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I've been thinking about Onkens/Fonkens and mulling around some different approaches but this idea would take the thread I've been using for that discussion in a new direction so I thought I'd be brave and start afresh.
The question is, can this design be realized using multiple-pipes instead of the traditional approach ?
To some it might look like a BR design gone mad, but I've not yet seen a BR design with so many ports or with ports specifically designed to be long enough so that they are 'resistive'.
Inspired of course by the Fonken of Planet10 fame here's my proposal ... (and no doubt your imagination like mine can see the variations possible)
The question is, can this design be realized using multiple-pipes instead of the traditional approach ?
To some it might look like a BR design gone mad, but I've not yet seen a BR design with so many ports or with ports specifically designed to be long enough so that they are 'resistive'.
Inspired of course by the Fonken of Planet10 fame here's my proposal ... (and no doubt your imagination like mine can see the variations possible)
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there's a cheerful design ! - perhaps offers more symmetry ?
but it can't tell if these ports are 'resistive'. My limited understanding of the Onken is that it has a much larger total area of vents than would be suggested by a traditional BR design. The vents or ports are designed to have a relatively high resistance (i.e. they are long) and then something about 'aperiodic' - I can see I have to do some more reading.
Or am I to learn that the Onken is just a BR by another name ?
but it can't tell if these ports are 'resistive'. My limited understanding of the Onken is that it has a much larger total area of vents than would be suggested by a traditional BR design. The vents or ports are designed to have a relatively high resistance (i.e. they are long) and then something about 'aperiodic' - I can see I have to do some more reading.
Or am I to learn that the Onken is just a BR by another name ?
An Onken is indeed a BR variation.
What you were looking at there was the original Bass Reflex enclosure drawing from Thuras's 1932 patent where the total vent CSA = the driver's Sd for maximum efficiency at Fb (rarely employed these days). The Onken has a similar take on distributed vents (albeit moved out to the sides), but often employs a somewhat different form of tuning. The aspect ratio of the vents makes them more resistive to airflow than, say, a single circular vent of the same CSA; the length of this will depend on the desired tuning of course. An Onken variation is often tuned somewhat lower than a more regular BR alignment (like QB3 etc) to provide, in conjunction with the greater resistance of the vents, a better damped LF.
What you were looking at there was the original Bass Reflex enclosure drawing from Thuras's 1932 patent where the total vent CSA = the driver's Sd for maximum efficiency at Fb (rarely employed these days). The Onken has a similar take on distributed vents (albeit moved out to the sides), but often employs a somewhat different form of tuning. The aspect ratio of the vents makes them more resistive to airflow than, say, a single circular vent of the same CSA; the length of this will depend on the desired tuning of course. An Onken variation is often tuned somewhat lower than a more regular BR alignment (like QB3 etc) to provide, in conjunction with the greater resistance of the vents, a better damped LF.
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The lights are starting to go on !
So let me see if I understand from a few hours of web searching.
The BR approach creates a Helmholtz resonator out of the enclosure by adding a port.
You 'tune' the enclosure to a low base frequency to artificially boost the LF output of the overall system. For a particular enclosure volume, driver performance etc. and assuming you pick a suitable resonant frequency, you know what port cross-section area (A) and port length (L) to use. It's best keep the port somewhat longer than it's diameter so that all the approximations in the math are reasonable.
Now, the resistance of the port is a measure of how easily air flows through it and for Laminar flow it is related to L/(A^2). We already know that A/L is determined by the resonant frequency, i.e. it's fixed, so the resistance of the port in our design depends only on 1/A. A smaller but longer port gives a higher resistance and vice versa.
For the Onken design, one uses long ports to minimize the total port area and thereby raise the resistance of the ports.
Why is a higher resistance better for sound ? - I can only assume that it lowers the Q of the resonator so the artificial boost to the LF response is not peaked at one frequency. It reduces the risk that the cone is unloaded below resonance which is known to contaminate the mid-range.
Am I getting this right ???
[interestingly there is a chap I found who claims a patent and from what I can see, it is based on forming a restriction in the diameter of the port at some convenient place and this thereby improves the sound - seems to me it's just a way of increasing the resistance of the port: http://www.wipo.int/pctdb/en/wo.jsp?IA=US1981000554&DISPLAY=DESC]
So let me see if I understand from a few hours of web searching.
The BR approach creates a Helmholtz resonator out of the enclosure by adding a port.
You 'tune' the enclosure to a low base frequency to artificially boost the LF output of the overall system. For a particular enclosure volume, driver performance etc. and assuming you pick a suitable resonant frequency, you know what port cross-section area (A) and port length (L) to use. It's best keep the port somewhat longer than it's diameter so that all the approximations in the math are reasonable.
Now, the resistance of the port is a measure of how easily air flows through it and for Laminar flow it is related to L/(A^2). We already know that A/L is determined by the resonant frequency, i.e. it's fixed, so the resistance of the port in our design depends only on 1/A. A smaller but longer port gives a higher resistance and vice versa.
For the Onken design, one uses long ports to minimize the total port area and thereby raise the resistance of the ports.
Why is a higher resistance better for sound ? - I can only assume that it lowers the Q of the resonator so the artificial boost to the LF response is not peaked at one frequency. It reduces the risk that the cone is unloaded below resonance which is known to contaminate the mid-range.
Am I getting this right ???
[interestingly there is a chap I found who claims a patent and from what I can see, it is based on forming a restriction in the diameter of the port at some convenient place and this thereby improves the sound - seems to me it's just a way of increasing the resistance of the port: http://www.wipo.int/pctdb/en/wo.jsp?IA=US1981000554&DISPLAY=DESC]
Maybe too late, since Dave has already chimed in, but:
Since the Fonken name was cited, if I may add tuppence to the discourse, - one of the main features of all variants of the Planet10 Fonken family of designs is the resistive nature of the narrow slotted ports.
A smaller member of this continually expanding family went though a couple of iterations - basically as an exercise to find how small the box volume could be shrunk while retaining as much of the sonic signature of the Prime as possible.
The first draft had a simple round port ( as Dave does all the math and working drawings, I of course forget the dimensions) and was literally a couple of hours start to finish for the rough prototype. As it showed promise, the next step was to rework for the slotted ports, and play with aesthetics / dimensions and bracing. Quite simply put, we found the narrow slot ports sounded better.
For reasons that Dave, Scott, or others could describe, the more resistive ports retain the enclosure tuning over a wider range of internal pressure levels (i.e. sound volumes) than a simple round port, particularly those of larger diameter.
A couple of years ago we built a minor variation (bracing and dimensions) of Fostex's factory recommended BR for the FE167E, with the round port. While it certainly demonstrated the extra sensitivity and more deeply extended low frequency response that you'd expect from this driver, it was outclassed in terms of articulation, tonality and textural detail from the lower mid-bass on down by the FE127 Fonken Prime.
edit: Dave -maybe you could address the highlight point - as you had discussed with clients on Sunday.
final (?) edit: Gareth - on closer review of your drawing, I'd be more concerned about the effect of what appears to be a large lateral brace behind the driver. Even with extensive holes, I think you'd find this to generate a lot of very early and high pressure reflections back to the driver. As we found out in an unrelated design, this can cause more problems than the extra cabinet bracing cures.
Since the Fonken name was cited, if I may add tuppence to the discourse, - one of the main features of all variants of the Planet10 Fonken family of designs is the resistive nature of the narrow slotted ports.
A smaller member of this continually expanding family went though a couple of iterations - basically as an exercise to find how small the box volume could be shrunk while retaining as much of the sonic signature of the Prime as possible.
The first draft had a simple round port ( as Dave does all the math and working drawings, I of course forget the dimensions) and was literally a couple of hours start to finish for the rough prototype. As it showed promise, the next step was to rework for the slotted ports, and play with aesthetics / dimensions and bracing. Quite simply put, we found the narrow slot ports sounded better.
For reasons that Dave, Scott, or others could describe, the more resistive ports retain the enclosure tuning over a wider range of internal pressure levels (i.e. sound volumes) than a simple round port, particularly those of larger diameter.
A couple of years ago we built a minor variation (bracing and dimensions) of Fostex's factory recommended BR for the FE167E, with the round port. While it certainly demonstrated the extra sensitivity and more deeply extended low frequency response that you'd expect from this driver, it was outclassed in terms of articulation, tonality and textural detail from the lower mid-bass on down by the FE127 Fonken Prime.
edit: Dave -maybe you could address the highlight point - as you had discussed with clients on Sunday.
final (?) edit: Gareth - on closer review of your drawing, I'd be more concerned about the effect of what appears to be a large lateral brace behind the driver. Even with extensive holes, I think you'd find this to generate a lot of very early and high pressure reflections back to the driver. As we found out in an unrelated design, this can cause more problems than the extra cabinet bracing cures.
Bigun said:
All that math! It may be right but my method is more like this... i model a bass reflex with a roll-off curve of a specific shape -- i play until it looks/feels right -- then i fit high ratio/long ports, build it, listen, modify as required, loop, until done.
One thing i can say re that last quoted bit is that the a port gets longer when you make the ports bigger.
Now my designs aren't necessarily official Onkens, they just look like it. (Jeff ran some of them thru an Onken spreadsheet and says they do or are at least close.
dave
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Thanks guys ! -it's great to have mentors to lean on.
I'm a bit of an engineer at heart and just can't resist needing to know a bit of what's going on and why the designs are the way they are.
And that patent I referred to for placing a restriction on the port looks most like adding a resistor to the 'inductance' of the port which would be vastly simpler than the Fonken approach as well as being tunable. Clearly I would need to do some experiments to further this, but I'm not sure if time constraints will allow. If I do I'll be sure to get back to you.
In the meantime...
My two FE127e drivers shipped today so I need to settle on the box. Well, of course I could (should) pick one of the many proven designs and get cracking. Home Depot for some Birch Ply this weekend.
Chris - good advice about the poorly positioned brace, I think I put it there to hold the 'pipes' but this isn't necessary of course.
I'm a bit of an engineer at heart and just can't resist needing to know a bit of what's going on and why the designs are the way they are.
And that patent I referred to for placing a restriction on the port looks most like adding a resistor to the 'inductance' of the port which would be vastly simpler than the Fonken approach as well as being tunable. Clearly I would need to do some experiments to further this, but I'm not sure if time constraints will allow. If I do I'll be sure to get back to you.
In the meantime...
My two FE127e drivers shipped today so I need to settle on the box. Well, of course I could (should) pick one of the many proven designs and get cracking. Home Depot for some Birch Ply this weekend.
Chris - good advice about the poorly positioned brace, I think I put it there to hold the 'pipes' but this isn't necessary of course.
chrisb said:
I neglected to address this (a couple posts, including Chris' happened while i was pecking out the reply above)... one thing that many people, including professionals, forget is that the T/S parameters used to design boxes change depending on how high you turn the wick (ie in a BR the tuning is really only perfect for one volume level). More insidious is that the tuning changes as the dynamics of the music you play change (with the volume at the same level)... making the ports resistive tends to ameriolate this... the box still uses the back wave to reinforce the bass but is less sensitive to the tuning shifting around.
note that the shifting of the parameters will also be affected by the weather.
As to the laterel brace, the one parallel to the baffle, i'd loose it as Chris suggests. A brace should always be edge on to the driver. Further this points out something you are giving up by going to multiple round ports... the slot ports on the side, by their very construction, create an incredibly stiff box panel, they are inherntly self braced. As well choice of the speacer used can further enhance this. We use a dissimilar material, most often MDF, to further isolate the outer wall from the inner wall, the different impedance of each layer of material -- one of the reasons for using plywood (9 layers in 12mm, 11 lauers in 15mm, 13 layers in 18mm BB) is that at each layer inteface, you get an impedance change and therefore any vibration is dissipated/damped.
In other words with the Onken-style ports the lateral brace is unnecessary.
dave
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