M frames are dipole constructions half way between H frame and W frame. Usually they look like this:
First important reason to go for a M frame is force cancellation: Since the angle between both drivers is 90°, half of the in/out movement of the drivers is cancelled in the up-down direction, and only the other half is expressed in forward/backward movement of the frame.
The other reason is the different ratio of frame size to lambda/4 resonance. Let´s look at the response of the shown M frame, measured at the crosspoint of the red lines:
The same response measured with another software, microphone and room:
As red line I have included the most applicable simulation of an H frame with the U & H Frame Woofer Worksheet:
http://www.pvconsultants.com/audio/boxmodel/uhframe.htm
This corresponds to a H frame depth of 21 cm (one chamber). Don´t worry about the big difference between 25-250 Hz. Measurement includes floor boost and was taken for the two-driver-frame, while the simulation is in free space and accounts for 1 driver only (and 1/2 of the opening area of the M frame).
Now let´s look how H frames of 28 cm depth (30 cm - 2 cm for baffle) and 14 cm depth would behave:
Red line is for the 28 cm chamber, purple for 21 cm (corresponding to the measured M frame) and blue for a 14 cm chamber.
To me this suggests that a M frame of a given depth could retain some of the bass efficiency of an H frame twice its depth by sacrificing the higher resonance frequency, a H frame of the same depth would provide.
Looks like a valuable compromise to me.😉
An externally hosted image should be here but it was not working when we last tested it.
First important reason to go for a M frame is force cancellation: Since the angle between both drivers is 90°, half of the in/out movement of the drivers is cancelled in the up-down direction, and only the other half is expressed in forward/backward movement of the frame.
The other reason is the different ratio of frame size to lambda/4 resonance. Let´s look at the response of the shown M frame, measured at the crosspoint of the red lines:
An externally hosted image should be here but it was not working when we last tested it.
The same response measured with another software, microphone and room:
An externally hosted image should be here but it was not working when we last tested it.
As red line I have included the most applicable simulation of an H frame with the U & H Frame Woofer Worksheet:
http://www.pvconsultants.com/audio/boxmodel/uhframe.htm
This corresponds to a H frame depth of 21 cm (one chamber). Don´t worry about the big difference between 25-250 Hz. Measurement includes floor boost and was taken for the two-driver-frame, while the simulation is in free space and accounts for 1 driver only (and 1/2 of the opening area of the M frame).
Now let´s look how H frames of 28 cm depth (30 cm - 2 cm for baffle) and 14 cm depth would behave:
An externally hosted image should be here but it was not working when we last tested it.
Red line is for the 28 cm chamber, purple for 21 cm (corresponding to the measured M frame) and blue for a 14 cm chamber.
To me this suggests that a M frame of a given depth could retain some of the bass efficiency of an H frame twice its depth by sacrificing the higher resonance frequency, a H frame of the same depth would provide.
Looks like a valuable compromise to me.😉
Rudolf,
Thanks for the info. I've got a set of M-frames + extra U extension that I need to tweak into submission. You have me scratching my head trying to figure out how deeper extension that a same depth H is possible. The M is still a dipole, so where is the extra propagation delay for the rear wave? Sure some of the output comes from deeper in the rear cavities, but the same applies in front making the average D still the depth of the cab. What am I missing or what is the simulator missing?
Thanks for the info. I've got a set of M-frames + extra U extension that I need to tweak into submission. You have me scratching my head trying to figure out how deeper extension that a same depth H is possible. The M is still a dipole, so where is the extra propagation delay for the rear wave? Sure some of the output comes from deeper in the rear cavities, but the same applies in front making the average D still the depth of the cab. What am I missing or what is the simulator missing?
If the measurements are near field, centered on the red "X", I think that the output from one side of the enclosure would dominate and the dipole influence would be supressed. If the simulation was run in the same manner then this would also be the case. That is my guess to explain what is shown in the plots. If I am correct, the correlation looks pretty good between simulation and test.
Yes, the simulation was done for the front plane of the H baffle. The only difference is that the simulation was done for a quadratic opening with one driver.MJK said:
@JohninCR:
Everything is strictly nearfield of course - with no influence of the rear wave. I wasn´t interested in the dipole behaviour in the first place, but in the H frame equivalent of that M frame.
@MJK:
Since I do not have comparable measurement data for low frequencies: Is it valid to assume that a M frame with a given opening area and resonance will have the same low frequency response as a H frame with the same data? When the M frame apparently needs only 2/3 of the equivalent H frame depth, this would be a real achievement.
As far as I understand it this would be the same question as: Has a tapered TML with the same Smouth and Fres the same output as a straight one?
Rudolf,
OK, I think I got it. Just to summarize the M-frame advantages over an H-frame to be sure:
1. 50% mechanical vibration.
2. Smaller size due to the fold.
and most importantly
3. The fundamental resonance for an M-frame occurs at a higher frequency than an H-frame of the same depth. This allows us to build an M approx 50% deeper for the resonance to occur at the same point as an H, so deeper bass is possible for the same XO limit.
Did I summarize correctly?
Now let me throw a possible monkey wrench in there, and maybe why I'm having problems with mine (the larger opening is to the rear). What about the resonances on the other side with 2 openings at 45deg angles? Are they higher or lower than a corresponding H? Also, if they don't correspond with the same opposite phase resonance in the front, do they become more problematic, since there's no dipole cancellation of the resonances?
OK, I think I got it. Just to summarize the M-frame advantages over an H-frame to be sure:
1. 50% mechanical vibration.
2. Smaller size due to the fold.
and most importantly
3. The fundamental resonance for an M-frame occurs at a higher frequency than an H-frame of the same depth. This allows us to build an M approx 50% deeper for the resonance to occur at the same point as an H, so deeper bass is possible for the same XO limit.
Did I summarize correctly?
Now let me throw a possible monkey wrench in there, and maybe why I'm having problems with mine (the larger opening is to the rear). What about the resonances on the other side with 2 openings at 45deg angles? Are they higher or lower than a corresponding H? Also, if they don't correspond with the same opposite phase resonance in the front, do they become more problematic, since there's no dipole cancellation of the resonances?
Rudolf,
To be honest, I have not spent much time looking at H, U, M, Ripole, or any of the other interesting dipole styles of boxes to be able to answer your question. These are just something that I have not had the time, or given enough priority, to look at in any detail.
MJK said:
Martin,
Are your spreadsheets accurate for such short pipes at very low frequencies, or is behavior different so close to the driver that it becomes significant in relation to the short length? If they're accurate in that zone, then we can easily model the simple versions of these cavity shapes using your existing sheets.
Is 0dB the driver's reference sensitivity?
If so, a modest M frame could provide dipole bass without the need for EQ- use a short frame and a quad of 8"s to get enough motor to get the efficiency up. And by going with such a short frame, you push the resonance up pretty high too making for less coloration...
Sounds like a win-win, I think I'll try it with my next OB.
If so, a modest M frame could provide dipole bass without the need for EQ- use a short frame and a quad of 8"s to get enough motor to get the efficiency up. And by going with such a short frame, you push the resonance up pretty high too making for less coloration...
Sounds like a win-win, I think I'll try it with my next OB.
John
The worksheet algorithm should be accurate enough for short pipe enclosures. The only easily modeled geometry at this time is the U frame. In addition, I am in the process of updating all of the worksheets to include a new baffle diffraction calculation and probably a polar response plot which should make the worksheets even better for the U frame simulation. But at this time, I do not believe I have a worksheet available that will accurate model the H, M, or Ripole geometry.
The worksheet algorithm should be accurate enough for short pipe enclosures. The only easily modeled geometry at this time is the U frame. In addition, I am in the process of updating all of the worksheets to include a new baffle diffraction calculation and probably a polar response plot which should make the worksheets even better for the U frame simulation. But at this time, I do not believe I have a worksheet available that will accurate model the H, M, or Ripole geometry.
MJK said:
Martin,
I was thinking of the TQWP sheet for modelling just one side of the tapered cavity shape like with these M-baffles.
John,
The only worksheets I would consider as being remotely applicable are the "TL Open End" and the "TL Offset Driver" worksheets and of course the U frame version from the FRD Consortium site. Again, these will only simulate a U frame and not one of the other geometries.
I hope you guys don't mind me reviving this thread.
I am currently using a pure open baffle and am looking at the various frame options to improve the performance. Specifically, I am using dual Eminence Kappalite 3015LF drivers in the OBs. I know they are not optimum drivers based on the T/S parameters, but I had six of them left over from my previous ported enclosure builds.
I currently use an actively crossed system with Oris horns for the HF, the OB for the range from 250-40Hz and a Klipsch RSW-15 sub for the range below 40Hz.
Do you recommend changing out the drivers?
What would be the effect on performance if the M-frame is extended by 7"?
I would appreciate any suggestions on how to improve things. I have given up the idea of being able to cover the whole audible spectrum with a true two way system, so am just trying to improve what I am using now.
I am currently using a pure open baffle and am looking at the various frame options to improve the performance. Specifically, I am using dual Eminence Kappalite 3015LF drivers in the OBs. I know they are not optimum drivers based on the T/S parameters, but I had six of them left over from my previous ported enclosure builds.
I currently use an actively crossed system with Oris horns for the HF, the OB for the range from 250-40Hz and a Klipsch RSW-15 sub for the range below 40Hz.
Do you recommend changing out the drivers?
What would be the effect on performance if the M-frame is extended by 7"?
I would appreciate any suggestions on how to improve things. I have given up the idea of being able to cover the whole audible spectrum with a true two way system, so am just trying to improve what I am using now.
Today I built a prototype to see what results I could get from an M-frame design using my Eminence Kappalite 3015LF drivers.
The results, compared to the current open baffles, were positive. Overall, I get slightly better performance with the M-frame vs. the pure open baffle.
Measurements were taken in my listening room, so I'm sure there are some room modes present in the results. Plot is raw data, no smoothing. BTW, my room has extensive treatments, so I don't normally have too many room modes to fight.
I built the M-frame with a 7" lip in front and behind the edges of the necessary structure to see if I could get improved LF response. Compared to a previous build, there was not much difference.
If I go with this frame design, it will be to reduce the size of the current open baffle. The slight performance improvement is a bonus.
The results, compared to the current open baffles, were positive. Overall, I get slightly better performance with the M-frame vs. the pure open baffle.
Measurements were taken in my listening room, so I'm sure there are some room modes present in the results. Plot is raw data, no smoothing. BTW, my room has extensive treatments, so I don't normally have too many room modes to fight.
I built the M-frame with a 7" lip in front and behind the edges of the necessary structure to see if I could get improved LF response. Compared to a previous build, there was not much difference.
If I go with this frame design, it will be to reduce the size of the current open baffle. The slight performance improvement is a bonus.
Attachments
You don't say, what you want to improve specifically with that M-frame. Is it the frame height, more bass response, lower Fs, better impulse compensation or what else?
The M frame is sort of a mid point between H frame and W frame. By closing the angle between both drivers you move from the first to the second - in every respect: Fs decreases, usable upper frequency limit decreases, efficiency decreases, box volume decreases, impulse compensation increases. Choose your poison. 🙂
Rudolf
I can't say that I understand exactly what you have done here 😕
The M frame is sort of a mid point between H frame and W frame. By closing the angle between both drivers you move from the first to the second - in every respect: Fs decreases, usable upper frequency limit decreases, efficiency decreases, box volume decreases, impulse compensation increases. Choose your poison. 🙂
Rudolf
After building and testing basic prototypes of each, I can see that there is no optimum answer, as each frame had different benefits and slightly different problems.
The 7" lip I referred to is simply a 7" depth in front of, and behind the basic M-frame. It was a starting point to see what improvements/problems I could measure by adding depth to the frame.
Like most of us, I wanted to find a 'perfect' frame, decrease Fs, improve efficiency, and get down to 20Hz. Needless to say, you can't have it all.
I will be building H-frames, which seems to meet my needs better than any of the other frames I have built for testing.
Part of the issue I am dealing with is that i wish to keep the drivers I already have, and use active EQ to compensate for any major problems. I currently have 6 Eminence Kappalite 3015LF drivers and wish to avoid te expense of going to something like AE or Hawthorne drivers.
The 7" lip I referred to is simply a 7" depth in front of, and behind the basic M-frame. It was a starting point to see what improvements/problems I could measure by adding depth to the frame.
Like most of us, I wanted to find a 'perfect' frame, decrease Fs, improve efficiency, and get down to 20Hz. Needless to say, you can't have it all.
I will be building H-frames, which seems to meet my needs better than any of the other frames I have built for testing.
Part of the issue I am dealing with is that i wish to keep the drivers I already have, and use active EQ to compensate for any major problems. I currently have 6 Eminence Kappalite 3015LF drivers and wish to avoid te expense of going to something like AE or Hawthorne drivers.
Could this M arrangement also provide second harmonic reduction, as in PPSL enclosures?
Rudy, what happened with your H frames? Did them sound better than the M ones?
Rudy, what happened with your H frames? Did them sound better than the M ones?
H2 reduction is possible, if drivers are positioned in different directions and wired mirrored (push-pull). The the unlinearities get balanced. This arrangement is used by S Linkwitz and many others.
More info Subwoofer Origami
More info Subwoofer Origami
Of all the various OB arrangements I tested, I found the H frame to be the best setup with the fewest problems. Each design had places along the frequency spectrum where they excelled, but at the cost of one problem or another. That made the H frame the best overall compromise with the best overall response.
I have been using the H frames since and am very happy with them.
Thanks for the info.
I also tried simple baffle, U and H frame. At the moment i have H frame and they sound good to me.
What crossover point and slope you use between the horn and H frames?
I also tried simple baffle, U and H frame. At the moment i have H frame and they sound good to me.
What crossover point and slope you use between the horn and H frames?
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