Dr. Geddes,
Could you comment on whether there were errors from the perspective of the Webster style theory? What I mean is if one didn't use your more modern approach to horn theory, and only viewed it from the older more basic approach is the information still riddled with errors? (kind like from the perspective of newtonian mechanics versus modern physics)
I have never read any thing on this subject before, so I found it enlightening. Especially since some of the math was hinted at (I took a few PDE courses in school). I would find it useful to understand what aspect of it you find flawed generally.
Could you comment on whether there were errors from the perspective of the Webster style theory? What I mean is if one didn't use your more modern approach to horn theory, and only viewed it from the older more basic approach is the information still riddled with errors? (kind like from the perspective of newtonian mechanics versus modern physics)
I have never read any thing on this subject before, so I found it enlightening. Especially since some of the math was hinted at (I took a few PDE courses in school). I would find it useful to understand what aspect of it you find flawed generally.
JoshK said:
I just finished my rebuttal paper for AudioXPress and maybe I'll post it on my web site after I send it out (in case its too inflamatory and they won't print it).
The Newtonian analogy is not appropriate. Newtonian Physics is completely accurate for our everyday real world experinces. Websters equation is not. It is not accurate in 90% of the cases where it is applied.
The paper is accurate in that it accurately reflects the thinking at the time, as erronious as it was. But then when he gets to my work the errors are quite distinct and do not correctly reflect my theories or others works coincident with mine.
Hope this helps till my more complete discussion is available.
Lynn Olson said:
Sorry but this is a loaded question. It is impossible to correctly answer because there are just too many variables. Many, many, variables ranging from the horn, to the mating of the lower range driver, to what I ate for breakfast to the room, to the crossover, to the amplifier, to the piece of music to......
JoshK said:
I have posted my response on my web site www.gedlee.com check it out. Its too big to attach here.
Dr. Geddes, you've written an very interesting and illuminating letter to Ed Dell. I have no idea if it will be reprinted in AudioXpress magazine, but I am deeply appreciative we can read it for ourselves on your website.
I always like to read a good patent - your Phase Plug with Optimized Shapes, Patent #7095868 is a good read, along with the referenced 1936 classic by Wente (Patent #2037817), and Henricksen's 1977 "Tangerine" variant (Patent #4050541).
I fully agree that diffraction and internal reflection is the primary problem in traditional, Webster-theory horns, and impulse measurements with substantial amounts of stored energy confirm that - and very likely the reason that impulse measurements are rarely seen in the horn literature, while they are much more commonly seen with direct-radiators, going back to the mid-1970's.
It is significant that 1950's-era Altec and JBL high-efficiency loudspeakers have complex, diffraction-inducing acoustical paths - phase-plugs exits with a 50% reflective metal-mesh bug screen a short distance away, a mismatch between the flare rate at the exit of the compression driver and the entrance to the throat of the horn, and in the more notorious JBL designs, another diffraction grating at the mouth of the horn, in the form of an "acoustic lens". Two decades later, more diffraction in the Altec Manta-Ray and JBL Bi-Radial horns for theater and studio-monitor use.
Plainly enough, time-domain performance was not a design consideration, with such a casual approach to intentional induction of diffraction. The attitude in professional designs circles seems to have been that any problem can be equalized away, and if the time domain is further degraded, it doesn't really matter all that much. This is the part of the prosound legacy that needs to be discarded.
I always like to read a good patent - your Phase Plug with Optimized Shapes, Patent #7095868 is a good read, along with the referenced 1936 classic by Wente (Patent #2037817), and Henricksen's 1977 "Tangerine" variant (Patent #4050541).
I fully agree that diffraction and internal reflection is the primary problem in traditional, Webster-theory horns, and impulse measurements with substantial amounts of stored energy confirm that - and very likely the reason that impulse measurements are rarely seen in the horn literature, while they are much more commonly seen with direct-radiators, going back to the mid-1970's.
It is significant that 1950's-era Altec and JBL high-efficiency loudspeakers have complex, diffraction-inducing acoustical paths - phase-plugs exits with a 50% reflective metal-mesh bug screen a short distance away, a mismatch between the flare rate at the exit of the compression driver and the entrance to the throat of the horn, and in the more notorious JBL designs, another diffraction grating at the mouth of the horn, in the form of an "acoustic lens". Two decades later, more diffraction in the Altec Manta-Ray and JBL Bi-Radial horns for theater and studio-monitor use.
Plainly enough, time-domain performance was not a design consideration, with such a casual approach to intentional induction of diffraction. The attitude in professional designs circles seems to have been that any problem can be equalized away, and if the time domain is further degraded, it doesn't really matter all that much. This is the part of the prosound legacy that needs to be discarded.
Near the bottom of page 4 of his letter to Audio Express, Dr. Geddes states that a spherical cap that vibrates radially does not exist.
Would not the Murata 80mm piezo-electric midrange dome qualify? It operates from 350 Hz up to around 15 kHz in this design:
http://www.murata.com/speaker/es024.html
The "super tweeter" by Murata, which has a bandwidth extending to 100 kHz on axis, has what is described as a "breathing" motion on its surface. This is their attempt to describe a radial vibration, I think.
quote from their information page: "... Another unique feature of the ES103 diaphragm is its unusual vibration mode. With conventional speakers, the diaphragm generates a front-back piston vibration. But the semispherical ceramic diaphragm in the ES103 expands and contracts like a balloon--a "breathing" vibration mode that actually realizes the point-type tone generation ideal for a speaker. This unique vibration mode offers wide directionality, enabling ideal listening from a wide range of positions. ...."
Would not the Murata 80mm piezo-electric midrange dome qualify? It operates from 350 Hz up to around 15 kHz in this design:
http://www.murata.com/speaker/es024.html
The "super tweeter" by Murata, which has a bandwidth extending to 100 kHz on axis, has what is described as a "breathing" motion on its surface. This is their attempt to describe a radial vibration, I think.
quote from their information page: "... Another unique feature of the ES103 diaphragm is its unusual vibration mode. With conventional speakers, the diaphragm generates a front-back piston vibration. But the semispherical ceramic diaphragm in the ES103 expands and contracts like a balloon--a "breathing" vibration mode that actually realizes the point-type tone generation ideal for a speaker. This unique vibration mode offers wide directionality, enabling ideal listening from a wide range of positions. ...."
Russell Dawkins said:
As I said, in theory such a thing is possible and indeed this MAY be one. But it would have to be measured to see just how accurate it is at pure radial motion. Piezo is generally a very low displacment process which works well in high impedance medium like water, but not very well in air. So, yes, there is a candidate, but surely an unproven one in terms of cost and performance effectivness.
And then there is the point that the spherical cap must subtend the same angle as the waveguide. The one shown would only work in a flat baffle (a 180° waveguide), not a waveguide of some finite angle. So in reality, a real "waveguide" source still does not exist - but certainly could be made. But piezo-electric is just not a very good material for an audio loudspeaker.
high excursion musical instrument driver
I've been lurking on this site for a while just trying to get caught up. There is a lot of good information. But when I saw the BOM 12 I just had to chime in.
This site has an interesting musical instrument driver using Dan Wiggin's XBL2 motor tech. Perhaps it may be worth considering for the OB that Lynn is designing.
http://www.acousticdev.com/html/products__adi.html
It has over 16mm of linear stroke. Might this work? If so it would require a group buy since they tend to want to sell to OEMs.
I've been lurking on this site for a while just trying to get caught up. There is a lot of good information. But when I saw the BOM 12 I just had to chime in.
This site has an interesting musical instrument driver using Dan Wiggin's XBL2 motor tech. Perhaps it may be worth considering for the OB that Lynn is designing.
http://www.acousticdev.com/html/products__adi.html
It has over 16mm of linear stroke. Might this work? If so it would require a group buy since they tend to want to sell to OEMs.
Re: high excursion musical instrument driver
The flat diaphragm is ideal for use on a waveguide - the first test that I did used a flat diaphragm driver.
But I am not a fan of XBL2 and excursion is seldom an issue in waveguides. I have my doubts about 16 mm in a driver that size, but thats simply a waste of BL when one will only use a few mm. Is such a driver available with normal voice coil drives?
Killjoy said:
The flat diaphragm is ideal for use on a waveguide - the first test that I did used a flat diaphragm driver.
But I am not a fan of XBL2 and excursion is seldom an issue in waveguides. I have my doubts about 16 mm in a driver that size, but thats simply a waste of BL when one will only use a few mm. Is such a driver available with normal voice coil drives?
Re: Re: high excursion musical instrument driver
Hello Gedlee,
Can you add some more thought to why you are not a fan of XBL2?
Also can you comment on why there aren't any large compression drivers used in waveguides. I'm talking about a 12" driver using a 2 or 3:1 compression and run through a waveguide to reproduce low frequencies--kind of along the lines patent--US5537481.
BTW, were you referring to the BOM 12 (musical instrument driver) or the LAW subwoofer with the flat diaphragm? I noticed the BOM 12 has a 4 inch voicecoil (assumed to be a single voicecoil).
gedlee said:
Hello Gedlee,
Can you add some more thought to why you are not a fan of XBL2?
Also can you comment on why there aren't any large compression drivers used in waveguides. I'm talking about a 12" driver using a 2 or 3:1 compression and run through a waveguide to reproduce low frequencies--kind of along the lines patent--US5537481.
BTW, were you referring to the BOM 12 (musical instrument driver) or the LAW subwoofer with the flat diaphragm? I noticed the BOM 12 has a 4 inch voicecoil (assumed to be a single voicecoil).
Hello Earl,
I just read you response to the A-ex article. Well written, but if I may I would make one suggestion that may improve the odds of publication. I found the opening paragraph somewhat confrontational. Having served as a reviewer for technical journals in the past, I might suggest that you "depersonalize" the opening some what. "Having spent the better part of my life studying these devices I was anxious to learn how they worked." in particular seems a rather sarcastic way to open the discussion.
I just read you response to the A-ex article. Well written, but if I may I would make one suggestion that may improve the odds of publication. I found the opening paragraph somewhat confrontational. Having served as a reviewer for technical journals in the past, I might suggest that you "depersonalize" the opening some what. "Having spent the better part of my life studying these devices I was anxious to learn how they worked." in particular seems a rather sarcastic way to open the discussion.
John Atwood has been kind enough to pass along the first constant-directivity horn paper, written by Abraham B. Cohen of University Sound in 1952.
The primitive nature of horn theory in the 1950's is all too evident - but remember, modern crossover filters using Butterworth functions and the first paper by Neville Theile (in Australia) was still more than a decade away when this was written.
I find the Lansing and Hilliard 1945 Theatre Speaker article more inspiring. The 515 and 288 drivers became the foundation of movie theatre sound for more than 30 years, and were used in more than 12,000 theatres across the world. Most notably, the 70mm Technicolor widescreen moves of the 1950's and 1960's used three fully discrete mag-track Altec A2 and A4 systems behind the giant curved screens.
The 515, 416, and 414 midbass drivers all use underhung voice coils, which resulted in notable midrange linearity due to well-controlled magnetic geometry in the gap. (It is much harder to control the linearity of the field lines with the more common overhung voice coil structure. Voice-coil cooling is less efficient as well, since the portion of the VC outside the gap is not in close proximity to the much cooler magnet, which acts as a heat sink for the voice coil.)
The 288 compression driver replaced the corrugated surround of the 283 Shearer-horn compression driver with tangential suspension, resulting in substantially lower distortion in the midrange. The 515 and 288 drivers represent good solid engineering that still has merit today - indeed, the titanium diaphragms and the JBL diamond surrounds used in modern theaters are arguably a step backward, with resonances starting at lower frequencies (4 kHz instead of 7 kHz), and far more aggressive use of HF boost equalization in the region where the driver is breaking up.
(The optical sound tracks of 1945-era movies rolled off above 8 kHz, thus not requiring the 10+ dB of boost EQ mandated by the modern THX specification. I strongly disagree with the idea of boosting power to a driver in breakup mode - that's what supertweeters are for.)
The primitive nature of horn theory in the 1950's is all too evident - but remember, modern crossover filters using Butterworth functions and the first paper by Neville Theile (in Australia) was still more than a decade away when this was written.
I find the Lansing and Hilliard 1945 Theatre Speaker article more inspiring. The 515 and 288 drivers became the foundation of movie theatre sound for more than 30 years, and were used in more than 12,000 theatres across the world. Most notably, the 70mm Technicolor widescreen moves of the 1950's and 1960's used three fully discrete mag-track Altec A2 and A4 systems behind the giant curved screens.
The 515, 416, and 414 midbass drivers all use underhung voice coils, which resulted in notable midrange linearity due to well-controlled magnetic geometry in the gap. (It is much harder to control the linearity of the field lines with the more common overhung voice coil structure. Voice-coil cooling is less efficient as well, since the portion of the VC outside the gap is not in close proximity to the much cooler magnet, which acts as a heat sink for the voice coil.)
The 288 compression driver replaced the corrugated surround of the 283 Shearer-horn compression driver with tangential suspension, resulting in substantially lower distortion in the midrange. The 515 and 288 drivers represent good solid engineering that still has merit today - indeed, the titanium diaphragms and the JBL diamond surrounds used in modern theaters are arguably a step backward, with resonances starting at lower frequencies (4 kHz instead of 7 kHz), and far more aggressive use of HF boost equalization in the region where the driver is breaking up.
(The optical sound tracks of 1945-era movies rolled off above 8 kHz, thus not requiring the 10+ dB of boost EQ mandated by the modern THX specification. I strongly disagree with the idea of boosting power to a driver in breakup mode - that's what supertweeters are for.)
john k... said:
John - off all of the lines in that paper, that was the one that I was most concerned with. I can't help but to try and have some fun when writtng those kind of things (they can be so dry) but I didn't mean this personally. I will strongly consider your point, but this is not a professional journal by any sense, so I might be more liberal with the attempted humor. (So you weren't offended by "evolution"? I'm surprised that slipped through.) As I told one editor I intended the tone to be "a bit sarcastic, but polite". I really do think my work was seriously short changed in that paper. Sarchasm has no place in a professional paper, but this was not intended to be a professional paper.
Re: Re: Re: high excursion musical instrument driver
I don't believe that cone excusion is anything more than a marketing scam. Its not a big factor to me, except in maybe a sub and even then my deisgns are not that sensitive to nonlinearity. The XBL2 seems to me to be simply a lot of add mass with no real net benefit.
Large drivers have very limited HF usage and as I said in my paper, I don't see the advantage of a horn for low frequencies. So there doesn't appear to me to be any advantage to a large diaphragm.
Killjoy said:
I don't believe that cone excusion is anything more than a marketing scam. Its not a big factor to me, except in maybe a sub and even then my deisgns are not that sensitive to nonlinearity. The XBL2 seems to me to be simply a lot of add mass with no real net benefit.
Large drivers have very limited HF usage and as I said in my paper, I don't see the advantage of a horn for low frequencies. So there doesn't appear to me to be any advantage to a large diaphragm.