Anglo said:
If you read carefully Magnetar's post (and also scroll back a couple of pages and read his comments on the LeCleah profile) his main objection is radiation pattern, with larger drives (resp. horn) beaming where the ribbon has its wides lateral dispersion (Magnetar, pls. correctly if I'm wrong).
Now, the _real_ question is: Besides the subjective objections of a small sweet spot (i.e. "having 2m tall headphones") are there any _other_ drawbacks of this compromise ?
Again, both larger drivers and horns+CDs have "suffer" this in mating with the RAAL (again, when attempting to cover same/similar freq. range).
Wrt direct radiatiors Lynn mentioned somewhere in this thread (IIRC) the possibility of bringing in different drivers (of the 6ND410 quad he already purchased two months ago) at different freq. Not sure what was the followup on that...
Thanks for the response. I have been following the thread and once again this beaming thing comes up again. For me the beaming isn't that much of an issue. Now, does that make me ignorant to better sound? Maybe... What, other than head in a vice does beaming do to the sound? When it becomes really directional, is it also breaking up? Is it changing tone? My ribbon tweeter "overdisperses" but isn't cutoff where the wideband begins its beaming.
If I understand this correctly, I should, to avoid beaming, cut my wideband at 1Khz and integrate my ribbon; right? Then all my loove for my wideband went out the door and I have replaced the tone density of wideband for a light weight metallic mid range that, however, doesn't beam.
I'd love to know the main reason for avoiding beaming in my case, consider weight in the mid from my wideband, impact, SPL slam, agility and tone density.
I am very open minded on hearing the +'s and -'s of a different approach.
If I understand this correctly, I should, to avoid beaming, cut my wideband at 1Khz and integrate my ribbon; right? Then all my loove for my wideband went out the door and I have replaced the tone density of wideband for a light weight metallic mid range that, however, doesn't beam.
I'd love to know the main reason for avoiding beaming in my case, consider weight in the mid from my wideband, impact, SPL slam, agility and tone density.
I am very open minded on hearing the +'s and -'s of a different approach.
John_E_Janowitz said:
Obviously great effort has been made to avoid motor induced IM as far as possible with this design.
The main difference between this and Doppler induced intermodulation is that it occurs at different points of the SPL pressure curve.
Picture a simple two sine wave mix and taking the lower frequency as the reference - motor induced IM has its maximum at the maximum excursion = at the minimal pressure points whereas Doppler induced IM has its maximum IM where VC speed is maximum e.g. at the points of peak pressure of the low frequency sine wave.
Motor induced IM - at the very first - is of the same type as produced by non linear circuits (VC heating effects not taken into account ), whereas Doppler induced IM isn't .
Might be that our ear brain system distinguishes between this two completely different forms of IM distortion easily – don't know.
Greetings
Michael
mige0,
One experiment of interest done in the past lead to the lengthening of the phase plugs on the 15" drivers. Paul Butterfield found that when playing a 1Khz tone simultaneously with a 10Hz tone at high excursions, you could audibly hear the 1KHz tone being modulated. This was found to be due to the "loading" of the cone being different as the coil got closer to the end of the phase plug. The solution was to lengthen the phase plug by nearly 1.5". The same test could be performed again with the same driver and longer phase plug, but now this modulation of the 1KHz frequency was no longer audible.
John
One experiment of interest done in the past lead to the lengthening of the phase plugs on the 15" drivers. Paul Butterfield found that when playing a 1Khz tone simultaneously with a 10Hz tone at high excursions, you could audibly hear the 1KHz tone being modulated. This was found to be due to the "loading" of the cone being different as the coil got closer to the end of the phase plug. The solution was to lengthen the phase plug by nearly 1.5". The same test could be performed again with the same driver and longer phase plug, but now this modulation of the 1KHz frequency was no longer audible.
John
Well that sounds interesting
– do you have any kind of measurement protocol about the test conditions like Xp-p-linear of the driver / Xp-p for F-low & F-high independently?
– if Butterfield's solution was to lengthen the phase plug to as it is now, does this mean that what we can see as a silver phase plug is much more than just an eye catcher and for the ventilation of the VC? Is it made of iron plated with copper and actually part of the magnet system?
Greetings
Michael
– do you have any kind of measurement protocol about the test conditions like Xp-p-linear of the driver / Xp-p for F-low & F-high independently?
– if Butterfield's solution was to lengthen the phase plug to as it is now, does this mean that what we can see as a silver phase plug is much more than just an eye catcher and for the ventilation of the VC? Is it made of iron plated with copper and actually part of the magnet system?
Greetings
Michael
mige0 said:
The experiment was originally done playing a 10Hz tone at near peak-peak linear excursion, approximately 14mm one direction. 10Hz was chosen because it was not an audible tone in itself so the 1KHz tone being modulated was easy to hear. I'm not sure what the level on the 1KHz tone was and there were not technical measurements on the modulation. From my understanding it was a clearly audible modulation that was completely inaudible after the lengthening of the phase plug.
The phase plug is a solid piece of polished aluminum weighing just over 1lb. The copper sleeve on the pole helps to pull heat from the VC where the heat is then transfered more effectively to the steel pole. The phase plug then is attached by a threaded set screw to the top of the pole and heat sink grease is placed to help transfer heat to the phase plug where it can then be dissipated better into the air. You can heat the phase plug to the point where you can burn your finger if touching it, but the VC stays quite cool with little measurable increase in DCR.
John
"I'd love to know the main reason for avoiding beaming in my case, consider weight in the mid from my wideband, impact, SPL slam, agility and tone density."
Hello Anglo
Me I like CD type waveguide presentation. I like to be able to move around and not have the frequency balance change. I also like using CD waveguides for HT because the coverage in the setting area is very uniform and you get the same frequency balance over the entire listening area. Can't do that with a beamy set-up as your balance changes too much as you go off axis.
If you are solo and you have a small sweet spot I doesn't matter as much providing you can get them set-up to work well in your listening space. With a driver set-up that are highly directional they tend to limit your placement options and also define your toe in and seating height.
As far as weight, impact, SPL and slam and the rest there are lots of ways to get there. I like multidriver limited bandwidth systems. 4 ways for the most part. I would never run a 15" woofer up that high. In my set-ups they are crossed over at 300 hz to 10" midranges but that's my preference. I have a coax center that takes a 15' up to 800-900hz and that's limit for me. Depending on the driver compliment you can get beaming in those systems just as easilly as with a larger fullrange driver. That doesn't mean they can't sound good when you are in the "zone"
Rob
Hello Anglo
Me I like CD type waveguide presentation. I like to be able to move around and not have the frequency balance change. I also like using CD waveguides for HT because the coverage in the setting area is very uniform and you get the same frequency balance over the entire listening area. Can't do that with a beamy set-up as your balance changes too much as you go off axis.
If you are solo and you have a small sweet spot I doesn't matter as much providing you can get them set-up to work well in your listening space. With a driver set-up that are highly directional they tend to limit your placement options and also define your toe in and seating height.
As far as weight, impact, SPL and slam and the rest there are lots of ways to get there. I like multidriver limited bandwidth systems. 4 ways for the most part. I would never run a 15" woofer up that high. In my set-ups they are crossed over at 300 hz to 10" midranges but that's my preference. I have a coax center that takes a 15' up to 800-900hz and that's limit for me. Depending on the driver compliment you can get beaming in those systems just as easilly as with a larger fullrange driver. That doesn't mean they can't sound good when you are in the "zone"
Rob
Re: Very Brief History of Compression Drivers
Hi Lynn,
There are a few points that you've repeated in this thread that I heartily agree with and also some that I take exception to.
Absolutely. This makes perfect sense and is just a question of tradeoffs. Of course, for this project we are trying to minimize the tradeoffs (which usually involves increasing the costs).
Yes, of course. Using resonances to extend the "working range" of *any* reproducer is nothing new. That's what happens in virtually any non-metallic cone or dome (eg, paper, cloth, polypropylene, et cetera). Not to mention things like whizzer cones, or even dust caps, for that matter. So nothing new here.
That's not to say that it's a *good* thing. In my estimation, *any* resonances are bad things that color the sound. But to pick on this particular one seems to be overlooking the sins of the many....
Again, absolutely true. All metals lose strength as the flex. This is known as fatigue. Nearly all metals will lose about half of their initial strength over time EXCEPT ALUMINUM.
Aluminum is peculiar, because it loses strength *without* limit! In other words, if you flex it lightly, eventually it will *always* fail no matter how lightly it is loaded. It is just a question of how long you want it to last.
Now this is where I have to part ways with you. I have never seen *any* studies that show any meaningful differences between the internal damping of titanium and aluminum. I just don't buy off on this one. I could give dozens of counter-examples, but don't even see the point.
Similarly, the claim for a lower breakup frequency for titanium is completely unsubstantiated. Titanium is actually stiffer than aluminum. It is also denser, so the breakup frequencies end up being quite similar. Probably within 5% or closer. I have no idea why someone told you this stuff, but it is simply untrue. Either they were making stuff up or they simply misremembered the facts.
Again, completely untrue. This apparently comes from a summary of an interview with Doug Button. And either he misspoke or is being intentionally misleading. Titanium will last longer due to its greater fatigue strength, but the breakup frequencies are virtually identical to aluminum.
Thanks for the interesting thread.
Best regards,
Charles Hansen
Founder of Avalon Acoustics, and (to the best of my knowledge) first US manufacturer to use metal dome tweeters.
Hi Lynn,
There are a few points that you've repeated in this thread that I heartily agree with and also some that I take exception to.
Lynn Olson said:
Absolutely. This makes perfect sense and is just a question of tradeoffs. Of course, for this project we are trying to minimize the tradeoffs (which usually involves increasing the costs).
Lynn Olson said:
Yes, of course. Using resonances to extend the "working range" of *any* reproducer is nothing new. That's what happens in virtually any non-metallic cone or dome (eg, paper, cloth, polypropylene, et cetera). Not to mention things like whizzer cones, or even dust caps, for that matter. So nothing new here.
That's not to say that it's a *good* thing. In my estimation, *any* resonances are bad things that color the sound. But to pick on this particular one seems to be overlooking the sins of the many....
Lynn Olson said:
Again, absolutely true. All metals lose strength as the flex. This is known as fatigue. Nearly all metals will lose about half of their initial strength over time EXCEPT ALUMINUM.
Aluminum is peculiar, because it loses strength *without* limit! In other words, if you flex it lightly, eventually it will *always* fail no matter how lightly it is loaded. It is just a question of how long you want it to last.
Lynn Olson said:
Now this is where I have to part ways with you. I have never seen *any* studies that show any meaningful differences between the internal damping of titanium and aluminum. I just don't buy off on this one. I could give dozens of counter-examples, but don't even see the point.
Similarly, the claim for a lower breakup frequency for titanium is completely unsubstantiated. Titanium is actually stiffer than aluminum. It is also denser, so the breakup frequencies end up being quite similar. Probably within 5% or closer. I have no idea why someone told you this stuff, but it is simply untrue. Either they were making stuff up or they simply misremembered the facts.
Lynn Olson said:
Again, completely untrue. This apparently comes from a summary of an interview with Doug Button. And either he misspoke or is being intentionally misleading. Titanium will last longer due to its greater fatigue strength, but the breakup frequencies are virtually identical to aluminum.
Thanks for the interesting thread.
Best regards,
Charles Hansen
Founder of Avalon Acoustics, and (to the best of my knowledge) first US manufacturer to use metal dome tweeters.
Re: Re: Very Brief History of Compression Drivers
Hello,
Everyone may agree that the stiffness defined as ratio between Young's modulus and density is quite the same for aluminum and titanium (well in fact something like 5%-6% lesser for titanium).
This means that for the same stiffness we have to use for titanium a diaphragm the thickness of which is half the thickness of the aluminum diaphragm.
In French language we have two different definitions:
"rigidité" which is the "intrinsic" stiffness of the material and "raideur" which is the stiffness of the part (here a diaphragm having a spherical or parabolic cap shape).
The "raideur"/stiffness of the cap diaphragm may differ with its shape. With a thickness halved, does the titanium diaphragm will present similar modes of vibration and strain as the aluminum diaphragm? Can this explain the somewhat harsh sound mentionned by Lynn?
I used to perform a lot of measurements on the Beyma 850ND compression driver and I could see at the resonance frequency a kind of antiresonance that I interpreted (may be wrongly) as the result of a (0,2) mode on large displacement of the titanium diaphragm.
I never saw a similar behavior with aluminum or beryllium diaphragms.
Best regards from Paris, France
Jean-Michel Le Cléac'h
Hello,
Everyone may agree that the stiffness defined as ratio between Young's modulus and density is quite the same for aluminum and titanium (well in fact something like 5%-6% lesser for titanium).
This means that for the same stiffness we have to use for titanium a diaphragm the thickness of which is half the thickness of the aluminum diaphragm.
In French language we have two different definitions:
"rigidité" which is the "intrinsic" stiffness of the material and "raideur" which is the stiffness of the part (here a diaphragm having a spherical or parabolic cap shape).
The "raideur"/stiffness of the cap diaphragm may differ with its shape. With a thickness halved, does the titanium diaphragm will present similar modes of vibration and strain as the aluminum diaphragm? Can this explain the somewhat harsh sound mentionned by Lynn?
I used to perform a lot of measurements on the Beyma 850ND compression driver and I could see at the resonance frequency a kind of antiresonance that I interpreted (may be wrongly) as the result of a (0,2) mode on large displacement of the titanium diaphragm.
I never saw a similar behavior with aluminum or beryllium diaphragms.
Best regards from Paris, France
Jean-Michel Le Cléac'h
Charles Hansen said:
Something I learned making classical guitars:
Stiffness to weight and thickness.
If I remember correctly, if you double the thickness it will be four times as stiff, and of course twice as heavy.
If titanium was twice as stiff as aluminum it would still not be as stiff for the same mass because aluminum can be allmost twice the thickness for the same mass.
Please correct me if I am wrong.
Stiffness to weight and thickness.
If I remember correctly, if you double the thickness it will be four times as stiff, and of course twice as heavy.
If titanium was twice as stiff as aluminum it would still not be as stiff for the same mass because aluminum can be allmost twice the thickness for the same mass.
Please correct me if I am wrong.
john k... said:
Yes, that has to be the case. My guess is that the "touch" test can be misleading on at least two accounts. Once the power is off, the coil will cool much faster than the phase plug, due to its much lower mass. Also, the coil is coated with an insulator, which will make it seem a little cooler to the touch than bare metal at the same temperature.
Sheldon
Re: Re: Re: Very Brief History of Compression Drivers
This is an interesting data point, but there is insufficient information to draw any conclusions.
I have very little experience with compression drivers. Nearly all of my work has been with direct radiators. When working with dome tweeters, one of the things that you learn quickly is that there are many resonances caused *not* by the diaphragm, but instead by the various acoustic elements of the air chambers.
For example, many companies make two versions of what are essentially the same tweeter. One has a solid pole piece and the other has a hollow pole piece that leads to a rear chamber. (Usually ferrofluid damping is used in these designs, and it tends to mask the acoustic resonances. If you remove the ferrofluid, it becomes much easier to see what is actually happening.) The chambered rear tweeter will exhibit one large resonance, usually within an octave of 1 kHz. This resonance is of the Helmholtz type, with the compliance provided by the air in the chamber and the mass provided by the "slug" of air in the hollow pole piece. If you perform detailed studies, you will usually find several more acoustic resonances caused by the cavities behind the dome/surround assembly.
In a compression driver, these effects are much worse as the acoustic structure is much more complex. To get an idea of the problem, just look at the impedance curve of a compression driver. Each peak is caused by some sort of resonance.
Without knowing more about the driver you examined, I would first suspect some type of acoustic resonance rather than attributing some mysterious property to titanium that is lacking in both aluminum and beryllium.
I could be wrong, as I have no direct experience with the devices you are discussing. However, please see my reply to another post below for more information.
Jmmlc said:
This is an interesting data point, but there is insufficient information to draw any conclusions.
I have very little experience with compression drivers. Nearly all of my work has been with direct radiators. When working with dome tweeters, one of the things that you learn quickly is that there are many resonances caused *not* by the diaphragm, but instead by the various acoustic elements of the air chambers.
For example, many companies make two versions of what are essentially the same tweeter. One has a solid pole piece and the other has a hollow pole piece that leads to a rear chamber. (Usually ferrofluid damping is used in these designs, and it tends to mask the acoustic resonances. If you remove the ferrofluid, it becomes much easier to see what is actually happening.) The chambered rear tweeter will exhibit one large resonance, usually within an octave of 1 kHz. This resonance is of the Helmholtz type, with the compliance provided by the air in the chamber and the mass provided by the "slug" of air in the hollow pole piece. If you perform detailed studies, you will usually find several more acoustic resonances caused by the cavities behind the dome/surround assembly.
In a compression driver, these effects are much worse as the acoustic structure is much more complex. To get an idea of the problem, just look at the impedance curve of a compression driver. Each peak is caused by some sort of resonance.
Without knowing more about the driver you examined, I would first suspect some type of acoustic resonance rather than attributing some mysterious property to titanium that is lacking in both aluminum and beryllium.
I could be wrong, as I have no direct experience with the devices you are discussing. However, please see my reply to another post below for more information.
Peter M. said:
The proof is in the pudding. Nearly all dome tweeters are made with a very similar geometry. When the diaphragm has very little damping (ie, any metallic or metal oxide material), it becomes quite easy to see the resonant modes, either by inspecting the frequency response, the impedance curve, or a waterfall plot.
Assuming that the dome is driven at the periphery (not the case with the Focal or Accuton inverted domes), then there are only four factors that affect the breakup frequency:
- The diameter of the dome.
- The thickness of the dome.
- The material of the dome.
- The profile of the dome.
If you look at any 1" aluminum dome tweeter, they all have a first breakup mode between 26 and 27 kHz, with only one exception that I am aware of. That is the aluminum dome version of the ScanSpeak Revelator tweeter. That dome has a breakup mode up around 33 kHz, due the the small radius of curvature that creates a very "steep" and "high" dome profile. (This causes other problems at the same time that it increases the frequency of the breakup mode.)
And if you look at any 1" titanium dome tweeter, they all have a breakup mode between 25 and 26 kHz, just slightly lower than that of an aluminum dome.
The bottom line is that when practical thicknesses of materials are used (and the dome diameter and profile are quite similar), aluminum has only a *very* small performance advantage over titanium in this regard.
As I noted in a previous post, aluminum *will* eventually fatigue and fail with extended use. It may be that in a speaker designed for domestic applications that it may take many decades (or even centuries!) before this would happen. Whereas in professional use it appears that the failures would happen quickly enough to cause the shift to titanium over aluminum.
john k... said:
Hi John,
My point was to illustrate the ability of heat to be transfered from the VC to the copper then steel, then to the phase plug. The coils we use won't fail until over 600F. You can burn yourself at much lower temperatures than this. The minimum temperature for skin to burn is considered 130F. The phase plugs can get much hotter than this, often in the 200F range.
If we take that 200F temperature we see about 73C above an ambient room temp of 20C. Copper roughly increases resistance at .393% per degree C. So 73C * .00393 = 28% rise in DCR. Say a 5.5ohm coil, the resistance goes up to 7.04. This accounts to about 1.08dB in power compression. Measurable, but not by any means audible. Keep in mind this would take EXTREME amounts of power to the the entire system heated to this level.
While under power, yes, temperature in the VC clearly needs to be higher than the temperature of the surrounding material. Being in close proximity, within .015" of the copper sleeve on the pole we have a couple things going on here. It can't simply be modeled as conduction, convection, or radiation individually. There is a layer of air insulating between the coil form and the copper on the pole, but being very thin we can't just take this as simple radiation to the air, but there isn't direct contact with the copper so it's not just conduction either. It's a fairly complex model to try to estimate by formulas. I will hopefully have some good models soon as well as the real world tests that will verify the model. Heat will not be absorbed by the copper as quickly as it would be if it was in direct contact with the VC, but the tighter the gap, the more effectively heat is transfered.
In any case, the copper mounted to the pole will absorb heat much quicker than would the steel alone. The copper sleeve is nearly 2x as heavy as most of the larger coils we use, so by mass would absorb 2x as much heat as the coil. Nearly instantly we are distributing the same amount of heat over 3x the mass of copper, reducing the overall temperature in any given part to 1/3 what it would be without the copper. From there, the copper does transfer heat directly to the steel pole through conduction. As you mentioned, heat will flow from hot to cool, so until the steel gets to the point where it is at equilibrium with the copper, heat will continue to be pulled from the copper to the steel, and as a result from the VC to the copper as it is now lower in temperature than the coil. The same then goes to the phase plug that is attached to the pole piece and connected via heat sink grease.
My point was not that the phase plug gets hotter than the VC as this would be a quite rare occurrence, likely only after the power was totally cut off to the driver and it would simply take longer for the larger mass to give up the heat. The phase plug does however prove effective as a further heatsink to pull heat from the pole and slightly to dissipate heat to the outside world. Again this is not a simple calculation either as to how the phase plug removes heat. There is convection around a cylinder as well as simple radiation, and in addition to that there is the motion of the cone which further moves air forward and backward cooling the phase plug. It porves to be effective at pulling heat from the steel pole and is another piece to the entire heatsinking ability of the driver.
In the case of the apollo upgrade, you have about 1500grams of aluminum and 125grams of copper to absorb heat vs from 35-60grams of copper/alum in the VC. It takes much more power to heat all this mass to the point where the coil DCR will rise noticeably than it would for simply the coil alone to get to this level.
John
Robh3606 said:"I'd love to know the main reason for avoiding beaming in my case, consider weight in the mid from my wideband, impact, SPL slam, agility and tone density."
Hello Anglo
Me I like CD type waveguide presentation. I like to be able to move around and not have the frequency balance change. I also like using CD waveguides for HT because the coverage in the setting area is very uniform and you get the same frequency balance over the entire listening area. Can't do that with a beamy set-up as your balance changes too much as you go off axis.
If you are solo and you have a small sweet spot I doesn't matter as much providing you can get them set-up to work well in your listening space. With a driver set-up that are highly directional they tend to limit your placement options and also define your toe in and seating height.
As far as weight, impact, SPL and slam and the rest there are lots of ways to get there. I like multidriver limited bandwidth systems. 4 ways for the most part. I would never run a 15" woofer up that high. In my set-ups they are crossed over at 300 hz to 10" midranges but that's my preference. I have a coax center that takes a 15' up to 800-900hz and that's limit for me. Depending on the driver compliment you can get beaming in those systems just as easilly as with a larger fullrange driver. That doesn't mean they can't sound good when you are in the "zone"
Rob
...interesting, we could have a lot more conversation on what was said here. I cross my Altec 515's at 120hz at 18DB per octave.
Charles Hansen said:
Here's a direct quote from the Doug Button interview:
However, it was recognized that there were compromises with the new diaphragms. Titanium does not have the internal damping of aluminum and thus has marginally higher distortion levels. The diamond surrounds, while extending frequency response, do so at the expense of transient response. Further, due to its lower stiffness, titanium goes into breakup at a lower frequency.
Maybe JBL (and Doug Button) are just pulling our chain with the usual big-business marketese, but their titanium diaphragms are the world standard in movie theaters and small to stadium-size SR applications, so JBL certainly has no reason to sell it short. Titanium-based diaphragms in JBL speakers pays the rent, keeps the lights on, and pays the employees. They put titanium drivers on the map for the prosound world, and continue to do very well selling them.
Personally, I have no dog in this fight. I'm not a fan of any metal-dome diaphragm. Don't much care for horns either, frankly, at least the ones I hear in movie theaters and SR systems. Too much tin-can coloration, which I seem to be very sensitive to, although not as much as Karna.
What I'm trying to find out is the source of the colorations I dislike, and keep them to a minimum, so I can go ahead with the high-efficiency, high-headroom goal. I'm not one of the mix-n-match tweak types - that just goes round and round in circles, and leads to pointless argumentation. Loudspeakers sound the way they do for very good technical reasons - the hard part is finding out what they are.
I don't often hear horn systems I like, and when I notice them, they take me by surprise - one was a partly-restored Altec A5 I heard at the Tube Fest in San Francisco. Very crude loudspeaker with a primitive crossover, but the sound of the Altec 288 driver and the 1505 multicell was really quite remarkable. There's something going on with the Western Electric/Jim Lansing/Altec technology that is quite interesting, and that seems to have been lost in modern SR and movie systems.
Audio is a funny business. Entire technologies can be forgotten, sometimes for many decades. Western Electric/Bell Labs invented negative feedback, but they only used it for their entry-level theater amplifier, the low-power 91A. The top-of-the-line amplifier (the 86 and 92A) did NOT use negative feedback, but the very unusual PP-only Harmonic Balancer instead. This was swept aside by the Williamson in 1947, and the Harmonic Balancer was forgotten until a group of triode nuts (myself included) re-discovered it only a few years ago.
From what I can tell, the majority of R&D in the professional sound business since the Seventies has been about power, coverage, and durability. This is understandable considering the parallel growth of stadium rock and the Dolby/THX/Lucasfilm conversion of movie theaters - louder, louder, louder, with more and more acoustic watts coming out of the compression drivers.
This is why I think it is interesting to read the original papers and see the reasons why XYZ technology became the industry standard - the 288 and 515 in 1945, the Williamson in 1947, the Lin output-transistor topology in the Sixties, op-amps in the Seventies, diamond-pattern surrounds, titanium diaphragms, and plastic surrounds at the beginning of the Eighties, and the widespread adoption of constant-directivity horns and site-specific equalization during the Eighties and through the present.
Here's a link to the patent for the diamond surrounds. It's an interesting read and is what Doug was speaking about. The original diaphrams were aluminum.
http://www.google.com/patents?id=FL...&as_miny_is=2007&as_maxm_is=1&as_maxy_is=2007
Rob
http://www.google.com/patents?id=FL...&as_miny_is=2007&as_maxm_is=1&as_maxy_is=2007
Rob