I've read the white paper from KEF about this concept, but I still have questions.
Can you help ?
What were the real drivers ? Low distortion ? Transient response ? Design simplicity ? Extension ? Phase accuracy ?
What ?
Reading as I do a lot of hi-fi mags, modern ported designs offer some clever strategies. In my case, I have the 105/3's which from the measured extension are a little off the pace in terms of extension compared to smaller designs.
Examples of modern ported designs show near 20hz extension from the port (eg Tannoy DC 8T/ 10T).
So there we have the question. Apart from novelty, what were the drivers (excuse the pun !) behind the coupled cavity concept ? I cannot see that bass extension is one of them but I'm curious to learn from those who know more than me !
Thanks for any pointers
Can you help ?
What were the real drivers ? Low distortion ? Transient response ? Design simplicity ? Extension ? Phase accuracy ?
What ?
Reading as I do a lot of hi-fi mags, modern ported designs offer some clever strategies. In my case, I have the 105/3's which from the measured extension are a little off the pace in terms of extension compared to smaller designs.
Examples of modern ported designs show near 20hz extension from the port (eg Tannoy DC 8T/ 10T).
So there we have the question. Apart from novelty, what were the drivers (excuse the pun !) behind the coupled cavity concept ? I cannot see that bass extension is one of them but I'm curious to learn from those who know more than me !
Thanks for any pointers
htHi,
The basic premise was that most most music material with no real low bass
the maximum bass levels occur in the upper bass levels and coupled cavity
bass is about supporting maximum SPL levels rather than bass extension.
I've never liked it and still don't, but the principle works well for small HT,
its not really hifi in my book, and that probably why it has not stuck.
I prefer using all box volume for bass extension at lower levels.
rgds, sreten.
The basic premise was that most most music material with no real low bass
the maximum bass levels occur in the upper bass levels and coupled cavity
bass is about supporting maximum SPL levels rather than bass extension.
I've never liked it and still don't, but the principle works well for small HT,
its not really hifi in my book, and that probably why it has not stuck.
I prefer using all box volume for bass extension at lower levels.
rgds, sreten.
Last edited:
4th order bandpass designs are essentially a ported box where the output of the woofer is "captured" in a sealed box. The sealed box only acts to modify the driver's compliance.
The nice thing about 4th order bandpass is you have a lot of tradeoff flexibility. You can design for lower SPL and wide bandwidth or higher SPL and narrower bandwidth, the gain x bandwidth product is essentially constant. You can also tilt the response up or down by changing tuning (or misalignment).
4th order bandpass can have the same extension as vented in a similar sized box, but with 12dB / octave rolloff on the low and high end instead of 24. Theoretically this gives better transient response, although the higher excursion at lower frequencies means that it will distort more than a vented system at the low end of its range.
These systems also roll off at 2nd order above their bandpass, although there is a peak in output due to port resonance that needs filtration...
The nice thing about 4th order bandpass is you have a lot of tradeoff flexibility. You can design for lower SPL and wide bandwidth or higher SPL and narrower bandwidth, the gain x bandwidth product is essentially constant. You can also tilt the response up or down by changing tuning (or misalignment).
4th order bandpass can have the same extension as vented in a similar sized box, but with 12dB / octave rolloff on the low and high end instead of 24. Theoretically this gives better transient response, although the higher excursion at lower frequencies means that it will distort more than a vented system at the low end of its range.
These systems also roll off at 2nd order above their bandpass, although there is a peak in output due to port resonance that needs filtration...
In the follow on series, (model 1, 2, 3 & 4) KEF added "interport" upgrades to the Coupled Cavity Design... i.e. they vented (via a tuned port) into the cavity.
This added additional and deeper bass but seemingly at the expense of an even higher rate of filter slope below the tuned frequency.
I'd like to add this to my 105/3's
any clues how to begin ?
Cheers
This added additional and deeper bass but seemingly at the expense of an even higher rate of filter slope below the tuned frequency.
I'd like to add this to my 105/3's
any clues how to begin ?
Cheers
Last edited:
Hi,
The standard 4th order bandpass uses 2nd order sealed highpass bass units
and adds a 2nd order low pass that boosts maximum SPL in the upper bass.
6th order bandpass uses a 4th order vented high pass and there are
two variants, ports external, and ports internal to the low pass cavity.
The alignments are extremely sensitive to driver parameters and volumes,
no way can you convert a Kef 4th order to 6th order, with any success.
rgds, sreten.
The standard 4th order bandpass uses 2nd order sealed highpass bass units
and adds a 2nd order low pass that boosts maximum SPL in the upper bass.
6th order bandpass uses a 4th order vented high pass and there are
two variants, ports external, and ports internal to the low pass cavity.
The alignments are extremely sensitive to driver parameters and volumes,
no way can you convert a Kef 4th order to 6th order, with any success.
rgds, sreten.
Last edited:
So, what you are suggesting is that when KEF did it (Reference 1 - 4) they changed everything.. driver, X/over.
If I were to (say) add this internal port (venting to cavity) and tuned it to (say) 30hz, and kept the crossover the same, what would be the worst effect ?
Too much excursion ?
Too little ?
screwed up impedance ?
End of the world as we know it ?
Thanks
If I were to (say) add this internal port (venting to cavity) and tuned it to (say) 30hz, and kept the crossover the same, what would be the worst effect ?
Too much excursion ?
Too little ?
screwed up impedance ?
End of the world as we know it ?
Thanks
Hi,
I'm pretty sure there is nowhere to add the internal ports easily, two
rear panel external ports are a far simpler idea, but not as elegant.
Quality flared ports should be used, to minimise the volume they take up,
and they should be tuned low, about half the Fbox resonant frequency.
(If Fbox RF is conjugated by the x/o, that won't work any more,
it would probably be best to remove any low bass conjugation.)
Internal ports (which then also use the bandpass port as well for total
port mass and tuning) would require ingenious modifications that are
difficult to tune, use up central volume and require main port retuning.
It can be done, but your destroying the value of your speakers unless
you get it very right, and that is very difficult with bandpass speakers.
rgds, sreten.
I'm pretty sure there is nowhere to add the internal ports easily, two
rear panel external ports are a far simpler idea, but not as elegant.
Quality flared ports should be used, to minimise the volume they take up,
and they should be tuned low, about half the Fbox resonant frequency.
(If Fbox RF is conjugated by the x/o, that won't work any more,
it would probably be best to remove any low bass conjugation.)
Internal ports (which then also use the bandpass port as well for total
port mass and tuning) would require ingenious modifications that are
difficult to tune, use up central volume and require main port retuning.
It can be done, but your destroying the value of your speakers unless
you get it very right, and that is very difficult with bandpass speakers.
rgds, sreten.
The answer to the bandpass is a mix of what Ron E and Sreten said.
We chose bandpass enclosures for the benefits of trading bandwidth for efficiency, and lower roll-off rates at low frequencies.
One main feature is that the minimum excursion frequency is at the center of the frequency range of the system. This is a good choice for a lot of music, but it not appropriate for a pure subwoofer application, particularly for movies rather than music.
Coupled cavity designs are more difficult to optimise than vented , and the driver parameter requirements are very different, hence the drive units need to be designed specifically for the particular application. They are also more sensitive to leaks between front and back cavity, and to driver paramater variation.
From a usaeage viewpoint, in a passive system, they are very difficult to design with. because the tuning is at the center of the band, the double impedance peaks are spread either side of the tuning point, putting the upper peak right at the crossover frequency. This peak MUST be fully conjugated to enable the xover to work properly. Often times, depending on the bandwidth, the lower frequncy peak must be conjugated also. This makes for a very complex xover. Luckily, with all of our home brewed optimisation programs, this was not so difficult to design, just costly!
When I started work on the interport coupled cavity, ie adding vents between the front and rear cavities, the extra tuning possibilities allowed for a big reduction in driver excursion along with improvements in output capability, but now the desin became much more sensitive to driver paramters. There is no free lunch, and conjugation became even more critical.
If you are DIY'ing a speaker with coupled cavity and passive xover, my advice is DONT!! and certainly you cannot convert a standard coupled cavity into an interport couple cavity using a common driver. As I already said, driver parameters have to be matched to the intended application.
Andrew
We chose bandpass enclosures for the benefits of trading bandwidth for efficiency, and lower roll-off rates at low frequencies.
One main feature is that the minimum excursion frequency is at the center of the frequency range of the system. This is a good choice for a lot of music, but it not appropriate for a pure subwoofer application, particularly for movies rather than music.
Coupled cavity designs are more difficult to optimise than vented , and the driver parameter requirements are very different, hence the drive units need to be designed specifically for the particular application. They are also more sensitive to leaks between front and back cavity, and to driver paramater variation.
From a usaeage viewpoint, in a passive system, they are very difficult to design with. because the tuning is at the center of the band, the double impedance peaks are spread either side of the tuning point, putting the upper peak right at the crossover frequency. This peak MUST be fully conjugated to enable the xover to work properly. Often times, depending on the bandwidth, the lower frequncy peak must be conjugated also. This makes for a very complex xover. Luckily, with all of our home brewed optimisation programs, this was not so difficult to design, just costly!
When I started work on the interport coupled cavity, ie adding vents between the front and rear cavities, the extra tuning possibilities allowed for a big reduction in driver excursion along with improvements in output capability, but now the desin became much more sensitive to driver paramters. There is no free lunch, and conjugation became even more critical.
If you are DIY'ing a speaker with coupled cavity and passive xover, my advice is DONT!! and certainly you cannot convert a standard coupled cavity into an interport couple cavity using a common driver. As I already said, driver parameters have to be matched to the intended application.
Andrew
Thanks so much for the detailed reply Andrew and I suspect, you used to work at KEF by your comments. It's really great to have this feedback and steer.
So how come my old Mission 765's (smaller sized, 2 x 8" bass units, single vented design,) reach a lot lower but have the same efficiency ? I realise that designers all strive for "the best" so there are definate compromises with ported designs as well, but in terms of the 2 parameters mentioned Mission (much less experienced that KEF, I venture) seem to have achieved the same.
Now, I do not suggest for a moment that "quality" and "musicality" etc are identical between the designs, but even before I had the 765's modified [new bass units, new port, new filling material, revised crossover] they had good LF extension and went loud quickly.
I'm curious to know if this is why the bass units were paper and the surround foam - this all points to low mass. Is this correct ?
What happens if not ? Loss of control ? Excessive current ? Again, not being too familiar with the exact need for precise conjugation, I'm wondering what the practical outcomes are if you get it wrong !
Again, just curious.
Also, thanks for the "don't" advice w.r.t. even thinking of converting my speakers to interport (or even room venting) designs. Clearly, my lack of detailed understanding of loudspeaker designs is probably why I'm doing something else to earn a living !
They say a little knowledge can be dangerous and the thought of me attacking my 105/3's with a couple of ports seemed to make sense in my mind. I still wonder however what would happen in real life if I did that !
Thanks again
Steve
So how come my old Mission 765's (smaller sized, 2 x 8" bass units, single vented design,) reach a lot lower but have the same efficiency ? I realise that designers all strive for "the best" so there are definate compromises with ported designs as well, but in terms of the 2 parameters mentioned Mission (much less experienced that KEF, I venture) seem to have achieved the same.
Now, I do not suggest for a moment that "quality" and "musicality" etc are identical between the designs, but even before I had the 765's modified [new bass units, new port, new filling material, revised crossover] they had good LF extension and went loud quickly.
I'm curious to know if this is why the bass units were paper and the surround foam - this all points to low mass. Is this correct ?
What happens if not ? Loss of control ? Excessive current ? Again, not being too familiar with the exact need for precise conjugation, I'm wondering what the practical outcomes are if you get it wrong !
Again, just curious.
Also, thanks for the "don't" advice w.r.t. even thinking of converting my speakers to interport (or even room venting) designs. Clearly, my lack of detailed understanding of loudspeaker designs is probably why I'm doing something else to earn a living !
They say a little knowledge can be dangerous and the thought of me attacking my 105/3's with a couple of ports seemed to make sense in my mind. I still wonder however what would happen in real life if I did that !
Thanks again
Steve
I’m not sure if it was a design aim, but one advantage of the coupled cavity (aka 4th order bandpass) woofer system is that it is in fact an acoustic filter. This enables it to effectively filter out driver distortion and other non-harmonically related driver noises, something electronic LP filters can’t do. This feature is even more advantageous when the low end is boosted or equalized(as KEF did with the KUBE) which often results in pushing the driving beyond its linear range and the amplifier into clipping. It has always amazed me how far into clipping you can push an amplifier driving a bandpass woofer and yet the output still sounds clean.
The issue of conjugation has to do with crossover response. With the typical woofer you will have a couple of Octaves between the bass electrical peaks and the crossover point. With a coupled cavity system theacoustical bandpass is narrow and the upper bass electrical peak is just below the crossover point. The lower peak is maybe just an Octave lower.
Even if your coupled cavity design achieves a perfect bandpass you will have interaction between your crossover and the electrical bumps, undoing your good work.
David S.
Even if your coupled cavity design achieves a perfect bandpass you will have interaction between your crossover and the electrical bumps, undoing your good work.
David S.
Sir Andrew: Can u point me to a link where I can download some form of "interport
coupled cavity" modeling program. I already use Unibox/WinIsd/FRD Tools but they all are lacking on the fancy Order enclosures. WinIsd has a Parallel tuned but I prefer interport design like ABC design and modeling software.
coupled cavity" modeling program. I already use Unibox/WinIsd/FRD Tools but they all are lacking on the fancy Order enclosures. WinIsd has a Parallel tuned but I prefer interport design like ABC design and modeling software.
Basspbox 6 pro can handle interport coupled cavity design.
To be clear, all of these bass loading techniques need drivers specifically designed for them. You cannot really get the design to work optimally without, or else you are left with just accepting the best of what can be achieved with an off the shelf driver.
And to re-iterate my previous posting, the more complex the bass loading technique, the more sensitive the design is to driver parameter variation. Also, sealing the compartments to prevent leaks is critical.
As Dave said, for a passive system, you MUST fully conjugate the driver impedance prior to adding the low pass filter. The upper impedance peak is right at the rolloff frequency, and the xover cannot work without the conjugation.
This was the biggest design issue we faced when using the bandpass designs, conjugating the load, designing the xover, and keeping the impedance reasonable. The xover got very big, complex and expensive.
To be clear, all of these bass loading techniques need drivers specifically designed for them. You cannot really get the design to work optimally without, or else you are left with just accepting the best of what can be achieved with an off the shelf driver.
And to re-iterate my previous posting, the more complex the bass loading technique, the more sensitive the design is to driver parameter variation. Also, sealing the compartments to prevent leaks is critical.
As Dave said, for a passive system, you MUST fully conjugate the driver impedance prior to adding the low pass filter. The upper impedance peak is right at the rolloff frequency, and the xover cannot work without the conjugation.
This was the biggest design issue we faced when using the bandpass designs, conjugating the load, designing the xover, and keeping the impedance reasonable. The xover got very big, complex and expensive.
Thx for replying AndrewJ. I do understand the fickle nature of the Coupled Cavity design but the SPL payoff is what I'm after. My project is the 184 Sub from SpeakerPlans.com using the RCF L18 P540. My setup: Bass:1 18" | Mids:2 FaitalPRO 10's | Top's: 1/2 horns. This driver is very sensitive and does not gobble up much power. I only have 400watts @ 8 ohm's to play with. I want near/over 120 db spl @ 32/38 Hz outdoors/my backyard. I have 2 Loud FaitalPRO 10's to take over from 80Hz up. Question is... What type of xover would best mate that sub with my mid's? I was thinking of a passive 2nd order xover but u guys are saying it will be expensive/complex/DONT do diy'r feat. Any advise as to my xover options?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.