Is there any rule of thumb I might apply to understand what driver/box combination might most benefit from using EQ to boost the low end, as taught by Keele, Thiele/Small, etc.?
I have a small subwoofer design I've prototyped on which I worked pretty hard to get its passive, unequalized performance where I wanted it. Cone excursion and passive radiator excursion were close to Xmax at full power, etc. Using WinISD, I was able to apply a peaking 2nd order highpass filter (3 dB) and extend F3 by about 1/3 octave (63 Hz -> 52 Hz). Max cone excursion on both the driver & passive radiator was no worse, although the latter moved up in frequency towards F3, which seems like it indicates I have less margin. (No free lunch.)
What I'm struggling to understand is under what "normal" design considerations would greater benefit be had? My gut tells me it's a way to increase bass output on designs that have some excursion margin at full power, but am I missing anything?
I have a small subwoofer design I've prototyped on which I worked pretty hard to get its passive, unequalized performance where I wanted it. Cone excursion and passive radiator excursion were close to Xmax at full power, etc. Using WinISD, I was able to apply a peaking 2nd order highpass filter (3 dB) and extend F3 by about 1/3 octave (63 Hz -> 52 Hz). Max cone excursion on both the driver & passive radiator was no worse, although the latter moved up in frequency towards F3, which seems like it indicates I have less margin. (No free lunch.)
What I'm struggling to understand is under what "normal" design considerations would greater benefit be had? My gut tells me it's a way to increase bass output on designs that have some excursion margin at full power, but am I missing anything?
Sure, but if you read Keele he does indicate that such extension can come at almost no excursion penalty. (My skepticism and anecdotal evidence led me to my "no free lunch" statement.)
I guess this is what I'm really getting at: is there is some reference or guidance out there which might help one identify drivers that are more optimally suited for low frequency EQ, perhaps beyond their optimal "un-equalized" alternatives? In my case, I already saw that the 6 dB peaking filter was too much for my application based on simple excursion limits.
I guess this is what I'm really getting at: is there is some reference or guidance out there which might help one identify drivers that are more optimally suited for low frequency EQ, perhaps beyond their optimal "un-equalized" alternatives? In my case, I already saw that the 6 dB peaking filter was too much for my application based on simple excursion limits.
I had luck.
Djk referred to it as 6th order assisted.
Works well for low qts drivers.
Small box, high efficiency, super high output.
Basically you take a qts driver (say qts .32), port it at fs (that will be box f9), then use a high pass filter with a q of 2 at tuning. That way you are applying eq at tuning (where driver isn't moving hardly at all), that gives you 6db of boost there and a subsonic filter all at the same time. Now that sub seems to go way louder.
But, it does eat up some power.
If you tune to 27hz, max excursion frequency is near 40hz.
Djk referred to it as 6th order assisted.
Works well for low qts drivers.
Small box, high efficiency, super high output.
Basically you take a qts driver (say qts .32), port it at fs (that will be box f9), then use a high pass filter with a q of 2 at tuning. That way you are applying eq at tuning (where driver isn't moving hardly at all), that gives you 6db of boost there and a subsonic filter all at the same time. Now that sub seems to go way louder.
But, it does eat up some power.
If you tune to 27hz, max excursion frequency is near 40hz.
FWIW, Altec's 'take' on Keele's 6th order assisted: http://alteclansingunofficial.nlenet.net/publications/techletters/TL_264A.pdf
GM
GM
DJK (RIP) notes:
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Thanks (0) Thanks(0) Quote _djk_ Quote Post ReplyReply Direct Link To This Post Posted: 01 August 2010 at 1:09am
The classic B6 assisted alignment is best with a Qts=0.312
This requires a high=pass filter of a Q=2 at Fb, and the Fb=Fs.
Because the moderate boost is at Fb, cone excursion doesn't go up a lot, and distortion is low.
The quick calculator fudge (Qts=0.312) is Vas*0.4 = box size.
For different Qts values, use (Qts^2)*Vas*4.5 = box size. Fb should be (Fs/Qts)*0.312
Trick:
If the Qts is a bit low, just pretend it is a bit higher (no more than 20% or so). This works because you are pretending the motor is weaker than it actually is (obviously doesn't work the other way if the Qts is too high). Also, when driving hard the Bl goes down and the cabinet is now correctly tuned!
The quick B4 calculator fudge (Qts=0.4) is Vas*0.8 = box size, Fb=Fs.
http://www.xlrtechs.com/dbkeele.com/PDF/Keele (1975-07 AES Published) - New Set of VB Alignments.pdf
A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).
http://www.xlrtechs.com/dbkeele.com...ES Preprint) - Direct LF Driver Synthesis.pdf
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Thanks (0) Thanks(0) Quote _djk_ Quote Post ReplyReply Direct Link To This Post Posted: 01 August 2010 at 1:09am
The classic B6 assisted alignment is best with a Qts=0.312
This requires a high=pass filter of a Q=2 at Fb, and the Fb=Fs.
Because the moderate boost is at Fb, cone excursion doesn't go up a lot, and distortion is low.
The quick calculator fudge (Qts=0.312) is Vas*0.4 = box size.
For different Qts values, use (Qts^2)*Vas*4.5 = box size. Fb should be (Fs/Qts)*0.312
Trick:
If the Qts is a bit low, just pretend it is a bit higher (no more than 20% or so). This works because you are pretending the motor is weaker than it actually is (obviously doesn't work the other way if the Qts is too high). Also, when driving hard the Bl goes down and the cabinet is now correctly tuned!
The quick B4 calculator fudge (Qts=0.4) is Vas*0.8 = box size, Fb=Fs.
http://www.xlrtechs.com/dbkeele.com/PDF/Keele (1975-07 AES Published) - New Set of VB Alignments.pdf
A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).
http://www.xlrtechs.com/dbkeele.com...ES Preprint) - Direct LF Driver Synthesis.pdf
So when I go too small and too deep on box size, my vents end up ridiculously long. Any good references for taking vent parameters and converting to passive radiator parameters? Simply going with 2X Sd (or more correctly 2X Vd) as a ratio of passive-to-active driver displacement doesn't seem to be the magic bullet, as I can easily overdrive the passive radiators. There has to be more to this.
I've seen that article several times -- to say it's less than clear would be putting it mildly. I think I finally figured out one of the discrepancies. It jumps to a "12 inch" passive radiator with an "effective radius" of 5 inches, which makes no sense whatsoever.
BUT, it does appear to be saying that one should pick a PR somewhat larger than the driver, model a port of that same size, tune the box appropriately with that port by dialing in the right length, and calculate required PR mass based on the air volume of the port. Iterate as necessary. Is that the gist?
BUT, it does appear to be saying that one should pick a PR somewhat larger than the driver, model a port of that same size, tune the box appropriately with that port by dialing in the right length, and calculate required PR mass based on the air volume of the port. Iterate as necessary. Is that the gist?
Effective radius of 5" = effective diameter of 10".
Once you take into consideration the size of the surround and mounting flange for a 12" PR, the effective diameter can end up being closer to 10".
1.5-2x the area, but 2x is easier to quickly figure
, so either two of the same size as the driver or a single with a diameter at least 1.4142x the driver's, which with the same Xmax its Vd [Sd*Xmax] = 2x [+3 dB] the driver's and since we want at least +6 dB = 2x driver's Xmax.Of course you can use more smaller ones to get the same ratios or fewer much larger ones with less Xmax for those who want sheer size to impress.
GM
OK, thanks. So if I select a PR that has ~ 2.3X the Vd of my driver (43% higher Sd, 60% higher Xmax), model a port of that diameter and calculate an air mass, then when I simulate that PR system the excursion of the PR is excessive well below the driver's steady power limit. I can double that number of PR and excursion drops JUST below Xmax. That's not really where I wanted to be in terms of device count, but it is what it is. What would be the recommended direction I head?
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You're welcome!
Hmm, just been responding to easy Qs, but reading the thread combined with what's in the email update Vs your revision I'm a bit confused in that DJK's tutorial seems to answer your concerns as I understand them, though don't have a lot of faith in how accurate WinISD is re 6th order assisted PR apps, so for the moment all I can contribute is that the pioneers determined that a Vb = measured Vas/1.44 combined with actual Fs tuning yields the overall highest vented box efficiency and that the upper size limit for vents in Helmholtz resonant alignments [i.e. air mass has ~uniform particle density] is Sd = Av. With today's stiffly sprung [low Vas spec] drivers the response is typically a peaking one in its mid band, but better overall to shelve than boost.
In calculating the PRs and assuming a Q = 2 6th order = +6 dB alignment, then the PR system must be sized to handle this secondary load which is the equivalent of dropping tuning a full octave ergo a 4x increase in PR specs is required [+12 dB total] just as WinISD predicts.
GM
Hmm, just been responding to easy Qs, but reading the thread combined with what's in the email update Vs your revision I'm a bit confused in that DJK's tutorial seems to answer your concerns as I understand them, though don't have a lot of faith in how accurate WinISD is re 6th order assisted PR apps, so for the moment all I can contribute is that the pioneers determined that a Vb = measured Vas/1.44 combined with actual Fs tuning yields the overall highest vented box efficiency and that the upper size limit for vents in Helmholtz resonant alignments [i.e. air mass has ~uniform particle density] is Sd = Av. With today's stiffly sprung [low Vas spec] drivers the response is typically a peaking one in its mid band, but better overall to shelve than boost.
In calculating the PRs and assuming a Q = 2 6th order = +6 dB alignment, then the PR system must be sized to handle this secondary load which is the equivalent of dropping tuning a full octave ergo a 4x increase in PR specs is required [+12 dB total] just as WinISD predicts.
GM
Sorry about the disconnect between thread and email updates -- I edited my response.
I think what I'm finding is that any given PR might not have the Xmax to handle the duty, regardless of whether or not it can be tuned to the right frequency. If I ensure that PR Vd is 4X driver Vd I assume that will get me close. For the record, I am trying to "convert" a bass-assisted vented design (+ 6 dB 2nd order peaking EQ) to the same with a PR. The thing that gets me is that all looks "good" with the vented design, with the exception of the length of the vent itself, but when I try to convert to a PR variant both the PR and driver excursions can go wonky. I'll keep playing with it but it's my nature to want to understand the physics behind the simulation.
I think what I'm finding is that any given PR might not have the Xmax to handle the duty, regardless of whether or not it can be tuned to the right frequency. If I ensure that PR Vd is 4X driver Vd I assume that will get me close. For the record, I am trying to "convert" a bass-assisted vented design (+ 6 dB 2nd order peaking EQ) to the same with a PR. The thing that gets me is that all looks "good" with the vented design, with the exception of the length of the vent itself, but when I try to convert to a PR variant both the PR and driver excursions can go wonky. I'll keep playing with it but it's my nature to want to understand the physics behind the simulation.
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