Hi...am I being stupid!
I have been designing a simple cardioid bass unit with two channels, and a forward and rear facing driver. Reverse polarity to the rear.
They have a known separation distance and driven independently.
It would appear from simulation software that a delay of around 1.3ms creates the cancellation required to produce a cardioid pattern and this could be implemented simply in dsp of course.
But implementing a line level bessel filter (at 200Hz/2nd order in this case) provides the correct group delay as well. In fact it remains constant (as bessel does) over the bandwidth of interest and the reducing gd as frequency rises provides a smoother transition to forward directivity and polar plot if the simulation programmes (both agree) are to be believed.
So, please forums, am I missing something here because it seems too simple an analogue solution that it hasn't been done before??
I have been designing a simple cardioid bass unit with two channels, and a forward and rear facing driver. Reverse polarity to the rear.
They have a known separation distance and driven independently.
It would appear from simulation software that a delay of around 1.3ms creates the cancellation required to produce a cardioid pattern and this could be implemented simply in dsp of course.
But implementing a line level bessel filter (at 200Hz/2nd order in this case) provides the correct group delay as well. In fact it remains constant (as bessel does) over the bandwidth of interest and the reducing gd as frequency rises provides a smoother transition to forward directivity and polar plot if the simulation programmes (both agree) are to be believed.
So, please forums, am I missing something here because it seems too simple an analogue solution that it hasn't been done before??
I have some threads on doing passive cardioids, check my posts
And the "Dutch & Dutch" passive cardioids began on diyaudio. They're a commercial product now, but their beginnings are documented here.
And Kimo published some passive cardioid designs too.
Keep us posted on how yours turns out!
And the "Dutch & Dutch" passive cardioids began on diyaudio. They're a commercial product now, but their beginnings are documented here.
And Kimo published some passive cardioid designs too.
Keep us posted on how yours turns out!
Correct, until you start fiddling with crossover filters which introduce their own phase shifts and relative level differences.
Link to Kimmosto site PB mentioned: Cardioid bass
Few other nuggets I've found: auraaudio.fi - Technical insight #2 and Passive Cardioid Technology™ | Fulcrum Acoustic
The Fulcrum cabinet seems cool, uses only on driver. There is link to a patent in the article with hints how it works.
Also this comment by Gunnes here Cardioid using aperiodic ports (a la Fulcrum Acoustics) | Sound Forums
"
The difference between an "aperiodic port" design and our passive cardioids is the acoustical mass of the ports and the amount of resistance. The acoustical mass in our passive cardioids puts a second-order low-pass filter a little below the frequency below which we want cardioid behavior. The resistance is just enough to damp the helmholtz resonance to "slightly underdamped" (Q of about 1). An aperiodic port design has almost no acoustical mass and is dominated by resistance, so there is essentially no helmholtz resonance and the resulting first-order low-pass corner is well above the passband; so it operates simply as a resistive leak that damps the sealed box resonance. If you look at the impedance of our passive cardioid subs, you'll see a trace of the double peaks of a ported box, but the dip between them doesn't go nearly as low. That's the effect of the port resistance.There is still significant reduction of excursion around the helmholtz frequency, and the summation of the rear radiation and front radiation increases the output over about 1 1/2 octaves. If you try to stretch the cardioid behavior beyond 1 1/2 octave, there will be a penalty in low frequency extension and maximum output.
Adding "conventional" ports to the front of the cabinet would essentially short out the rear ports, which would destroy any cardioid behavior.
"
Few other nuggets I've found: auraaudio.fi - Technical insight #2 and Passive Cardioid Technology™ | Fulcrum Acoustic
The Fulcrum cabinet seems cool, uses only on driver. There is link to a patent in the article with hints how it works.
Also this comment by Gunnes here Cardioid using aperiodic ports (a la Fulcrum Acoustics) | Sound Forums
"
The difference between an "aperiodic port" design and our passive cardioids is the acoustical mass of the ports and the amount of resistance. The acoustical mass in our passive cardioids puts a second-order low-pass filter a little below the frequency below which we want cardioid behavior. The resistance is just enough to damp the helmholtz resonance to "slightly underdamped" (Q of about 1). An aperiodic port design has almost no acoustical mass and is dominated by resistance, so there is essentially no helmholtz resonance and the resulting first-order low-pass corner is well above the passband; so it operates simply as a resistive leak that damps the sealed box resonance. If you look at the impedance of our passive cardioid subs, you'll see a trace of the double peaks of a ported box, but the dip between them doesn't go nearly as low. That's the effect of the port resistance.There is still significant reduction of excursion around the helmholtz frequency, and the summation of the rear radiation and front radiation increases the output over about 1 1/2 octaves. If you try to stretch the cardioid behavior beyond 1 1/2 octave, there will be a penalty in low frequency extension and maximum output.
Adding "conventional" ports to the front of the cabinet would essentially short out the rear ports, which would destroy any cardioid behavior.
"
I think this solution is not used that much (especially commercially), because the cost difference between an analog filter and a DSP is small, while the DSP allows for better tuning and even a variable dispersion pattern. DSPs are cheap if not already present in the system, so there is no reason not to use them if you already have paid for a second woofer and amplifier channel.
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Thanks for these comprehensive replies. Very grateful.
I am aware of both Kimmi's work and the Fulcrum products and methods.
Thank you windforce for the reassurance! I should have mentioned that the bessel filter to provide the group delay is in addition to the front/rear drivers having the same active crossover applied. So one channel has the 2nd order bessel and both channels have 24 dB/octave LP (active) at the same frequency to ensure the only "difference" is the bessel gd.
All the filters are line level (active or passive) as attempting to provide the delay as a LC network on the relatively complex load the loudspeaker presents produces inconsistent delay, certainly around the system resonance (even when damped with and rcl).
With this reassurance, I will advance the project beyond planning stage and keep all updated on this forum, cheers!
I am aware of both Kimmi's work and the Fulcrum products and methods.
Thank you windforce for the reassurance! I should have mentioned that the bessel filter to provide the group delay is in addition to the front/rear drivers having the same active crossover applied. So one channel has the 2nd order bessel and both channels have 24 dB/octave LP (active) at the same frequency to ensure the only "difference" is the bessel gd.
All the filters are line level (active or passive) as attempting to provide the delay as a LC network on the relatively complex load the loudspeaker presents produces inconsistent delay, certainly around the system resonance (even when damped with and rcl).
With this reassurance, I will advance the project beyond planning stage and keep all updated on this forum, cheers!
I never understood the idea of cardiod subs......
If you use a big horn stack where the combined mouth is large enough you will also get directivity....
And you will not lose power etc.....
Also I have never experienced a cardiod sub system that sounded really good.
Most of the time it lacks punch.
That been said if you do go the route of adding more amplifier channels for cardiod subs then the cost of DSP is nothing.... most touring amps start to have there own DSP build in.....
If you use a big horn stack where the combined mouth is large enough you will also get directivity....
And you will not lose power etc.....
Also I have never experienced a cardiod sub system that sounded really good.
Most of the time it lacks punch.
That been said if you do go the route of adding more amplifier channels for cardiod subs then the cost of DSP is nothing.... most touring amps start to have there own DSP build in.....
Thank you TBTL. You are of course right re:dsp of which I am a fan.
But it has it's own problems. On the rear processed channel you would just dial in delay but the decreasing gd of the bessel method actually enables one to use the bass unit up to a higher frequency before the polar response becomes unusable.
If you use dsp just to process the cardioid you will have a latency issue..although small, still relevant. If you derive your output channels to the "satellites" you have the dsp in circuit so the quality of the unit becomes an issue. I happen to be quite fond of the behringer unit but many would see this as a weak point. Inevitably then some may choose a much more expensive unit. Something I am trying to avoid.
And there are those who love analogue!!
My concept is as simple as possible, not just because of the negligible cost, but to implement the fewest components required.
Of course the system in cardioid mode will require eq and again this gives the user the opportunity to go either valve or solid state route. The valve route may appeal to many and with the Klark Teknik (behringer!!) units at a fraction of the price of the original Putec they emulate, might be an appealing choice?
But it has it's own problems. On the rear processed channel you would just dial in delay but the decreasing gd of the bessel method actually enables one to use the bass unit up to a higher frequency before the polar response becomes unusable.
If you use dsp just to process the cardioid you will have a latency issue..although small, still relevant. If you derive your output channels to the "satellites" you have the dsp in circuit so the quality of the unit becomes an issue. I happen to be quite fond of the behringer unit but many would see this as a weak point. Inevitably then some may choose a much more expensive unit. Something I am trying to avoid.
And there are those who love analogue!!
My concept is as simple as possible, not just because of the negligible cost, but to implement the fewest components required.
Of course the system in cardioid mode will require eq and again this gives the user the opportunity to go either valve or solid state route. The valve route may appeal to many and with the Klark Teknik (behringer!!) units at a fraction of the price of the original Putec they emulate, might be an appealing choice?
Thank you Vintage Audio. You are right about horns but they do need to be huge! Some believe the reverse polarity method for cardioid can produce audible effects due to the "start points" of each wave form.
But my design is intended more for domestic and studio than stage.
Additionally, just by changing the cable to the power amp driving the rear driver you can have monopole, dipole, cardioid or hyper-cardioid.
Not giving any more away..!!!
.
.
But my design is intended more for domestic and studio than stage.
Additionally, just by changing the cable to the power amp driving the rear driver you can have monopole, dipole, cardioid or hyper-cardioid.
Not giving any more away..!!!
.
.
Sorry, one more link to a text by mr. Gunnes why passive cardioid is good. Just for reference to future readers me included 🙂 https://www.fulcrum-acoustic.com/as...e Passive Cardioid Loudspeaker Technology.pdf
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The Fulcrum is a single driver cardioid using traditional methods to create rear cancellation...
If I understand correctly, what you want to do is to apply passive all-pass onto one of the two digitally LP-filtered drivers. This sounds reasonable. This is very rarely used from weird unknown reason? Directional response via phase manipulation between two spatially displaced sources can be obtained using two different methods - substractive and additive. It leads to different time, efficiency and frequency-related behaviour. I wrote about it somewhere here some time ago but it was left without response, too. It seems you are smart enough to practice it on your own and show us your findings. I keep thumbs up for you.
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Indeed you do Windforce! I would probably stay in the analogue domain though with the active crossover just to keep the whole chain analogue...
Sustaining well controlled directivity from passive networks will be struggling becouse of real parts parameters tolerances and variation under broad range of operational and environmental conditions. DSP is very reasonable and cheap way to experiment and in the end, to do it right in stable and efficient manner.
These are all at line level. I would not contemplate trying to achieve this without taking the loudspeaker load out of the equation...if that's what you meant?!
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