... They were rather small with 6 inch drivers and a VERY loose suspension as well as a small box and THEY did 38-42hz. Again, those drivers were nothing special.
So what was the secret sauce to getting acceptable sound from small-to-mid-sized woofers using run-of-the-mill enclosures and drivers?
Xmax sets it and if wanting to ensure not worrying about it, then IME use the pioneer's Fb = 1.56x Fs prosound tuning.
~0.4 is for an optimally max flat alignment (Vb = Vas, Fb = Fs), so this means that an increasingly lower tuning with increasing Qt and vice versa:
T/S max flat alignment:
Vented net volume (Vb) (L) = 20*Vas*Qts'^3.3
(Ft^3 = (Vb)/~28.31685)
Vented box tuning (Fb) (Hz) = 0.42*Fs*Qts'^-0.96
F3 (Hz) = Fs*0.28*Qts'^-1.4
Historically, high efficiency was the goal since high power was too big/costly, so high Vas with high effective Qt was required to get as much acoustic efficiency as practical for the intended app, but as power increased, everything else shrank to compensate.
@GM - Thanks a ton for this. Let me re-cap to ensure I understand things...
-> A Qts of .4 is IDEAL, at which point the tuning of the box (Fb) should (roughly) be the Fs [of the driver].
-> When the Qts is higher (as is the case with cheaper drivers), you CAN tune a bit below the drivers Fs, but this is where SQ starts to suffer I imagine
-> In a perfect world, we would operate a driver above it's Fs at all times
Sorry... I have built dozens of sealed enclosures over the years and have recently decided to try designing some ported enclosures more for practice than anything. I have figured out WinISD a bit but am learning the Do's and Dont's as far as driver selection, what the sane limits for tuning SHOULD be, etc.
I often hear of people with rather high-end subs (Fs of 18-24hz) tuning the boxes at 14-16hz. I have always assumed you essentially tune for whatever you want to squeeze out of the driver and go on with it.
-Dean
-> A Qts of .4 is IDEAL, at which point the tuning of the box (Fb) should (roughly) be the Fs [of the driver].
-> When the Qts is higher (as is the case with cheaper drivers), you CAN tune a bit below the drivers Fs, but this is where SQ starts to suffer I imagine
-> In a perfect world, we would operate a driver above it's Fs at all times
Sorry... I have built dozens of sealed enclosures over the years and have recently decided to try designing some ported enclosures more for practice than anything. I have figured out WinISD a bit but am learning the Do's and Dont's as far as driver selection, what the sane limits for tuning SHOULD be, etc.
I often hear of people with rather high-end subs (Fs of 18-24hz) tuning the boxes at 14-16hz. I have always assumed you essentially tune for whatever you want to squeeze out of the driver and go on with it.
-Dean
You're welcome!
Well, 'ideal' in that it yields the widest (max) flat vented alignment, but it's often anything but 'ideal' for a given app.
For instance, a max flat sub in a corner can be a ~ 'one note' boombox, but re-tune it (much) lower till it's flat in room.........whole different 'story', so in a round about way this is 'showing' us that an under-damped alignment (high Qt) tuned to its mathematical max flat lower tuning may/can be the best performer in this case! 😉 😎
The more (most) correct way to present this is that T/S theory only loads to the driver's upper mass corner (Fhm), so note that as Qts' increases, Fhm decreases, ergo the narrower the box loading BW gets for a given Fs.
upper: Fhm = 2*Fs/Qts'
lower: Flc = Fs*Qts'/2 (AFAIK only useful for reactance annulled BLHs)
Qts': 2*Fs/Fhm
(Qts'): (Qts) + any added series resistance (Rs)
All that said, you're right in that we do what we have to do to get the desired in room response and all the theory/math just quickly gives us starting points to fine tune either in advanced simming programs and/or by in room measurements.
Well, 'ideal' in that it yields the widest (max) flat vented alignment, but it's often anything but 'ideal' for a given app.
For instance, a max flat sub in a corner can be a ~ 'one note' boombox, but re-tune it (much) lower till it's flat in room.........whole different 'story', so in a round about way this is 'showing' us that an under-damped alignment (high Qt) tuned to its mathematical max flat lower tuning may/can be the best performer in this case! 😉 😎
The more (most) correct way to present this is that T/S theory only loads to the driver's upper mass corner (Fhm), so note that as Qts' increases, Fhm decreases, ergo the narrower the box loading BW gets for a given Fs.
upper: Fhm = 2*Fs/Qts'
lower: Flc = Fs*Qts'/2 (AFAIK only useful for reactance annulled BLHs)
Qts': 2*Fs/Fhm
(Qts'): (Qts) + any added series resistance (Rs)
All that said, you're right in that we do what we have to do to get the desired in room response and all the theory/math just quickly gives us starting points to fine tune either in advanced simming programs and/or by in room measurements.
I don't have any maths for you, only the following observations.
(1) Trying to get 30hz from a 6.5" driver is possible but the juice ain't worth the squeeze. @30Hz you're probably 10db down requiring 10x power to return to flat.
(2) Identical drivers sharing the same air-space smooths possible differences in actual specification.
(3) Mono is preferable for sub drivers in the same cabinet. The configuration offers more headroom. e.g. In a live recording with a bass instrument on one side of the stage, one driver may be maxed out when the other is receiving a substantially lower signal.
(1) Trying to get 30hz from a 6.5" driver is possible but the juice ain't worth the squeeze. @30Hz you're probably 10db down requiring 10x power to return to flat.
(2) Identical drivers sharing the same air-space smooths possible differences in actual specification.
(3) Mono is preferable for sub drivers in the same cabinet. The configuration offers more headroom. e.g. In a live recording with a bass instrument on one side of the stage, one driver may be maxed out when the other is receiving a substantially lower signal.