Back Again
Hi all, the reason I've been quiet for a couple of days is twofold:
A) John Atwood passed through Colorado on his way to the Montreal show (he loves to drive long distances). During the one-day visit, we had a lot of discussions ranging from an early four-tube phase-locked-loop circuit in the horizontal sync circuit of a 1951 RCA television, the impact of the 10 ~ 20% of the US population who are Dominionist/End Times fundamentalists will have on the future of the USA vs more-secular Asia and Europe, why Japanese Buddhist monks gave up celibacy and married (unlike Buddhist monks elsewhere in Asia), the way power distribution systems work in Vietnam, California, Nevada, and Colorado, the effect of the US combined household, government, and Asian-held Treasury Note debt on the long-term value of gold vs international currencies ... and a whole bunch of other things.
Since both of us have worked for big high-tech companies and have spent long stretches of time in Asia and Europe (and have many friends there), we had a lot to talk about besides audio. The area we were most concerned about was the de-industrialization of the USA over the last forty years - both of us had trouble imagining an economy where people do nothing more than investment-scheme paper-shuffling, making music-videos and video-games, working for Google, or flipping hamburgers, while real (physical) commodities are built elsewhere. It's an old principle of economics a village can't survive by taking in each other's washing - somebody has to grow the crops and make the stuff of everyday life.
B) It's still kind of awkward getting around the house - it takes me about ten minutes of hard work to get upstairs, so it's something I only do once or twice a day. The big Dual G5 Mac with twin 20" Samsung monitors is my preferred platform for Web surfing and chatting on the forums, but it's upstairs, and I don't get to it every day.
I'm surprised and impressed the way this thread has continued on its own momentum, instead of fizzling out and disappearing below the diyAudio event horizon.
Here are my own thoughts, which have been stimulated by the Johnincr's intriguing front-and-back symmetric horn I saw a few pages ago. Recalling the metaphor of the 1,2,3, and 4 variations of a surface, ranging from a flat baffle to a horn, and finishing with a pipe, I realized that's not the whole story.
The diaphragm is acoustically transparent, so the rear-wave expansion also affects the front. In fact, there are not one, but three sets of expansion, or surfaces, that are important:
1) The front expansion
2) The rear expansion
3) Any intervening surface between front and rear - specifically, the delay term between the edge of the front and rear surface. This is close to zero for a thin dipole, but is nonzero and significant for any other combination of front and rear surfaces.
I like to visualize a pair of expanding hemispheres coming out of the front and rear of the driver. These are reverse polarity, of course, and cancellation effects occur when they meet. The acoustic pressure of the expanding shockwave diminishes at an inverse-square rate if the expansion surface is flat, and at rates somewhat less than that if there's a horn. The pressure of the wave does not diminish at all inside a pipe, unless it is filled with absorbing material.
All of the surfaces - flat though horn through pipe - are subject to standing waves forming on their surface. The standing waves are created by surface-edge reflections; the standing waves are at their most intense if the surface-edges are radially symmetric with respect to the driver (circular). The standing waves can be reduced by:
i) Curving the edge of the surface, which is most effective at wavelengths smaller than the diameter of the curve.
ii) Lining area close to the edge with acoustically absorbent material, such as felt. Using felt in triangles similar to the Mamboni pattern or my 3-size variation is probably the best way to do this.
iii) Perforating the surface close to the edge with an array of holes, such that the holes are smaller than the shortest wavelength (1/4" or smaller), having the greatest density of holes closest to the edge, and the lowest density of holes farther away from the edge.
iv) Other surface treatments, such as roughening, patterning, EnABL, treatment, etc.
All of the standing-wave problems originate from the sharp boundary at the edge. At first blush, a doughnut or toroidal shape around the driver might be the best in terms of avoiding standing waves - if Johnincr's symmetric dual-horn had a flat surface connecting the front and rear horns, that would effectively be a near-toroid.
The tradeoffs with a horn is the increased energy at the edge of the horn compared to a flat surface - although as mentioned earlier, a dipole has double the energy at the energy at the surface-edge since the rear-wave is in opposite polarity, and doubles the pressure compared to unipolar radiator (a closed box with 100% internal absorption).
Some impulse measurements of the same driver in a dipole vs Johnincr's symmetric (front & rear) horn would be most interesting. Although the dipole might have more energy right at that first edge reflection - thanks to the hard boundary - there's an acoustic delay between the front and rear horn (energy diffracted around the edge of the rear horn has an additional delay before it appears around the edge of the front horn), so we might see several lower-energy reflections in the symmetric-horn system. Hard to say in the absence of A vs B impulse measurements with the same driver.
Sonically, they'll sound different as well, since the null region has a different shape, and almost certainly a different frequency spectra. The biggest differences, I guess, would be in the region between 180 and 120 degrees off-axis, with interesting distortions in time and frequency response for both types of dipole. Much to be learned here, far afield of commercial loudspeakers.
Variac said:I agree, I think our non-belligerent attitude around here attracts people like Lynn, so I would appreciate everyone keeping that in mind. Also let's not get off into general comments about perception and other topics that typically don't lead anywhere....
Lynn, when you want to start making your decisions, please feel free to start another thread that is limited to comments about your design only, as it is developed. We can keep this thread more for "Brainstorming"
Variac
Hi all, the reason I've been quiet for a couple of days is twofold:
A) John Atwood passed through Colorado on his way to the Montreal show (he loves to drive long distances). During the one-day visit, we had a lot of discussions ranging from an early four-tube phase-locked-loop circuit in the horizontal sync circuit of a 1951 RCA television, the impact of the 10 ~ 20% of the US population who are Dominionist/End Times fundamentalists will have on the future of the USA vs more-secular Asia and Europe, why Japanese Buddhist monks gave up celibacy and married (unlike Buddhist monks elsewhere in Asia), the way power distribution systems work in Vietnam, California, Nevada, and Colorado, the effect of the US combined household, government, and Asian-held Treasury Note debt on the long-term value of gold vs international currencies ... and a whole bunch of other things.
Since both of us have worked for big high-tech companies and have spent long stretches of time in Asia and Europe (and have many friends there), we had a lot to talk about besides audio. The area we were most concerned about was the de-industrialization of the USA over the last forty years - both of us had trouble imagining an economy where people do nothing more than investment-scheme paper-shuffling, making music-videos and video-games, working for Google, or flipping hamburgers, while real (physical) commodities are built elsewhere. It's an old principle of economics a village can't survive by taking in each other's washing - somebody has to grow the crops and make the stuff of everyday life.
B) It's still kind of awkward getting around the house - it takes me about ten minutes of hard work to get upstairs, so it's something I only do once or twice a day. The big Dual G5 Mac with twin 20" Samsung monitors is my preferred platform for Web surfing and chatting on the forums, but it's upstairs, and I don't get to it every day.
I'm surprised and impressed the way this thread has continued on its own momentum, instead of fizzling out and disappearing below the diyAudio event horizon.
Here are my own thoughts, which have been stimulated by the Johnincr's intriguing front-and-back symmetric horn I saw a few pages ago. Recalling the metaphor of the 1,2,3, and 4 variations of a surface, ranging from a flat baffle to a horn, and finishing with a pipe, I realized that's not the whole story.
The diaphragm is acoustically transparent, so the rear-wave expansion also affects the front. In fact, there are not one, but three sets of expansion, or surfaces, that are important:
1) The front expansion
2) The rear expansion
3) Any intervening surface between front and rear - specifically, the delay term between the edge of the front and rear surface. This is close to zero for a thin dipole, but is nonzero and significant for any other combination of front and rear surfaces.
I like to visualize a pair of expanding hemispheres coming out of the front and rear of the driver. These are reverse polarity, of course, and cancellation effects occur when they meet. The acoustic pressure of the expanding shockwave diminishes at an inverse-square rate if the expansion surface is flat, and at rates somewhat less than that if there's a horn. The pressure of the wave does not diminish at all inside a pipe, unless it is filled with absorbing material.
All of the surfaces - flat though horn through pipe - are subject to standing waves forming on their surface. The standing waves are created by surface-edge reflections; the standing waves are at their most intense if the surface-edges are radially symmetric with respect to the driver (circular). The standing waves can be reduced by:
i) Curving the edge of the surface, which is most effective at wavelengths smaller than the diameter of the curve.
ii) Lining area close to the edge with acoustically absorbent material, such as felt. Using felt in triangles similar to the Mamboni pattern or my 3-size variation is probably the best way to do this.
iii) Perforating the surface close to the edge with an array of holes, such that the holes are smaller than the shortest wavelength (1/4" or smaller), having the greatest density of holes closest to the edge, and the lowest density of holes farther away from the edge.
iv) Other surface treatments, such as roughening, patterning, EnABL, treatment, etc.
All of the standing-wave problems originate from the sharp boundary at the edge. At first blush, a doughnut or toroidal shape around the driver might be the best in terms of avoiding standing waves - if Johnincr's symmetric dual-horn had a flat surface connecting the front and rear horns, that would effectively be a near-toroid.
The tradeoffs with a horn is the increased energy at the edge of the horn compared to a flat surface - although as mentioned earlier, a dipole has double the energy at the energy at the surface-edge since the rear-wave is in opposite polarity, and doubles the pressure compared to unipolar radiator (a closed box with 100% internal absorption).
Some impulse measurements of the same driver in a dipole vs Johnincr's symmetric (front & rear) horn would be most interesting. Although the dipole might have more energy right at that first edge reflection - thanks to the hard boundary - there's an acoustic delay between the front and rear horn (energy diffracted around the edge of the rear horn has an additional delay before it appears around the edge of the front horn), so we might see several lower-energy reflections in the symmetric-horn system. Hard to say in the absence of A vs B impulse measurements with the same driver.
Sonically, they'll sound different as well, since the null region has a different shape, and almost certainly a different frequency spectra. The biggest differences, I guess, would be in the region between 180 and 120 degrees off-axis, with interesting distortions in time and frequency response for both types of dipole. Much to be learned here, far afield of commercial loudspeakers.
ctrevith said:
1. Yes, you will need a steep-slope (18dB/oct or more) active crossover, active equalization, and a good-quality solid-state power amp for the 18-inch driver.
2. All you need are mirror-imaged pairs. I'd put the narrow (vertical) edge closest to the center, although I'd try it both ways just to be sure.
3. Yup. Diffraction happens everywhere, although I worry more about the integrity of the front wave, since the rear waves are destined for early reflections, not the direct sound.
4. No question about it, the big drivers will REALLY vibrate things. This is where a symmetric W-baffle for the monster bass drivers is a good idea, since they can be mounted in pairs that oppose each other mechanically, thus cancelling almost all of the reaction-force vibration.
5. Good idea. If you want to be clever, you can have aluminum or brass L-beam support not just the magnet, but the side-mounting screws for the full-range driver as well. For that matter, the front baffle can be mechanically isolated from the driver, and only have a compliant gasket where the edge of the driver basket meets the cutout of the baffle.
6. BAD idea, no donut. I can't think of a worse bass amp - particularly for a monster 18-driver that generates huge back-EMFs - than a tiny little 8-watt SET amp. The bass will be VERY murky and slow, and degrade the entire spectrum with all the sludge and distortion. Controlling bad-*** woofers takes an aggressive amp with lots of damping factor, a serious high-current supply, and lots of DC-coupled power. Think of a good-quality PA amplifier.
7. Retaining the quality of the widerange portion of the system is the hard part. The low-level high-pass capacitor preceding the SET amp needs to be of superlative quality, or if the amp is RC-coupled, you can play games with the value of the RC-coupling.
Setting the high-pass crossover frequency is something you might want to do by ear, although your initial suggestion is a good starting point. The subjective tradeoff is murk vs hardness.
8. Pimm's experience was the abominable electrolytic caps in the Behringer *must* be replaced with polypropylene films, at the minimum, and the op-amps swapped out for decent-quality parts. Despite the popularity of this gizmo in Europe, my audition told me it's not suitable for the high-quality part of the signal path - but there's no problem using it for low-bass crossovers and low-frequency room EQ.
hi lynn, would love to be a fly on the wall for some of those interesting and wide ranging discussions of yours!!!
Good to see you back, however I must admit that most of it still zooms way over my head ha ha ha.
Quick question, ctrevith's question made me prick my ears up, as I'm currently considering doing something a bit similar, namely to use my 18 in PHL's as a dipole bass unit. I have heard only two dipoles previously, one was the alon4's and the other the orions. Admittedly the alons only run a sealed 12 for the bass, so in the scheme of things perhaps not a good model by which to evaluate the dipole alignment.
Got to say, the orions didn't do it for me, for the life of me I couldn't pick the amazing imaging etc that is often used to describe dipoles, but that is neither here nor there and only my opinion.
HOWEVER, the bass dipole of the orions was lluuuvvvverly! So, my thinking about using the 18's only as dipole, and like ctrevith, I run my system using the deqx and as such his post had even more interest for me.(I would leave the mid and tweeter as is).
However, again when you answered his post re using the deqx, you again mentioned the behringer, and that has left me a little confused as the deqx is a totally different beast to any behringer unit, and I'm wondering if you have gotten the two products mixed up. Indeed, did ctrevith himself mean a behringer unit or the deqx unit. Praps we had better clear all that up???
One last quick question for the gurus on my possible conversion, I can understand that if I run the PHL's as a dipole they would not go very low, and so to compensate I have every intention of adding a sub, and maybe only take the PHL's down to 50 hz or so. The problem that I may run into with the PHL nay not be at the bottom, but rather at the top of the FR, as I am unable to have the mids go any lower that 300 hz.
Is that where my best laid plans may come undone??? ie will I get all sort of wacky response from a dipole bass unit running that high??
Lastly, I often see multiple bass units used in dipole, and assume that is mainly for help in augmenting the bass response. However, is it also to help in cancelling vibrations??? In other words, is it indeed feasible to have a single 18 doing the dipole bass??
enjoying the learning experience here
Good to see you back, however I must admit that most of it still zooms way over my head ha ha ha.
Quick question, ctrevith's question made me prick my ears up, as I'm currently considering doing something a bit similar, namely to use my 18 in PHL's as a dipole bass unit. I have heard only two dipoles previously, one was the alon4's and the other the orions. Admittedly the alons only run a sealed 12 for the bass, so in the scheme of things perhaps not a good model by which to evaluate the dipole alignment.
Got to say, the orions didn't do it for me, for the life of me I couldn't pick the amazing imaging etc that is often used to describe dipoles, but that is neither here nor there and only my opinion.
HOWEVER, the bass dipole of the orions was lluuuvvvverly! So, my thinking about using the 18's only as dipole, and like ctrevith, I run my system using the deqx and as such his post had even more interest for me.(I would leave the mid and tweeter as is).
However, again when you answered his post re using the deqx, you again mentioned the behringer, and that has left me a little confused as the deqx is a totally different beast to any behringer unit, and I'm wondering if you have gotten the two products mixed up. Indeed, did ctrevith himself mean a behringer unit or the deqx unit. Praps we had better clear all that up???
One last quick question for the gurus on my possible conversion, I can understand that if I run the PHL's as a dipole they would not go very low, and so to compensate I have every intention of adding a sub, and maybe only take the PHL's down to 50 hz or so. The problem that I may run into with the PHL nay not be at the bottom, but rather at the top of the FR, as I am unable to have the mids go any lower that 300 hz.
Is that where my best laid plans may come undone??? ie will I get all sort of wacky response from a dipole bass unit running that high??
Lastly, I often see multiple bass units used in dipole, and assume that is mainly for help in augmenting the bass response. However, is it also to help in cancelling vibrations??? In other words, is it indeed feasible to have a single 18 doing the dipole bass??
enjoying the learning experience here
Welcome back Lynn,
The point missing from the trade imbalance discuss was that all we do is send over pieces of paper that are easy to print, and we get back real stuff. Where we get in trouble is when they send us cheap junk, and they use the paper to buy fleets of aircraft from us. Once they figure out all we send is paper, or they just decide to keep everything they make for themselves, then we're in deep trouble.
I'll be sure to include impulse responses when I start measuring. I have 2 other B200's, so I can do flat baffle too, or do I need to use the same driver?
2 things concern me about woofers mounted sideways. 1. You lose potential rear wave delay. 2. not firing the woofer at the listening position has to have consequences with open alignments. If you make it a manifold to fire essentially into a common air cushion, doesn't that affect impulse response.
I've gone both ways, but have adopted mounting the woofer to a base separate from the baffle with better success. In both alignments below, the woofer is isolated by foam rubber from the baffle, and the woofer energy goes into it's separate base, and isolated completely from the baffle structure and/or support structure of other drivers. While these are minimalist forms with limited max SPL, the same approach can be fitted to any baffle size, shape, or driver compliment. Once you don't have to put everything in a box, many things become much easier. Oh before I forget, AJ I don't have measurements for these either.
The point missing from the trade imbalance discuss was that all we do is send over pieces of paper that are easy to print, and we get back real stuff. Where we get in trouble is when they send us cheap junk, and they use the paper to buy fleets of aircraft from us. Once they figure out all we send is paper, or they just decide to keep everything they make for themselves, then we're in deep trouble.
I'll be sure to include impulse responses when I start measuring. I have 2 other B200's, so I can do flat baffle too, or do I need to use the same driver?
2 things concern me about woofers mounted sideways. 1. You lose potential rear wave delay. 2. not firing the woofer at the listening position has to have consequences with open alignments. If you make it a manifold to fire essentially into a common air cushion, doesn't that affect impulse response.
I've gone both ways, but have adopted mounting the woofer to a base separate from the baffle with better success. In both alignments below, the woofer is isolated by foam rubber from the baffle, and the woofer energy goes into it's separate base, and isolated completely from the baffle structure and/or support structure of other drivers. While these are minimalist forms with limited max SPL, the same approach can be fitted to any baffle size, shape, or driver compliment. Once you don't have to put everything in a box, many things become much easier. Oh before I forget, AJ I don't have measurements for these either.
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An externally hosted image should be here but it was not working when we last tested it.
Deqx question
"However, again when you answered his post re using the deqx, you again mentioned the behringer, and that has left me a little confused as the deqx is a totally different beast to any behringer unit, and I'm wondering if you have gotten the two products mixed up. Indeed, did ctrevith himself mean a behringer unit or the deqx unit. Praps we had better clear all that up???"
Thanks for the comments Lynn. Yes, as terry j mentions, the unit I was referring to was the Deqx unit: http://www.deqx.com/PDC26P-Preamp.html, not the Behringer.
"However, again when you answered his post re using the deqx, you again mentioned the behringer, and that has left me a little confused as the deqx is a totally different beast to any behringer unit, and I'm wondering if you have gotten the two products mixed up. Indeed, did ctrevith himself mean a behringer unit or the deqx unit. Praps we had better clear all that up???"
Thanks for the comments Lynn. Yes, as terry j mentions, the unit I was referring to was the Deqx unit: http://www.deqx.com/PDC26P-Preamp.html, not the Behringer.
Yes, I was thinking of the Behringer DQX, not the DEQX. Similar names are confusing!
Naturally, with an all-digital source, you can do any kind of digital signal bending you want, limited only by the bit depth of the computing engine, and you're free to choose high-quality DACs on the other end.
As for the bass unit, well ... if you take a purist all-flat-baffle dipole approach with no W or U-box for the bass, you're going to need a LOT of equalization or a REALLY BIG (wall-sized) dipole baffle. Folks who get cute and make a room-divider folding baffle with a piano hinge are sort of kidding themselves - small air leaks through the hinge reduce the effective size to not much larger than the hinge-to-baffle-edge distance.
I still like the idea of a small, fairly deep box with an open back and and mesh termination on the sides and top. This has the merit of three surfaces available for driver mounting, a straight air-path for the front drivers, and an air-path with only one relatively unconstrained 90-degree turn for the side drivers.
Yes, the backwave is radiating into a more constrained environment, but it emerges into a combination of resistive damping material and mesh side walls. This is quite a different animal than a six-sided box with resistive filling, or a transmission line, which is a tube with resistive filling.
The diagram a few pages back with the rear-wave proportionally split between an open-back box and a folded transmission line really got my attention. It would have never have occured to me to split the rear-wave into two quite different kinds of rear loading.
However - that does prompt a thought. Instead of having an enclosure that splits the backwave into two very different lengths, why not create a continuously lossy transmission line, say with a mesh wall going all the way down the line?
So you've got a typical 1/4 wave line tuned to 45 Hz and six feet long, folded as many times as you want, but the whole thing is filled with recycled cotton waste and has a heavily resistive felt-covered mesh replacing one of the side walls. The thing "leaks" along its entire length, but that's actually a design feature!
This is kind of the inverse of the TQWT - instead of an intentionally resonant pipe with a bit of horn-gain, it's a semi-transparent tube with losses along its entire length. The gain comes from the driver array, not horn-gain, so it's a different kind of sound than a TQWT or a transmission line.
You can play with the TL vs resistive-loss characteristics by having a clear air-path go through the center of the line, or filling the the entire pipe with filling material, as you wish. That controls the ratio of the energy coming out the distant end vs the leaks over the entire length - easily measured, by the way, by measuring the spectrum and magnitude of the nearfield output at the far end of the pipe. Alert readers will notice the similarity to automotive muffler design, using lossy baffles and filling material.
Naturally, with an all-digital source, you can do any kind of digital signal bending you want, limited only by the bit depth of the computing engine, and you're free to choose high-quality DACs on the other end.
As for the bass unit, well ... if you take a purist all-flat-baffle dipole approach with no W or U-box for the bass, you're going to need a LOT of equalization or a REALLY BIG (wall-sized) dipole baffle. Folks who get cute and make a room-divider folding baffle with a piano hinge are sort of kidding themselves - small air leaks through the hinge reduce the effective size to not much larger than the hinge-to-baffle-edge distance.
I still like the idea of a small, fairly deep box with an open back and and mesh termination on the sides and top. This has the merit of three surfaces available for driver mounting, a straight air-path for the front drivers, and an air-path with only one relatively unconstrained 90-degree turn for the side drivers.
Yes, the backwave is radiating into a more constrained environment, but it emerges into a combination of resistive damping material and mesh side walls. This is quite a different animal than a six-sided box with resistive filling, or a transmission line, which is a tube with resistive filling.
The diagram a few pages back with the rear-wave proportionally split between an open-back box and a folded transmission line really got my attention. It would have never have occured to me to split the rear-wave into two quite different kinds of rear loading.
However - that does prompt a thought. Instead of having an enclosure that splits the backwave into two very different lengths, why not create a continuously lossy transmission line, say with a mesh wall going all the way down the line?
So you've got a typical 1/4 wave line tuned to 45 Hz and six feet long, folded as many times as you want, but the whole thing is filled with recycled cotton waste and has a heavily resistive felt-covered mesh replacing one of the side walls. The thing "leaks" along its entire length, but that's actually a design feature!
This is kind of the inverse of the TQWT - instead of an intentionally resonant pipe with a bit of horn-gain, it's a semi-transparent tube with losses along its entire length. The gain comes from the driver array, not horn-gain, so it's a different kind of sound than a TQWT or a transmission line.
You can play with the TL vs resistive-loss characteristics by having a clear air-path go through the center of the line, or filling the the entire pipe with filling material, as you wish. That controls the ratio of the energy coming out the distant end vs the leaks over the entire length - easily measured, by the way, by measuring the spectrum and magnitude of the nearfield output at the far end of the pipe. Alert readers will notice the similarity to automotive muffler design, using lossy baffles and filling material.
Hi Lynn,
I agree with you about mesh/felt sides.
I tried similar and reported good findings to Fullrangedriver Forum, also the much improved reproduction from mounting a full-ranger against a two-three turns pipe of heavy carpet, itself 'soft' plugged with carpet.
When there are solid sides behing a driver there is no getting away from rear pressure and reflections causing cone re-excitation apparent from the front.
For 'sides' I used steel mesh (of the type used for open mezanine floors) covered with heavy carpet, and the front of driver output was very much improved. Only thing was I did not have enough materials or the capabilities to make an entire cabinet. I also wondered how the construction could be made long term acceptable in a home environment.
If the side of a TL is to have controlled leaky damping does the far end still need to be open ?
Cheers .......... Graham
I agree with you about mesh/felt sides.
I tried similar and reported good findings to Fullrangedriver Forum, also the much improved reproduction from mounting a full-ranger against a two-three turns pipe of heavy carpet, itself 'soft' plugged with carpet.
When there are solid sides behing a driver there is no getting away from rear pressure and reflections causing cone re-excitation apparent from the front.
For 'sides' I used steel mesh (of the type used for open mezanine floors) covered with heavy carpet, and the front of driver output was very much improved. Only thing was I did not have enough materials or the capabilities to make an entire cabinet. I also wondered how the construction could be made long term acceptable in a home environment.
If the side of a TL is to have controlled leaky damping does the far end still need to be open ?
Cheers .......... Graham
Graham Maynard said:
Yes. I see no performance advantages to a closed-end TL, just more obnoxious resonance. The concept here is an enclosure that is transitional between a traditional TL and an open-baffle, as insane as that sounds. The transition is controlled by the resistance of the mesh and the percentage of the open aperture of the transmission line from front to back.
You can go even further by having a Mesh TL that is 100% TL closest to the driver (rigid walls and open space along the centerline of the pipe), and gradually becoming 100% mesh (mesh walls and all filling in the middle) by the time you get to the end. This would give the desired progressive-leak characteristic - for once, a truly non-resonant line.
How on earth did I miss this thread..?
My thoughts have been along similar lines for a good while now, but the efficiency area is where we're differing.
My concept uses a smallish fullrange driver, and the OB baffle rolloff to provide the low pass filtering, and a simple 1st order or so crossover at say 10K to a reasonable tweeter. The target efficiency for this part of the system is 90 or so db/watt. Bass is augmented by a 12" prosound woofer also on an OB, and a simple inductor high pass. Target efficency of the bass stage is 96 or so db.
For a 300Hz OB (1 foot or so wide), the XO of the bass stage will be in the range of 150 - 225Hz (give or more likely take a little), taking real world bass response down to around 100Hz, with low compression, and low equalisation. Add on the fact that the bass driver could be supplemented below this by a pair of drivers, using another low pass roll off at something like 50Hz (again run OB), you'd take the 1/4 space efficiency gain of the lowest driver, and should go flat to 50 Hz, and 12 db roll off after that.
Thats as far as I've thunk on this, but If I build this, it'll be active.
Oh, as an aside, the center of the fullranger would be around 38" from the floor in something like this - ideal for some-one like me (tall)
Just the results of my addled brain.
I also like the look of something like the eminence alpha 6 as a high efficiency OB wide-ranger - as it would have the prosound headroom, and efficiency, the downside is then the bass section would need to be more efficient
Owen
My thoughts have been along similar lines for a good while now, but the efficiency area is where we're differing.
My concept uses a smallish fullrange driver, and the OB baffle rolloff to provide the low pass filtering, and a simple 1st order or so crossover at say 10K to a reasonable tweeter. The target efficiency for this part of the system is 90 or so db/watt. Bass is augmented by a 12" prosound woofer also on an OB, and a simple inductor high pass. Target efficency of the bass stage is 96 or so db.
For a 300Hz OB (1 foot or so wide), the XO of the bass stage will be in the range of 150 - 225Hz (give or more likely take a little), taking real world bass response down to around 100Hz, with low compression, and low equalisation. Add on the fact that the bass driver could be supplemented below this by a pair of drivers, using another low pass roll off at something like 50Hz (again run OB), you'd take the 1/4 space efficiency gain of the lowest driver, and should go flat to 50 Hz, and 12 db roll off after that.
Thats as far as I've thunk on this, but If I build this, it'll be active.
Oh, as an aside, the center of the fullranger would be around 38" from the floor in something like this - ideal for some-one like me (tall)
Just the results of my addled brain.
I also like the look of something like the eminence alpha 6 as a high efficiency OB wide-ranger - as it would have the prosound headroom, and efficiency, the downside is then the bass section would need to be more efficient
Owen
In 1951 W.A.Hartley of Hartly-Turner was singing the praises of his 215 full range loudspeaker in a Boffle, which basically was an 18 inch open backed cube, with nine internal divisions of heavy felt, spaced approximately 1 7/8 inches apart. Each division had a different sized hole, with the final one having no hole to form the back. He claimed the sound was improved because of the non-resonance design of this enclosure.
He produced two frequency response curves for the same speaker, one mounted in a large flat wall, the other in the boffle. The curves above 100 Hz were very similar indeed, but below that the frequency response of the boffle fell at double the rate of the wall, about 10db per octave from memory, as might be expected.
Many years later I built a boffle with a cheapish 12" unit that ideally required a five cub.ft BR cabinet, to compare the sound of the two cabinets, (hoping to get a box of more acceptable size). The sound from normal listening position was obviously cleaner than the BR, but also obviously had less bass output. Only a low level rumble could be heard with an ear placed close to the back.
The same unit in a large OB was quite unsatisfactory. I did not follow this line of thought, but moved to TL's via Voight QW designs.
It seems to me that Lynn's suggestion of a leaky boffle, with modern equalisation may well be worth pursuing.
He produced two frequency response curves for the same speaker, one mounted in a large flat wall, the other in the boffle. The curves above 100 Hz were very similar indeed, but below that the frequency response of the boffle fell at double the rate of the wall, about 10db per octave from memory, as might be expected.
Many years later I built a boffle with a cheapish 12" unit that ideally required a five cub.ft BR cabinet, to compare the sound of the two cabinets, (hoping to get a box of more acceptable size). The sound from normal listening position was obviously cleaner than the BR, but also obviously had less bass output. Only a low level rumble could be heard with an ear placed close to the back.
The same unit in a large OB was quite unsatisfactory. I did not follow this line of thought, but moved to TL's via Voight QW designs.
It seems to me that Lynn's suggestion of a leaky boffle, with modern equalisation may well be worth pursuing.
Sorry, I meant H.A.Hartley. The firm at that time was H.A.Hartley Co based in London. It produced only one design, a full range 4 ohm unit with an 8" cone using an alinco magnet. The cone was curvalinear, and divided by a compliance at about the three inch diameter giving a mechanical crossover, and the voice-coil isolated with a latex coating over an aluminium former. The surround was untreated cloth. A very interesting design.One recently sold on ebay for big money.
The firm later became Hartley-Turner, producing a range of sizes including 12" and I think 15" for sale in the States.
The firm later became Hartley-Turner, producing a range of sizes including 12" and I think 15" for sale in the States.
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Joined 2003
I just finished a boffle-like midbass baffle for testing this weekend.
The top and bottom are closed (to sit on a woofer module and support a tweeter module above). The sides and back are open, with wool felt in a semi-circle side to side. Felt density is adjustable by changing the number of layers and position around the semi-circle.
I'll post a picture if interested.
The top and bottom are closed (to sit on a woofer module and support a tweeter module above). The sides and back are open, with wool felt in a semi-circle side to side. Felt density is adjustable by changing the number of layers and position around the semi-circle.
I'll post a picture if interested.
Is it established that this is true for low frequencies?
You won't be "fooling yourself" if you use the "Clear Flat Hinges" here:
http://www.outwatercatalogs.com/2006_master/lg_display.cfm?page_number=186&catalog=otm
They fit into a saw kerf in the edge of the panel.
planet10 said:
handy : http://www.pat2pdf.org/
btw that 434,563 is combined latch and lock
useful- but for other type of boxes
It's very difficult to get you guys out of box mode. Sure if the hinged wings are folded sharply back, pressure is contained in the cavity and some may leak through the hinges, but the effect is going to be far less than you think. The rear wave is going to take the path of least resistance for the most part. I've played woofers with about 1/2" of space between the driver and the baffle all the way around and it had surprizingly little effect compared to being well sealed.
I'm sure there are all kinds of imaginative ways we can come up with to absorb the rear wave and retain an open look to the thing, but doing so to obtain bass means a shift toward monopole and detracts from the biggest advantage of open alignments. Plus things like a "boffle" will have an effective range sloped in the wrong direction. We need more attenuation the lower the frequency, ideally at a 6db/oct slope.
Before I tried the dual pathway with a Helmholtz slot as a divider to send only LF to the longer pathway, I tried a series of low tuned helmholtz traps for the rear wave. I never could get that method to make more than a barely audible difference, and I had much more success with the slot arrangement feeding an expanding pathway.
Back then I was wanting to create a single driver speaker with an open alignment that didn't need the help of a sub, a true minimalist OB. It works well with a 15" coax, but they're too big for most. I also tried it with a B200, which didn't achieve the bass levels I wanted. I really need a 10"coax with a mid Q and 7mm of more of useable Xmax. Here's the B200 version, same concept as before, just a different layout:
I'm sure there are all kinds of imaginative ways we can come up with to absorb the rear wave and retain an open look to the thing, but doing so to obtain bass means a shift toward monopole and detracts from the biggest advantage of open alignments. Plus things like a "boffle" will have an effective range sloped in the wrong direction. We need more attenuation the lower the frequency, ideally at a 6db/oct slope.
Before I tried the dual pathway with a Helmholtz slot as a divider to send only LF to the longer pathway, I tried a series of low tuned helmholtz traps for the rear wave. I never could get that method to make more than a barely audible difference, and I had much more success with the slot arrangement feeding an expanding pathway.
Back then I was wanting to create a single driver speaker with an open alignment that didn't need the help of a sub, a true minimalist OB. It works well with a 15" coax, but they're too big for most. I also tried it with a B200, which didn't achieve the bass levels I wanted. I really need a 10"coax with a mid Q and 7mm of more of useable Xmax. Here's the B200 version, same concept as before, just a different layout:
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An externally hosted image should be here but it was not working when we last tested it.
Hi Variac,
I'm not sure where your 'hinges' quote comes from.
It has been my experience that gapping a solid baffle from the wings (as with hinges) does much to relieve the increased rear of cone pressure from deleteriously affecting the forward response.
I would not imagine the boffle design releasing the immediate rear pressure, though what Lynn has described could.
I can also imagine John's B200 OB-RLH design sounding better in the forward direction because the rear pressure has an 'escape' route, yet one which is then put to good use by augmenting the overall bass response.
Cheers .........Graham.
I'm not sure where your 'hinges' quote comes from.
It has been my experience that gapping a solid baffle from the wings (as with hinges) does much to relieve the increased rear of cone pressure from deleteriously affecting the forward response.
I would not imagine the boffle design releasing the immediate rear pressure, though what Lynn has described could.
I can also imagine John's B200 OB-RLH design sounding better in the forward direction because the rear pressure has an 'escape' route, yet one which is then put to good use by augmenting the overall bass response.
Cheers .........Graham.