I've stated in several posts I like overdamped vented alignments
so I thought I try and illustrate why with a typical Vifa 165mm driver.
One version of overdamped is detuning the port by root 2.
Classic vs. overdamped.
C is flat to 60hz, O droops by 3db at 60hz.
C is -3dB at 44Hz, O is -3dB at 60Hz, nearly half an octave.
They are both -6dB at 38Hz.
Of particular interest is the dB loss between 30 and 40Hz,
for the classic it is 8dB, whilst for overdamped just over 3dB.
this makes a big difference in IMO.
The white line on the graph shows a 6dB/octave slope.
What should be obvious is that the classic alignment is rolling
off at 12dB/octave at F3 and will sound like it, i.e. the bass
cutoff will appear to be around 44Hz.
The overdamped alignment does not begin to approach 12/dB
per octave until 26Hz, the apparent bass cut-off being much
less linked to a particular frequency, I'd say just under 30Hz.
Transient response of the O vs. C is by definition much better.
When you factor in room gain and placement IMO a clear
winner emerges, the overdamped alignment is much more
flexible, the classic alignment must be mounted in free space.
sreten.
edit : picture deleted
so I thought I try and illustrate why with a typical Vifa 165mm driver.
One version of overdamped is detuning the port by root 2.
Classic vs. overdamped.
C is flat to 60hz, O droops by 3db at 60hz.
C is -3dB at 44Hz, O is -3dB at 60Hz, nearly half an octave.
They are both -6dB at 38Hz.
Of particular interest is the dB loss between 30 and 40Hz,
for the classic it is 8dB, whilst for overdamped just over 3dB.
this makes a big difference in IMO.
The white line on the graph shows a 6dB/octave slope.
What should be obvious is that the classic alignment is rolling
off at 12dB/octave at F3 and will sound like it, i.e. the bass
cutoff will appear to be around 44Hz.
The overdamped alignment does not begin to approach 12/dB
per octave until 26Hz, the apparent bass cut-off being much
less linked to a particular frequency, I'd say just under 30Hz.
Transient response of the O vs. C is by definition much better.
When you factor in room gain and placement IMO a clear
winner emerges, the overdamped alignment is much more
flexible, the classic alignment must be mounted in free space.
sreten.edit : picture deleted
Correction : paragraph 7 should read :
What should be obvious is that the classic alignment is rolling
off at at near 24 dB/octave at F3 and will sound like it, i.e.
the bass cutoff will appear to be around 44Hz.
Any room gain below 44Hz cannot be gainfully used.
Correction : paragraph 8 should read :
The overdamped alignment does not begin to approach 24/dB
per octave until 26Hz, and has a whole region near 6dB/octave
the apparent bass cut-off being much less linked to a particular
frequency, I'd say just under 30Hz.
This allows room gain to be used effectively.
F3 is 30Hz with 6dB of room gain at 30Hz, F6 becomes ~ 26Hz.
sreten.
What should be obvious is that the classic alignment is rolling
off at at near 24 dB/octave at F3 and will sound like it, i.e.
the bass cutoff will appear to be around 44Hz.
Any room gain below 44Hz cannot be gainfully used.
Correction : paragraph 8 should read :
The overdamped alignment does not begin to approach 24/dB
per octave until 26Hz, and has a whole region near 6dB/octave
the apparent bass cut-off being much less linked to a particular
frequency, I'd say just under 30Hz.
This allows room gain to be used effectively.
F3 is 30Hz with 6dB of room gain at 30Hz, F6 becomes ~ 26Hz.
sreten.
Seconded.
If my TAD 1601a's were tuned for flat FR (in 125l box), it would be 3db @ 42Hz and roll off at 24db/oct below that. This alignment has an fb of ~40Hz. I've lowered the fb to ~30Hz, which is how I run it, and the f3 raises to >47Hz, but the f6 goes from 38Hz to 32Hz. I also have a boost circuit that can be used depending on the room gain/source material/mood. The boost will bring the total FR back to flat with f3 = ~28Hz and a brick wall roll off below that.
If my TAD 1601a's were tuned for flat FR (in 125l box), it would be 3db @ 42Hz and roll off at 24db/oct below that. This alignment has an fb of ~40Hz. I've lowered the fb to ~30Hz, which is how I run it, and the f3 raises to >47Hz, but the f6 goes from 38Hz to 32Hz. I also have a boost circuit that can be used depending on the room gain/source material/mood. The boost will bring the total FR back to flat with f3 = ~28Hz and a brick wall roll off below that.
I do something similar with 3 paralleled 2226J's in my basement blasters, where the box is tuned to 26 hz but I have a series C, shunt L circuit (1320 uF from 4 330uF Solen polypropylenes in parallel & 30 mH) in front of them that resonates at around the same frequency providing 4 db boost between 30 hz and 40 hz passively with the boost tapering off until about 70 hz, plus giving a little boost below 26 hz by partly 'transforming' the bass reflex impedance peaks into amplitude gain. An advantage of this approach as compared to active boost, IMO, is that a voltage amplitude at the speaker terminals in the boost range that considerably exceeds that of the amplifier output capability is achievable without clipping, plus at least the upper load impedance magnitude peak is minimized. Cone excursion at a constant input voltage is maximum at around 40 hz with this setup that also blocks most energy below about 20 hz.
Glad to see you point this out Sreten. As I have been timid about purchasing some subs for the rear deck/hatshelf of my daily driver, that are not properly damped for a 11 cu ft. trunk. And trying to hypothesize the cabin-gain effects.
The driver in question is the Tangband W69-1042.
FS: 35hz
Vas: 1.13 cu. ft.
QTS: 0.33
I dont have a way to post the graph, so I'll have to explain. If someone wants to post it for me, send me an e-mail.
Using 2 drivers:
In a classic vented alignment the graph is -3db @ 35hz, -8db @ 29hz, and -12db @25hz.
In an 11 cu ft. trunk the graph is -3db @ 90hz, -8db@42hz, and -12db @ 25hz.
Basically, the drivers are rolling off at 6db per octave in the trunk alignment.
If I remember right, Dickason noted up to 20db at 20hz for cabin gain in one instance. I wont be going that low with these drivers and I'll be using an 24db subsonic at 25hz.
They'll be placed in the hatshelf, and firing straight up into the rear glass - FYI for gain-effect considerations.
I also have active bass equalization of up to +9db - if needed - post cabing gain. I also plan on using my LPF at 80hz.
One last note - Since these are paper cones, I'll also be looking to coat them for protection. The windows are tinted, but they will see some UV-rays, and get all of the temp exposure. This will likely raise the QTS slightly. (a hundreth or two?)
So - any comments/suggestions on how these drivers may work in this application?
Thanks.
The driver in question is the Tangband W69-1042.
FS: 35hz
Vas: 1.13 cu. ft.
QTS: 0.33
I dont have a way to post the graph, so I'll have to explain. If someone wants to post it for me, send me an e-mail.
Using 2 drivers:
In a classic vented alignment the graph is -3db @ 35hz, -8db @ 29hz, and -12db @25hz.
In an 11 cu ft. trunk the graph is -3db @ 90hz, -8db@42hz, and -12db @ 25hz.
Basically, the drivers are rolling off at 6db per octave in the trunk alignment.
If I remember right, Dickason noted up to 20db at 20hz for cabin gain in one instance. I wont be going that low with these drivers and I'll be using an 24db subsonic at 25hz.
They'll be placed in the hatshelf, and firing straight up into the rear glass - FYI for gain-effect considerations.
I also have active bass equalization of up to +9db - if needed - post cabing gain. I also plan on using my LPF at 80hz.
One last note - Since these are paper cones, I'll also be looking to coat them for protection. The windows are tinted, but they will see some UV-rays, and get all of the temp exposure. This will likely raise the QTS slightly. (a hundreth or two?)
So - any comments/suggestions on how these drivers may work in this application?
Thanks.
"It sounds as if you are using resonance to amplify the bass. If this is the case I would say - in the nicest possible way - that this is the opposite of the original idea.
or did I miss something ?"
Compared to the standard 40hz BR tuning JBL recommends for the 2226, I get a comparable overall frequency response with as good or better group delay (transient response) down to 40 hz and a much slower initial rolloff below that frequency than with the standard tuning. Also, the characteristic bass reflex port transient anomalies are pushed down to about 25hz where they are less objectionable to me. And of course, there's the useable response down to about 20 hz. Down to 40 hz or so, the sound resembles that of a closed box more than a ported system to me, and even though it starts 'slowing down' due to the overall sixth order characteristic below that frequency, the advantages of not needing an external box eq with its active circuitry which would add a second order resonance anyway, along with the enhanced maximum output from 30-70hz appear to be advantages for my application.
or did I miss something ?"
Compared to the standard 40hz BR tuning JBL recommends for the 2226, I get a comparable overall frequency response with as good or better group delay (transient response) down to 40 hz and a much slower initial rolloff below that frequency than with the standard tuning. Also, the characteristic bass reflex port transient anomalies are pushed down to about 25hz where they are less objectionable to me. And of course, there's the useable response down to about 20 hz. Down to 40 hz or so, the sound resembles that of a closed box more than a ported system to me, and even though it starts 'slowing down' due to the overall sixth order characteristic below that frequency, the advantages of not needing an external box eq with its active circuitry which would add a second order resonance anyway, along with the enhanced maximum output from 30-70hz appear to be advantages for my application.
So, the red/blue lines are similiar in rolloff shape to the closed box rolloff (white) shape.
Our real advantage in this example is a much better transient response, extended bass response (as per closed boxes) but in a much smaller fb--one-sixth, in fact. All we have to rely on is room gain to bring our spl back up to where the classic reflex box would have been. Correct?
Mos
Our real advantage in this example is a much better transient response, extended bass response (as per closed boxes) but in a much smaller fb--one-sixth, in fact. All we have to rely on is room gain to bring our spl back up to where the classic reflex box would have been. Correct?
Mos
mikelm said:
Not really, its taking it one step further. Overdamping achieves
good extension and in most rooms will give good bass, but in
some cases the bass becomes too lean.
The solution is to go to sixth order alignments. Basically add
a high pass filter q=>1 to <3 tuned to the port frequency.
An active filter with adjustable Q is best, allowing the bass
to be tuned to the room, quite stunning extension can be
acheived this way.
But note the box reflex alignment must have good damping,
i.e. be thoroughly overdamped for this to work, otherwise
bass will just be fat and slow.
sreten.
Btw, I should mention that the component values I specified with the passive approach I described will essentially cause no boost at the BR resonant frequency itself although it does at higher and to some extent at slightly lower frequencies (which is one reason I set the port tuning so low in the first place) - if I chose the LC values to give boost at this frequency which is also an impedance minimum, the speaker impedance would quickly drop from its nominal 4-6 ohm range at this frequency which I don't want for this application (not to mention which the series capacitance value would get significantly larger - not a negligible cost factor when I'm using a number of large value polypropylenes in parallel to achieve that value).
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