Wide range dipoles have two major challenges - low bass below say 60Hz and treble above 2kHz.
The bass issue comes from dipole principle, "loss by summation of opposite polarity of waves" When wavelength grows, it gets very difficult to compensate. Another problem comes when we put the radiator in a room with some walls and other boundaries. Easily it happens that backwave gets boosted when bouncing back from front wall, still polarity remains same (negative) if distance to wall is short relative to wavelength. Then at certain frequency the bounced wave is again in opposite phase to fronside wavefront and nulling happens. Above that boosting-nullings happens periodically.
So, a dipole bass only changes the frequency range of destructive interferences! Member kimmosto has measured cardioid vs dipole bass in a room
and John Kreskowsky tells theory
Basics told:
Speaker Placement 101: How to Fight Boundary Interference (monopole source)
Dan Russell's Acoustics and Vibration Animations
This is why I decided to use monopole bass below 200Hz. When monopole is combined with dipole, there is cardioid pattern in crossover region. This seems to work well in practise. Same principle is used for Gradient 1.x and Martin Logan panels.
Craw woofer
http://www.musicanddesign.com/VariabDF.html
The bass issue comes from dipole principle, "loss by summation of opposite polarity of waves" When wavelength grows, it gets very difficult to compensate. Another problem comes when we put the radiator in a room with some walls and other boundaries. Easily it happens that backwave gets boosted when bouncing back from front wall, still polarity remains same (negative) if distance to wall is short relative to wavelength. Then at certain frequency the bounced wave is again in opposite phase to fronside wavefront and nulling happens. Above that boosting-nullings happens periodically.
So, a dipole bass only changes the frequency range of destructive interferences! Member kimmosto has measured cardioid vs dipole bass in a room
and John Kreskowsky tells theory
Basics told:
Speaker Placement 101: How to Fight Boundary Interference (monopole source)
Dan Russell's Acoustics and Vibration Animations
This is why I decided to use monopole bass below 200Hz. When monopole is combined with dipole, there is cardioid pattern in crossover region. This seems to work well in practise. Same principle is used for Gradient 1.x and Martin Logan panels.
Craw woofer
http://www.musicanddesign.com/VariabDF.html
Let me give you an example of an H-frame that I built and measured as a test bed early last year (currently disassembled). This used an 18" 90dB/W high-Xmax subwoofer in an H-frame dimensioned 24"W, 24"H and 30"D.
I measured the response at the front and rear openings, in the plane of the opening, in the middle of the opening. From that I can create a model of the dipole without having try and measure it directly. This is because at low frequencies wavelengths are long and you can model the front and rear as two monopoles, the rear one reversed in polarity, and separated by D=30". The first plot shows the SPLs of front (cyan) rear (yellow) and the dipole response (dark blue) at a distance of 2m in front of the H-frame. The levels have been normalized so that the front SPL is 0dB in the region of 30-50Hz. I also compensated for the extra distance the rear wave must travel to the listening position in its SPL, and this results in about a 3dB lower level for it at the listening position as shown in the plot. To make the blue (system) curve I take the FRD files and apply an inversion and delay, plus the -3dB SPL distance correction, to the rear data before summing them.
The CYAN curve is the measured response at the front opening.
The YELLOW curve is the measured response at the rear opening.
The BLUE curve is the synthesized H-frame response (its dipole response) at the listening position, 2m distant and in front of the front opening.
Looking at the plot above, we can observe the following features:
- The driver response, e.g. at the front opening of the H-frame, is relatively flat to 30Hz.
- There are some resonances in the front and rear response between 70Hz and 150Hz.
- The 1/4 wavelength transmission line resonance of the H-frame creates a null around 375Hz and the response starts falling into this around 150Hz.
- The H-frame response has a dipole peak around 140Hz, and this is +6dB WRT the 0dB level
- Below the dipole peak there is a region with a 6dB/oct slope from about 40Hz to 120Hz
- Below 40Hz the rolloff resulting from the driver's fundamental resonance kicks in and the response falls more quickly but only approaching 12dB/oct from 10Hz-40Hz
- At 40Hz the sensitivity of the dipole is only 3dB less than the response at the front, which is essential the 90dB/W free air sensitivity of the driver, so 87dB/W@1m
In the above curve for the correction filter I am applying 3dB of boost at 40Hz. Boost increases below 40Hz to lift the response and only a LP filter type rolloff is used to both compensate for the dipole peak and create a LP crossover filter. This is possible because the Q of the dipole peak is not too high and the necessary correction is simple. It is a similar approach to the one that Martin King used in his paper comparing an OB, U-frame, and H-frame.
Below is the resulting H-frame dipole subwoofer frequency response, at the listening position:
The response is flat to 20Hz. Even though the correction filter applies up to 12dB of boost, this happens at low frequencies and the relative program level near 20Hz is not as high as above 100Hz so this much boost is not so difficult to manage. Less boost can be used at the cost of some low frequency extension. The H-frame dimension D and tunnel cross section vs Sd has been chosen so that the dipole peak is about where you want to cross over to the next higher band, and the Q of the peak is not too high.
I did build this subwoofer and I knew the first time I ran an impulse sweep on it that it had some impressive bass. Even if you did not use boost (eg. a passive filter only) the response extends to 40Hz before rolling off. I took the data and did the modeling, and this is what I had planned to create but I had to disassemble the system. I hope to build it again soon with some nice, heavy materials to keep the H-frame in place and resist the cone movement.
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seriously though, it's nice to see some sanity in the audio world, for a change.The room was about 20x15 feet. I just tested the sub (made measurements on it) out in the middle of the room. It was never really set up for listening.
Also, keep in mind that I did not measure the response in room, I only predicted it from a model. It should be accurate, but the model did not include the room boundaries, room reflections, relative location in the room, and so on. But I did listen to the un EQd sub during testing and was impressed.
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Your use of a monopole for the lowest band, and the resulting cardioid pattern in the crossover band to the next higher one, is certainly one viable approach. It doesn't fit into my 20-20k dipole approach, however. That doesn't mean you are wrong or I am right, I am just trying to achieve a different goal than you have done.
I have seen lots made about how bad dipole are at low frequencies because of this and that. But personally I have heard several dipole systems with excellent bass. An (early?) version of John Bush's "Widowmaker" used a large planar baffle and two 18" woofers. It had quite astonishing low bass impact. Siegfried Linkwitz's LX521 uses two 10" Seas woofer in a short M-frame and that has lots of bass, too, even in a small listening room (e.g. at a Burning Amp, not Fort Mason). So it seems that these dire predictions just don't quite measure up in real life from what I have heard.
How to say this.... I have always found the belief that dipoles can not work well below 60 or so Hz to be curious. It just comes down to volume of displacement (VD). A pair of 18" woofers I was referring to in my previous post have, at 32 Hz, a useful one way displacement of 2,695 CC. A stereo pair is of course twice that at 5,390 CC! For comparison, a typical long throw 12" sub woofer is around 400-500 CC. A big OB just overwhelms the losses. I have demo'd OB's at a number of venues, small private to RMAF. Have yet to have anyone comment on the lack of or quality of bass. If anything, a few thought there was a bit too much. There are very few commercial speakers systems intended for residential use that can deliver an acoustic watt at 32 Hz. Most can't even approach that level of output.
Admittedly, a dual 18" OB is a bit of overkill and not for every one. Even my relatively small Manzanita with a single medium throw 12" or short throw 15" provides more and deeper low distortion bass than the average high end speaker. Most traditional box loaded (sealed or ported) start to roll off on the low end around 60 Hz. Very few are flat at or below 50 Hz. The Manzanita is flat at 42 Hz.
Admittedly, a dual 18" OB is a bit of overkill and not for every one. Even my relatively small Manzanita with a single medium throw 12" or short throw 15" provides more and deeper low distortion bass than the average high end speaker. Most traditional box loaded (sealed or ported) start to roll off on the low end around 60 Hz. Very few are flat at or below 50 Hz. The Manzanita is flat at 42 Hz.
Agree - it is a matter of volume displacement only. I employ 6 x 12" in a stereo H-Frame tower. Very shallow wings - 3". These drivers cost me $18 each - so I discount their 8mm peak xmax spec. Probably more like 8mm peak to peak. This would put volume displacement around 2k cubic centimeters. If it were not for the need to reproduce movie bass - I'd target smaller drivers (in an array) and get some extra bandwidth.
This is a dual purposed system - high quality audio 90% of the time and the occasional movie. Not for everyone of course but they are flat to 30 Hz with EQ.
The bass is so clean and authoritative that I tend to overemphasize it and set the gain for the low channels too high.
This is a dual purposed system - high quality audio 90% of the time and the occasional movie. Not for everyone of course but they are flat to 30 Hz with EQ.
The bass is so clean and authoritative that I tend to overemphasize it and set the gain for the low channels too high.
Originally Posted by fporegon View Post
Agree - it is a matter of volume displacement only.
Agreement with previous post from John Busch. Bass reproduction below 60 Hz is a matter of moving air. Multiple drivers or 18” long throw drivers may be needed, but there is no fundamental reason that a dipole speaker can’t perform down to 20 Hz.
Sorry if not clear.
Agree - it is a matter of volume displacement only.
Agreement with previous post from John Busch. Bass reproduction below 60 Hz is a matter of moving air. Multiple drivers or 18” long throw drivers may be needed, but there is no fundamental reason that a dipole speaker can’t perform down to 20 Hz.
Sorry if not clear.
I have always found the belief that dipoles can not work well below 60 or so Hz to be curious? huh ? open baffles struggle to compensate against the laws of physics; you know---the ole acoustic short circuit. I have noticed over the many, many years of reading this type of stuff, that denial of the front-to-back cancellation is quite common among OB enthusiasts.
Another thing, where do you get the notion that most traditional box loaded (sealed or ported) start to roll off on the low end around 60 Hz ? Woudn't the performance be tied to the design/and box size ?
I'll tell you what. With respect to your 2x18 inch drivers for open baffle. If you would compare those two, to JUST ONE 18 inch loaded in a properly designed bass reflex, the B.R. would blow the OB's out of the water, in terms of bass quality and efficiency.
I admire your enthusiasm, but I don't share it.
If only it were that simple, thankfully it isn't
It would have to be a very poorly implemented OB to be beaten in terms of quality by even the best implemented BR
Your biased opinion has been duly noted.
Now, how about actually trying ?
Do you know anybody who can properly build a bass reflex design ?
There shall be no sabotage allowed !
Best wishes for a healthy and prosperous new year !!
Best wishes for a healthy and prosperous new year !!
selective editing ?
Here ya go (in full)
Your biased opinion has been duly noted.
Now, how about actually trying ?
Do you know anybody who can properly build a bass reflex design ?
There shall be no sabotage allowed !
Best wishes for a healthy and prosperous new year !!
selective editing ?
Here ya go (in full)
Your biased opinion has been duly noted.
Now, how about actually trying ?
Do you know anybody who can properly build a bass reflex design ?
There shall be no sabotage allowed !
Best wishes for a healthy and prosperous new year !!
Scott, the BR has relatively poor transient response, even when "done right". For an 18" driver the box will need to be very large in order to keep the port length, and therefore port resonances, to a minimum. It must be the size of a big refrigerator if you want low tuning (e.g. 20Hz or less). And it will need some good bracing to keep the walls stiff. It won't be very portable...
SL in one of his last talks at Burning Amp mentioned how he strongly dislikes reflex designs and felt that they all had "boom", e.g. the resonator droned on excessively when excited by the driver. He probably never heard one "done right" I guess.
The excellent transient response of an OB and the lack of any port or internal box resonances give a big advantage to dipole bass. It's true that the "acoustic short circuit" must be dealt with, but as I have shown in my example this is not so difficult when you size the baffle to be large enough and fold it into a U- or H-frame to make it visually small(er).
I have tried to build "slim" open baffle loudspeakers and they pretty much matched your expectations of "thin sounding". Even when I applied lots of bass lift, it just couldn't really overcome the front-back cancellation. But I have found that by using an H-frame I can overcome this problem. I hope you will someday hear that kind of system, that is a "properly designed dipole subwoofer" system, without any "sabotage", as you say!
Another long winded reply from what's his name...
Scott…. Your thoughts and conclusions make a lot of sense. However, we need to focus on outcomes…. Yes, you can get a lot of output from a quality 18” woofer employed as a mono-pole source (closed, ported or horn loaded boxes of various alignments). I build a lot of box speaker designs too. Be they pro or residential.
My outcome goals for the bass end of any OB I design – build is natural low distortion bass in RESIDENTAIL sized spaces. Measures flat on axis in most residential environments. Minimum standing waves – room excitement - interaction. Reasonable efficiency that can be directly driven by any competent Hi-Fi stereo amplifier. (88-92 dbw) And be able to play loud enough and low enough to handle just about any musical input thrown at it.
The only exception is the DIY Manzanita which is a small OB with very low cost drivers that comes in at 85 DBW. This efficiency is pretty typical of a small to medium sized stand mounted commercial speakers that can play flat to around 60 Hz. The Manzanita will play flat to about 42 - 45Hz.
One thing I would like you to do, just for giggles. Design a subwoofer, 18” class driver of course, that will play flat to 32 Hz with a net efficiency of at least 1% (92 dbw) capable of delivering a minimum of 1 acoustic watt (109 db) at 32 Hz at one meter without exceeding Xmax. (Half space mesurement) Make sure it can do that sitting on the floor away from the rear wall, say 24” to minimize standing waves (exciting the room). Plug and play. No EQ other than passive. Something you can drive with any competent HiFi stereo amplifier.
I think you may be surprised at how large the box is going to be. And the bass quality it will deliver in a typical residential environment will be, based on many years of experience, grossly inferior to what an OB can provide. The room interaction from a sub that large is going to be significant. You can remediate some of the interaction problems with bass traps and other techniques. But you can’t come close to eliminating the issue. You basically have a resonating box playing in a larger resonating box! And the two do not play well together.
I think you are blinded by your perception of “acoustic short circuit” negatives, not the positives the “acoustic short circuit” can provide. My opinion of course. But to be fair, 18” woofers, when box loaded are designed to play loud in large spaces, not small ones. My basic point is there are a lot of ways around the barn. Each has its pluses and minuses. I think we can agree it is about quality of outcome. I get the sense you have yet to hear a properly designed Open Baffle. I have listened to the best of the Open Baffles, Horns, Sealed, Transmission lines and Ported box format speakers the world has to offer. Will stick with a properly designed OB for most residential environments.
BTW, if you look at the true anechoic response of most speakers, the vast majority start to roll off around 60 Hz. Not all of course. The JBL K2 starts to roll of at 60 Hz and is down at least 3 db at 48 Hz. This is a well regarded $40K a pair speaker that uses a 15” woofer. Stated efficiency is 93 dbw.
Scott…. Your thoughts and conclusions make a lot of sense. However, we need to focus on outcomes…. Yes, you can get a lot of output from a quality 18” woofer employed as a mono-pole source (closed, ported or horn loaded boxes of various alignments). I build a lot of box speaker designs too. Be they pro or residential.
My outcome goals for the bass end of any OB I design – build is natural low distortion bass in RESIDENTAIL sized spaces. Measures flat on axis in most residential environments. Minimum standing waves – room excitement - interaction. Reasonable efficiency that can be directly driven by any competent Hi-Fi stereo amplifier. (88-92 dbw) And be able to play loud enough and low enough to handle just about any musical input thrown at it.
The only exception is the DIY Manzanita which is a small OB with very low cost drivers that comes in at 85 DBW. This efficiency is pretty typical of a small to medium sized stand mounted commercial speakers that can play flat to around 60 Hz. The Manzanita will play flat to about 42 - 45Hz.
One thing I would like you to do, just for giggles. Design a subwoofer, 18” class driver of course, that will play flat to 32 Hz with a net efficiency of at least 1% (92 dbw) capable of delivering a minimum of 1 acoustic watt (109 db) at 32 Hz at one meter without exceeding Xmax. (Half space mesurement) Make sure it can do that sitting on the floor away from the rear wall, say 24” to minimize standing waves (exciting the room). Plug and play. No EQ other than passive. Something you can drive with any competent HiFi stereo amplifier.
I think you may be surprised at how large the box is going to be. And the bass quality it will deliver in a typical residential environment will be, based on many years of experience, grossly inferior to what an OB can provide. The room interaction from a sub that large is going to be significant. You can remediate some of the interaction problems with bass traps and other techniques. But you can’t come close to eliminating the issue. You basically have a resonating box playing in a larger resonating box! And the two do not play well together.
I think you are blinded by your perception of “acoustic short circuit” negatives, not the positives the “acoustic short circuit” can provide. My opinion of course. But to be fair, 18” woofers, when box loaded are designed to play loud in large spaces, not small ones. My basic point is there are a lot of ways around the barn. Each has its pluses and minuses. I think we can agree it is about quality of outcome. I get the sense you have yet to hear a properly designed Open Baffle. I have listened to the best of the Open Baffles, Horns, Sealed, Transmission lines and Ported box format speakers the world has to offer. Will stick with a properly designed OB for most residential environments.
BTW, if you look at the true anechoic response of most speakers, the vast majority start to roll off around 60 Hz. Not all of course. The JBL K2 starts to roll of at 60 Hz and is down at least 3 db at 48 Hz. This is a well regarded $40K a pair speaker that uses a 15” woofer. Stated efficiency is 93 dbw.