As i mentioned earlier, you can use the LInkwtz Transform circuit to compnesate for the drop below the resonant frequency and Rod Elliot (sound.au.com) produces a pcb for this purpose.
I am assembling one right now to repurpose a pair of ML Aeon LF sealed boxes (all the electrostatic wood work and panels having been removed)..
I measured them yesterday and found 59Hz to be the impedance peak so the LT circuit starts to boost the amp output from just below this.
https://www.linkwitzlab.com/thor-measmt.htm - well worth a read.
thanks what have you done with you Martin logins I think the linquitz I'd a good compromise potentially only requiring 4 amp channels I had a look at rod elliot he's got some eq projects on there as well look good
Just keep in mind that linkwitz transform was specifically designed for closed boxes (with 12 dB/oct roll off) and may not be exactly suitable for open baffle (6 dB/oct roll off).
I have started a new thread to discuss the Linkwitz Transform and will be posting some measurements showing it in action.
https://www.diyaudio.com/community/threads/sub-filters-linkwitz-transform-etc.404334/
https://www.diyaudio.com/community/threads/sub-filters-linkwitz-transform-etc.404334/
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Because the driver's response combines with the dipole cancellation the slope is not a constant 6dB per octave when you get below 200Hz or so, especially if the driver Qts is below 0.4. So the LT can be very useful for EQ in this region, although not an "exact" match to the rolloff.
This is how I picture it:
The U-shaped "Lambda" cabinet rolls off at about 60Hz, and the woofer and filter combine together to make a total response that's a bit fat on the low end and rolls off at 30dB per octave below 40Hz. The "bump" between 40-50Hz is somewhere between +2dB and +4dB depending on many things including the room, and distance between microphone and speaker. @mefistofelez @CharlieLaub
The U-shaped "Lambda" cabinet rolls off at about 60Hz, and the woofer and filter combine together to make a total response that's a bit fat on the low end and rolls off at 30dB per octave below 40Hz. The "bump" between 40-50Hz is somewhere between +2dB and +4dB depending on many things including the room, and distance between microphone and speaker. @mefistofelez @CharlieLaub
Hi CharlieLaub, perrymarshall,
if I understand the illustration and text correctly, the reason for being able to use only the peaking filter is the steep roll-off - 30dB/octave, below 40 Hz.
Kindest regards,
M
if I understand the illustration and text correctly, the reason for being able to use only the peaking filter is the steep roll-off - 30dB/octave, below 40 Hz.
Kindest regards,
M
I'm not sure I understand what you're trying to say. 12dB of that 30dB slope comes from the LC peaking filter itself. This is intentional because signals sent to this woofer below 40Hz would just be wasted anyway.
Hi perrymarshall,
post #48, CharlieLaub (emphasis supplied):
Kindest regards,
M
post #48, CharlieLaub (emphasis supplied):
So, my question was: "What feature of the cabinet design enables this?"
Kindest regards,
M
@mefistofelez I'd be interested to hear @CharlieLaub 's take on this, but the triangular "wings" on the side of the Walnut Dipoles mean that the distance from driver to cabinet edge in the u-frame is non-constant (very unlike a u-frame square / cube) so that transition frequency is spread across a wide range which is all the way from 80Hz to 200Hz. This means the "dipole peak" is also spread around and practically speaking has little effect. So all I have to deal with roughly is a -6dB/octave slope that starts around 60Hz. That makes extending response down to 40Hz a straightforward issue of adding a +5.5dB boost at 45.
Hi perrymarshall,
it seems that we are miscommunicating. What I understood from CharlieLaub's post was, that a feature of your cabinet design enables to use only the peaking filter without a need of the filter limiting the lower frequencies. His answer was in post 68.
Kindest regards,
M
it seems that we are miscommunicating. What I understood from CharlieLaub's post was, that a feature of your cabinet design enables to use only the peaking filter without a need of the filter limiting the lower frequencies. His answer was in post 68.
Kindest regards,
M
@mefistofelez Perry's cabinet design is really more like a U-frame than a pure dipole. The U-frame increases the front to back pathlength, and this helps to reduce the dipole loss and bass droop. It also gives rise to a resonance in the rear, but by making the wings in a triangular shape the Q of the resonance is usually reduced and/or the resonance frequency is moved up. So over the band that Perry uses the woofer in this cabinet he has gotten the response to be mostly flat, except at the very low end around 40Hz where there is still some downturn. His peaking passive filter is increasing the system output around the lowest end of the woofer passband and once octave above it or so.
If this was a true dipole, such as with a driver or drivers in a small plain baffle, the response would be falling at 6dB per octave over the entire passband. In that case Perry's passive filter would not help except below 80Hz, with the response at higher frequencies still tilted down. Thus my earlier comment to that effect.
If this was a true dipole, such as with a driver or drivers in a small plain baffle, the response would be falling at 6dB per octave over the entire passband. In that case Perry's passive filter would not help except below 80Hz, with the response at higher frequencies still tilted down. Thus my earlier comment to that effect.
Hi CharlieLaub,
I understand what you are saying.
Perhaps I misread your post #48, but even after re-reading it, I still understand it as arguing that Perry's cabinet design allows him to use a peaking filter only, without a need for a filter limiting the lower frequencies (note your contrasting the LT and Perry's approach in emphasizing the lower frequencies.)
However, as it seems that I cannot get my point across, let us abstain from beating the half-dead horse.
Kindest regards,
M
I understand what you are saying.
Perhaps I misread your post #48, but even after re-reading it, I still understand it as arguing that Perry's cabinet design allows him to use a peaking filter only, without a need for a filter limiting the lower frequencies (note your contrasting the LT and Perry's approach in emphasizing the lower frequencies.)
However, as it seems that I cannot get my point across, let us abstain from beating the half-dead horse.
Kindest regards,
M
A filter limiting the lower frequencies is an inherent part of Perry's peaking filter, which is a feature the Linkwitz transform does not possess.
An LT filter however, can address the 6db an octave rolloff across a much wider range of frequencies that the driver on a small flat baffle would have.
Perry's baffle does not experience this typical dipole rolloff starting at a much higher frequency and the tapered, folded wings are the features that mitigate it.
He only needs to boost a narrow range of frequencies down lower to get a flat response, and his peaking filter also has a built in "rumble filter".
Hope this helps. Jason
An LT filter however, can address the 6db an octave rolloff across a much wider range of frequencies that the driver on a small flat baffle would have.
Perry's baffle does not experience this typical dipole rolloff starting at a much higher frequency and the tapered, folded wings are the features that mitigate it.
He only needs to boost a narrow range of frequencies down lower to get a flat response, and his peaking filter also has a built in "rumble filter".
Hope this helps. Jason
I'm not sure if the physics behind this has been discussed, but I find it interesting.
The top plot is your 22uf/330uh/8ohm circuit. The amplifier voltage is Blue. The voltage across the 8 ohm resistor is Pink.
The second plot is impedance. The load on the amplifier is Green. The load on the 22uf capacitor is Red. Starting around 1.4khz the load on the capacitor is actually greater than the entire load on the amplifier. At this point the circuit is underdamped, and the voltage across the resistor will be higher than the voltage across the amplifier's output terminals.
At least I think this is what's happening.
The top plot is your 22uf/330uh/8ohm circuit. The amplifier voltage is Blue. The voltage across the 8 ohm resistor is Pink.
The second plot is impedance. The load on the amplifier is Green. The load on the 22uf capacitor is Red. Starting around 1.4khz the load on the capacitor is actually greater than the entire load on the amplifier. At this point the circuit is underdamped, and the voltage across the resistor will be higher than the voltage across the amplifier's output terminals.
At least I think this is what's happening.