They use flow resistance:
http://www.me-geithain.de/index2.html?eng
How this directional characteristics has been achieved and what consequences result for audio control rooms, graduate engineer Dieter Thomsen talked about it to Jochen Kiesler, owner and chief developer of ME-Geithain. The company, which is located close to Leipzig, was known already before the German reunion for the production of studio monitors with special coaxial arranged high- and mid-frequency transducers, satisfying highest professional demands. Without great PR activities they now start to conquer one of the front ranks in the high-end consumer segment.
Production Partner: Mr. Kiesler, how do you achieve cancellation of waves with wavelength in meter range, and this not only for a few discreet Hz but within a 1:10 bandwidth? After all, we talk about wavelength, which extend between 1 and 10 meters.
Jochen Kiesler: Well, for sure not by deceiving physics. That won’t do. To sum it up, the trick was to attach an acoustic phase-rotation line at the back of a loudspeaker which produces a phase-shift of 160 degrees. Thereby both, the phase and the amplitude must be kept constant within 5 degrees and – this is the biggest problem – irrespective of pressure conditions. After all, the range of pressure inside the box differs from 25 to 125 dB and for all that, the specific flow resistance has to be the same
PP: In other words, the flow resistance causes a wideband delay?
J.K.: You could say so. We use a combination of several materials which I would not like to discuss in detail now. Inside the loudspeaker box RL 901K there are two separated cabinets which close up with these flow resistances in their back. The dimensions of the flow resistance and of the membrane correspond to each other and if you choose the correct combination, it works for frequencies down to 30 Hz.
http://www.me-geithain.de/index2.html?eng
How this directional characteristics has been achieved and what consequences result for audio control rooms, graduate engineer Dieter Thomsen talked about it to Jochen Kiesler, owner and chief developer of ME-Geithain. The company, which is located close to Leipzig, was known already before the German reunion for the production of studio monitors with special coaxial arranged high- and mid-frequency transducers, satisfying highest professional demands. Without great PR activities they now start to conquer one of the front ranks in the high-end consumer segment.
Production Partner: Mr. Kiesler, how do you achieve cancellation of waves with wavelength in meter range, and this not only for a few discreet Hz but within a 1:10 bandwidth? After all, we talk about wavelength, which extend between 1 and 10 meters.
Jochen Kiesler: Well, for sure not by deceiving physics. That won’t do. To sum it up, the trick was to attach an acoustic phase-rotation line at the back of a loudspeaker which produces a phase-shift of 160 degrees. Thereby both, the phase and the amplitude must be kept constant within 5 degrees and – this is the biggest problem – irrespective of pressure conditions. After all, the range of pressure inside the box differs from 25 to 125 dB and for all that, the specific flow resistance has to be the same
PP: In other words, the flow resistance causes a wideband delay?
J.K.: You could say so. We use a combination of several materials which I would not like to discuss in detail now. Inside the loudspeaker box RL 901K there are two separated cabinets which close up with these flow resistances in their back. The dimensions of the flow resistance and of the membrane correspond to each other and if you choose the correct combination, it works for frequencies down to 30 Hz.
Russell Dawkins said:
Phase inversion, a large radiating vent(s) on the back side yet still maintaining tuning in the bass. Elegant but tricky
I have tried this but without this type of success
Here it is explained
The ME Geithain monitors use an acoustic delay line formed by two slots in the backside of the woofer cabinet that are filled with some special foam material (they don´t disclose what is used - some suspect that it may be a kind of Dacron fiber).
Siegfried Linkwitz describes the approach on his websites (http://www.hifi-forum.de/extURL.php?goURL=http://www.linkwitzlab.com/frontiers.htm#A) :
“A cardioid speaker can be made with two opposite polarity monopole sources separated by a distance D, and with the signal to one of the sources delayed by a time T = D/c. An implementation of this concept could be a driver in a box of depth D where the rear wall is an acoustic resistor R. At long wavelengths the box internal air volume behaves as a compliance or acoustic capacitor C. The acoustic output from the rear of the box is low-passed by the RC filter and delayed relative to the front output by T = RC. The acoustic resistor should be purely dissipative, with vanishing reactive component, and be independent of frequency. It also should be linear over the range of volume velocities encountered for high SPL. “
Obviously Geithain have found a way to achieve that (at least for the < 300 Hz region).
Siegfried Linkwitz describes the approach on his websites (http://www.hifi-forum.de/extURL.php?goURL=http://www.linkwitzlab.com/frontiers.htm#A) :
“A cardioid speaker can be made with two opposite polarity monopole sources separated by a distance D, and with the signal to one of the sources delayed by a time T = D/c. An implementation of this concept could be a driver in a box of depth D where the rear wall is an acoustic resistor R. At long wavelengths the box internal air volume behaves as a compliance or acoustic capacitor C. The acoustic output from the rear of the box is low-passed by the RC filter and delayed relative to the front output by T = RC. The acoustic resistor should be purely dissipative, with vanishing reactive component, and be independent of frequency. It also should be linear over the range of volume velocities encountered for high SPL. “
Obviously Geithain have found a way to achieve that (at least for the < 300 Hz region).
Cardioid response at low frequency is straight forward. It has been around for years. I use the acoustic resistance principle in my NaO II woofer system.. The idea has been around since the 50's (one of Lynn's mile stone???) and used in the 60's, and 70's. Holmes reviewed this in his '86 paper on the subject and Backman looked at it again with a simple model in '99. I present a simplified explanation of the idea here..
The acoustic resistance can be derived in any number of way, but it always comes down to damping resonances and low pass filtering of the rear response of the driver. It is easier to achieve a true cardioid using two monopole sources with a delay. At one time I offered a woofer system called the CRAW (Controlled Response Active Woofer) using dual 10" woofers which operate as a monopole, a dipole or a cardioid.
People like to talk about these things as though they are esoteric an difficult to build when in fact they are fairly simple if you understand them.
The acoustic resistance can be derived in any number of way, but it always comes down to damping resonances and low pass filtering of the rear response of the driver. It is easier to achieve a true cardioid using two monopole sources with a delay. At one time I offered a woofer system called the CRAW (Controlled Response Active Woofer) using dual 10" woofers which operate as a monopole, a dipole or a cardioid.
People like to talk about these things as though they are esoteric an difficult to build when in fact they are fairly simple if you understand them.
john k... said:
In the last several posts, I have been discussing the 200 ~ 800 Hz frequency band. I must disagree that "cabinet resonance aren't much of an issue here because the frequency is typically well below any cabinet resonance" because 200 ~ 800 Hz is classically where internal cabinet standing-waves are at their worst.
I very much regret that we seem to be talking past each other. You are discussing performance at LF cutoff, and I'm discussing minimization of stored energy in the 200 ~ 800 Hz region, which falls outside the scope of LF electroacoustic models.
It is certainly true that the majority of designers regard (monopole) cabinet performance in the 200 ~ 800 Hz region as a problem that has already been solved - a non-issue from the design perspective. I am not in that group. A minority of designers are pursuing the alternative of actively equalized dipole systems. This sidesteps the problem of stored energy in the (monopole) cabinet at the cost of equalization, system complexity and requiring substantially more amplifier power below the baffle peak.
The intermediate solution of a resistive-box or quasi-cardioid can be achieved in several ways - a filled box with the rear end open has been around for a long, long time, probably going back to the old console radios of the Thirties. What Gary Pimm and I will be looking at is not a box, but a resistive mesh - a trellis-like framework that holds lossy felt pads. This isn't new either - merely a variant of a microphone. Since acoustic absorbers have non-ideal performance, this will have to be measured, and several variants tried and discarded.
The two things I am most concerned about are mass-coupling to the cone (as seen in the lower white curve in the IMP family of curves you've posted) and shifting of the damping material at high levels - or worse, a gradual drift backwards under high drive, a sort of DC-rectification effect that Gary Pimm observed in early versions of his filled boxes. There is a reasonable probability that these problems are severe enough that I'll be forced to revert to an equalized dipole, but I won't know until we build and measure several variants.
Dr. Geddes, thanks for the pointer to re-read your excellent book. Much appreciated.
Just my 2 cents after futzing around with a 4-way OB/cardioid design the past 6 months -- The greatest transfer of energy in my system doesn't occur directly, i.e., from the 4 x12" drivers to their U-frame enclosure. Instead, it is the energy from these drivers that seemingly bypasses the U-frame and shakes the bejeezus out of the mid/treble baffle. This baffle is very narrow, with the front entirely covered in 3/8" felt, so it probably isn't contributing too much. However, if I were using a wider baffle, or something like the AH425, I would definitely be concerned.
I'll be looking into ways to decouple the two baffles in the future: I'm just not that motivated these days.
I'll be looking into ways to decouple the two baffles in the future: I'm just not that motivated these days.
Thanks for the heads-up. I plan to use an isolated steel frame to support the mid/HF drivers, with no mechanical coupling between the LF assembly and the mid/HF frame. Gary Pimm has warned me about the substantial reaction forces in his system, particularly in his equalized W-baffle subwoofer.
John K, I want to thank you for posting the measurements of the open-back box with various levels of filling. Where was the microphone placed - is this a nearfield measurement? My previous comment about mass-coupling is based on the family of curves in the 120 ~ 240 Hz region, where the white curve clearly departs from the rest.
The times I've seen this before it was caused by getting damping material (or fill) too close to the cone, adding effective mass to it, thus lowering efficiency. In subjective terms, the sound quality takes a big drop when this happens.
John K, I want to thank you for posting the measurements of the open-back box with various levels of filling. Where was the microphone placed - is this a nearfield measurement? My previous comment about mass-coupling is based on the family of curves in the 120 ~ 240 Hz region, where the white curve clearly departs from the rest.
The times I've seen this before it was caused by getting damping material (or fill) too close to the cone, adding effective mass to it, thus lowering efficiency. In subjective terms, the sound quality takes a big drop when this happens.
A note on Ultratouch
Hi Crew:
I was hoping to be able to buy just 1 batt of this when I picked it up, but had to buy a 5 batt (53.04 sqft) bundle. Still, much cheaper than acoustic foam, at about $60 for the bundle (Ganahl lumber, Anaheim CA).
And ask your supplier to check physical stock if it's a long drive. I called ahead to a store, they claimed to have stock, but upon arrival, they did not. Then the darned baltic birch delaminated during cutting! I've never heard of this with BB, since it's higher quality than cheapo ply. Heck, I've never seen delam like this with even the cheapest ply.
The new sheet seems fine, so it was probably just a fluke, but sure wasted my time somethin' fierce.
Anyway, confirm with your supplier that they have Ultratouch at the location, save some pain.
Hi Crew:
I was hoping to be able to buy just 1 batt of this when I picked it up, but had to buy a 5 batt (53.04 sqft) bundle. Still, much cheaper than acoustic foam, at about $60 for the bundle (Ganahl lumber, Anaheim CA).
And ask your supplier to check physical stock if it's a long drive. I called ahead to a store, they claimed to have stock, but upon arrival, they did not. Then the darned baltic birch delaminated during cutting! I've never heard of this with BB, since it's higher quality than cheapo ply. Heck, I've never seen delam like this with even the cheapest ply.
The new sheet seems fine, so it was probably just a fluke, but sure wasted my time somethin' fierce.
Anyway, confirm with your supplier that they have Ultratouch at the location, save some pain.
Lynn Olson said:
Gee's Lynn,
It's like trying to hit a moving target. In the post I quoted you clearly stated,
I didn't realize that a LF system was 200 to 800Hz.
If you want to build a damped trellis like box all around the driver back side what you have is a a different form of low pass filter effect apples to the rear radiation compared to a damped, open backed box (OBB) with solid walls. Ultimately the radiation from the rear will just be a low pass filtered version of what would radiate from the rear of an open baffle driver with the potential addition of resonant behavior. How it will sum to the front response is only a matter of the effective delay. Just go the Home Depot, buy some 1/2" wire mesh. Form it into a box and line it with felt. Add some internal baffles at what ever angles you like, also coved in felt. The only real difference between this and a solid walled OBB is that with solid walls you have direct control over the propagation delay along the length of the box.
Lynn Olson said:
Yes they are near filed measurements with the mic centered in the plane of the rear opening. The more heavily damped responses do reflect additional loading of the woofer, not because the damping is too close to the cone, but because the damping is so dense that it restricts the volume flow at low frequency and the box starts to look like a ported box with highly damped port (or a very leaky sealed box). I guarantee that you will see the same effect with your trellis like box if the felt pads are too restrictive and there isn't an open air path sufficient for low frequency volume displacement.
Actually I'm using something not all that removed from your proposals in the ICTA midrange.
Yes, basically a leaky transmission line, or more comically, a soaker hose for the backwave. Like a TL, if it's overfilled, it doesn't work as well. That's what I'd expect from a quasi-TL structure.
The biggest problem are the stupid-sounding names, reeking of marketese, for a simple structure to attenuate and time-disperse the backwave. I think of it as a resistive mesh for lack of anything better. Not quite a cabinet, not quite a dipole, somewhere in between.
The biggest problem are the stupid-sounding names, reeking of marketese, for a simple structure to attenuate and time-disperse the backwave. I think of it as a resistive mesh for lack of anything better. Not quite a cabinet, not quite a dipole, somewhere in between.
Lynn,
I think we may be finally on the same page. What threw me off was the use of the quasi-cardioid terminology. Then you say q-c my though go directly to the low frequency response, what would be below the 1/4 wave frequency of a short U-frame. That is the frequency range where a q-c response can be obtained. Above that frequency the front and rear wave are basically uncorrelated and the polar response it the typical dazy pedal. The point I would make is that for an mid, what ever the format of the enclosure, if not sealed damping the rear wave at higher frequencies isn't too much of an issue. But at lower frequency it becomes an issue. If you consider the enclosure to have L, C and R components thus you have the LP filter effect and, depending on L, a potential resonance structure. With the felt/mesh you will probably have little L so the enclosure will act as a LP filter. In my ICTA I am going with a standard U-frame format for the mids. I have my damping solution worked out.
I think we may be finally on the same page. What threw me off was the use of the quasi-cardioid terminology. Then you say q-c my though go directly to the low frequency response, what would be below the 1/4 wave frequency of a short U-frame. That is the frequency range where a q-c response can be obtained. Above that frequency the front and rear wave are basically uncorrelated and the polar response it the typical dazy pedal. The point I would make is that for an mid, what ever the format of the enclosure, if not sealed damping the rear wave at higher frequencies isn't too much of an issue. But at lower frequency it becomes an issue. If you consider the enclosure to have L, C and R components thus you have the LP filter effect and, depending on L, a potential resonance structure. With the felt/mesh you will probably have little L so the enclosure will act as a LP filter. In my ICTA I am going with a standard U-frame format for the mids. I have my damping solution worked out.
John
You listing says that you don't accept E-mail and your web site has no E-mail. Could you E-mail me at Egeddes@gedlee.com.
Thanks
You listing says that you don't accept E-mail and your web site has no E-mail. Could you E-mail me at Egeddes@gedlee.com.
Thanks
Having looked into some designs using resistive measures, a patent identifying Symmetric Air Friction design looked promising. I first saw it addressed in Popular Mechanics back in the 80's.Lynn Olson said:
Basically resistive material need to be of quite some length so that the energy is gradually dissipated. So making it small is really tricky.
Earl,
John's mail can be accessed via the "contact music @ design" link at his web site.
John,
In your post 5231 in the previous page, I think the 300Hz peak is the U-frame 1/4 wave resonance.
1) What caused the small peak at 80Hz and 100Hz for the yellow and white curives?
2) Are they measurements taken from between 80cm to 200cm in front of the U-frame? Floor bounce included or excluded?
Thanks and regards,
Bill
In your post 5231 in the previous page, I think the 300Hz peak is the U-frame 1/4 wave resonance.
1) What caused the small peak at 80Hz and 100Hz for the yellow and white curives?
2) Are they measurements taken from between 80cm to 200cm in front of the U-frame? Floor bounce included or excluded?
Thanks and regards,
Bill
Hi,
I have been following this thread for some time and am experimenting with open baffels for my Altecs.
I came across this post over at the Lansing Heritage site.
This type of open backed cab design is interesting.Does any one have details of this type of design?
"Hello,I had to jump into this thread,one of the first speakers I had was a 605b,open back cabinet,what was interesting about the open back was inside of the cabinet,it had layered frames with some type of wool tacked to each frame,they had about 6 frames,each frame had a single hole 15"-12"-10"-8"-6" and the last frame was covered with a soild sheet of wool.the gent I got them from said he built them in 1966,I also picked up the amp/preamp that he built,it was a knightkit,maybe 15 watt(mono)it was one of the sweetest sounding mono system that I ever had.I think the wool was used for damping,wish I'd taken pictures that little system roared(it had a stock x over)"
The link to the thread is
http://www.audioheritage.org/vbulletin/showthread.php?t=16700
Regards
Rajiv
I have been following this thread for some time and am experimenting with open baffels for my Altecs.
I came across this post over at the Lansing Heritage site.
This type of open backed cab design is interesting.Does any one have details of this type of design?
"Hello,I had to jump into this thread,one of the first speakers I had was a 605b,open back cabinet,what was interesting about the open back was inside of the cabinet,it had layered frames with some type of wool tacked to each frame,they had about 6 frames,each frame had a single hole 15"-12"-10"-8"-6" and the last frame was covered with a soild sheet of wool.the gent I got them from said he built them in 1966,I also picked up the amp/preamp that he built,it was a knightkit,maybe 15 watt(mono)it was one of the sweetest sounding mono system that I ever had.I think the wool was used for damping,wish I'd taken pictures that little system roared(it had a stock x over)"
The link to the thread is
http://www.audioheritage.org/vbulletin/showthread.php?t=16700
Regards
Rajiv