Certainly agree if the discussion is what is “best” .
But this thread is not about what is best. It’s about was is “difference”
Sure! The terminology can be a little confusing. Actually, my old colleague of 5 years from BG, Igor Levitsky, wrote a pretty good article about some of this for a piece in voicecoil magazine from August of last year. I’m going to excerpt it here. Though I haven’t asked for permission, it was kind of a marketing piece/editortial for the work he’s doing with Radian Audio/Dai Ichi on planar drivers and I’m sure that he/they are fine with it. He previously did planar drivers for HiVi, SLS (now dolby), Bohlender Graebener (now Christie Digital), Oppo (headphones) and now Radian and is a subject matter expert and someone that I learned a lot from in the 5 years that we worked closely together.
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Over the years, the audio community has acknowledged that speakers based on drivers with thin—film
diaphragms deliver exceptional sound quality in mid and high-frequency ranges. Many well-known loudspeakers from such brands as Infinity, Genesis, Apogee, Magnepan, Quad, KLH, MartinLogan, ESS, BG Radia, Wisdom Audio, SLS Loudspeakers, Adam Audio, Alcons, Philips, Foster, Sonigistix/Monsoon/n, Level 9/Eastech, and others have been praised for superior audio quality in mid and high-frequency bands. Similar transducer technology has proliferated over the past 10 to 12 years in headphones where brands such as Audeze, OPPO, Hifiman, Meze, Mr. Speakers, Stax, and Sonoma have taken leading positions in the high-performance headphone market. Even Sennheiser, a
juggernaut of headphones, when its engineers were tasked with developing the ultimate headphone, chose a thin—film electrostatic concept and developed its legendary flagship Orpheus, avoiding using any kind of voice coil-based drivers.
Thin Diaphragms
The key mechanism in thin-film drivers is a very thin diaphragm made from polymer film/aluminum foil laminate (or just pure aluminum foil ribbon) that is uniformly driven more or less by either electrostatic (based on forces acting on electric charge in electric field) or electro-magnetic (using Lorentz force on a current—carrying conductor in a magnetic field} motor system. Such diaphragms have extremely low mass, comparable with the mass of air load, vibrating along with the diaphragm. Mechanical and electro- acoustical mechanisms and relationships in those drivers differ in some fundamental ways from typical common cone or dome drivers.
Traditional drivers are, in general, a very good universal solution for a wide range of applications that has been tested by time. The first cone—based electrodynamic driver was invented by Chester Rice and Edward Kellogg at General Electric in 1925, and in principle remains the same to this
day. However, as almost always in audio, gains in one area require compromises in other areas. Thin-film drivers do not possess universality, they are very challenged when it comes to the reproduction of low-frequency signals. Some electrostatic and large panel planar designs lack ruggedness
and are not suitable for certain applications. They may have limited dynamic range and often have compromised dispersion characteristics.
On the other hand, since thin-film drivers can compromise on ultimate universality and applicability for the most demanding pro applications or bass reproduction, they get the advantage of using different principles and design solutions. Thin-film transducers use different technology and materials that provide less distortion and, in many cases, enable them to achieve better sound quality than
voice-coil based drivers, which have a “burden” to meet requirements in universality, extreme output levels, ease of mass production, low cost, and so forth.
The fundamental advantage that thin-film drivers possess over typical electrodynamic drivers is that they do not use round voice coils with multiple wire—turns immersed in a
magnetic gap. Such motors tend to have high inductance and magnet systems that are not symmetric and are prone to magnetic hysteresis, Eddy currents, and a slew of nonlinearities associated with inductance modulation when voice coil dynamically moves in the magnetic gap.
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Research Work
Several breakthrough research works conducted over the past 20 or so years analyzed the nature and identified causes of nonlinearities in loudspeakers based on new approaches, techniques, and tools. We make reference to several major research works here, but there is a number of others that point to similar results.
In articles that Dr. Wolfgang Klippel wrote for the Audio Engineering Society (AE5) [1], [2] and many other related articles and presentations, he specifically points out that nonlinearities associated with voice coil inductance and modulation of flux B in magnetic gap are predominant distortion mechanisms in speakers.
Those distortion components manifest themselves in much wider frequency ranges than nonlinearities related to displacement dependent nonlinearity of suspension stiffness Kms(x} and motor force B|(x), which mostly dominate below and slightly above fundamental driver resonance
Fs. Nonlinearitv of such transducer parameters as Le(i) inductance vs. current, Le{x) inductance vs. displacement, and BL(i) motor force, play major roles in generating distortions above driver resonance and at all signal levels even at the lowest ones. For many dome tweeters, however, the Kms(x) nonlinearity is inherent, even at low levels since they have a single-roll suspension that is always
asymmetric and its nonlinearity is more prevalent since it is not compensated by spider dominant compliance (usually more linear) at small displacements.
Alex Voishvillo—in his extensive range of papers [3],[4} and other presentations on nonlinearity in speakers, their audibility, and the latest methods of measurements and assessment - clearly shows distortion that manifests itself at low and moderate signal levels is easily noticeable and the
most objectionable. Similar conclusions are reported by Earl Geddes and Lidia Lee [5].
This has to do with the fact that music, unlike sine wave test tones, has signal amplitude probability distribution with peaks occurring rather infrequently, while lower-level signals are present in program material most of the time. In other words, the lower the level of the signal, the higher
the frequency of its occurrence. That is why distortion components that we observe only at higher levels are very often benign (up to a point), since they are masked by higher-level signals that generate those distortions in the first place. Those are typically generated by suspension
stiffness Kms(x} limiting {some dome tweeters have Kms(x) nonlinearity at low level as well) and the voice coil getting out of the magnetic gap, generating B|{x) nonlinearity.
On the other hand, distortions related to Le(i}, Le(x), Bl(i), and cone/dome mechanical break—up are generated at all signal levels and in a very wide frequency band. Such distortions, without the presence of strong masking signals, often extend beyond the masking band (difference, parametric, higher-order nonlinearities, and many intermodulation components). That’s when they become
very audible. The key here is that the masking effect depends on the level of the masking signal. The lower the signal, the weaker the masking effect and the more acute our ability to perceive distortions.
Our ear perceives low-level signals with much greater acuteness and resolution. This is a very strong survival mechanism that humans developed over a very long time. Besides, low-level signals and associated distortions are very easy to discern since there is no masking effect present.
This explains why having low distortion at moderate and low levels is crucial for sound quality.
For years, audio experts and enthusiasts paid attention to low level signal resolution and signal detail retrieval. In fact, this quality is one of the main criteria for high-end systems’ differentiation from budget and mass products. From the early days of digital audio, disappointing sound quality of
CDs was associated with quantization error and low-level digital noise. We were promised the perfect sound forever and there were claims that distortions would be vanishingly
small to non-existent, and yet, many early CDs sounded uninvolving, to say the least. Later, dither was introduced to reduce low-level noise and distortion. Even earlier, before digital, audio professionals noticed that transistor amplifiers, claiming an extremely low percentage of zero- crossing distortion, sounded very unpleasant.
On paper, they may have had rather low total harmonic distortion (THD} levels, but since those distortions were generated at low level input signals, they were very noticeable. No wonder, that many high-end amplifier manufacturers religiously choose Class-A topology even though it is very inefficient and limited in power. A Class-A amplifier has an extremely linear performance at low level
input signals and this is what really matters for sound quality.
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Comparing Drivers
Getting back to thin-film transducers, as with many other technical challenges, we see that once we narrow applicability and accept certain compromises (bass performance, costs, ease of mass production, etc.), and instead focus on low distortion, both linear and nonlinear, we can use thin-film
drivers‘ fundamental advantages to serve us in the pursuit of ultimate sound quality. Thin-film drivers/speakers are represented by several different and distinct types.
Electrostatic Speakers—There are some die-hard proponents of electrostats and that’s understandable. When it oomes to midrange reproduction, the best Quads and a
few other best electrostatic speakers can deliver superbtransparency, if they are set up correctly in a suitable room. However, it is very difficult to do so since all electrostatic speakers, as well as all large planar magnetic panels, despite various efforts, have very irregular dispersion pattern, lobing effects, and so forth. This makes them very sensitive to setup conditions and the “sweet spot" may be
rather limited in size. Besides, extra thin and very large Mylar/aluminum film tends to deteriorate with time and lose its tensioning. The aluminum layer in electrostatic diaphragm is less than one micron thick and is subject to arcing, metal drop outs, and corrosion {in the long term).
Electrostatic speakers have limitations in their dynamic range as well and require bass augmentation, which is very difficult to do right, due to many differences in driver characteristics and radiation patterns.
Large Planar Magnetic Speakers (Apogee, Magnepan} - These transducers can produce an amazing mid-range sound as well and can create a magnificent and palpable sound scape, which in the right setup could be stunning. However, as with electrostats, they have irregular dispersion due to their large size, limited dynamic range, and they require woofers to augment their limited bass with the same
set of integration challenges as mentioned above.
Not many people realize that all elongated thin-film speakers are not point sources but rather are extended line sources at least in mid-frequency and high-frequency bands. The same is fully applicable to popular discrete line arrays. As such, they do not follow a typical inverse square law of sound pressure level (SPL) drop off with distance. To further complicate things, this drop off is frequency
dependent. In most domestic conditions, listeners are positioned in the near field of those sources where their behavior is very complicated and varies dramatically in
space and frequency.
In other words, frequency response of such devices will vary with distance and position of a listener. There are no two spots in space with identical SPL response. To find that magic reference spot with truly balanced sound is often a challenging task for a listener. At any different spot such system will sound different. In practice, room reflections and reverberation help to alleviate this problem and make
such systems often sound big and spectacular, but the technical problem remains and could be a subject for discussion and arguments.
Aluminum foil ribbon tweeters (pure ribbon) - Pioneered by Stanley Kelly at London Decca, these designs use an extremely thin corrugated aluminum foil strip suspended in a relatively weak magnetic field and require an impedance matching transformer. There are only a few designs that
are exceptionally good and have superbly transparent and delicate high-frequency reproduction. Those are extremely expensive {RAAL), since such devices are difficult to manufacture with reliable consistency. They are rather fragile and the foil can easily lose its geometric stability over
time or when overdriven. They also have relatively limited dynamic range. In order not to jeopardize efforts of super expensive speaker cable enthusiasts, such devices should
probably use a special wire in their transformers to avoid cable controversy.
Air motion transformer drivers, (AMT, ESS Heil)—Invented by Dr. Oskar Heil, these very interesting designs proved to be more reliable and usable in domestic and even some light pro applications (studio monitors) since they do not require a matching transformer and use a laminated
diaphragm with polymer substrate instead of thin aluminum foil. The original ESS Heil driver was a special device with unusually clear, low distortion sound for that time. It was
large, heavy, and its integration with low-frequency drivers was…challenging.
Today, there are many very good AMT tweeters and they have become very popular. As in the original ESS design, they use a folded “accordion” diaphragm, but their magnetic system is much simpler, leveraging small high MGO neodymium magnets or magnetic steel poles, positioned in the front and at the back perpendicular to the conductors. Small AMTs, especially the ones made in Asia,
have been rather cost effective and successfully compete with dome tweeters even in budget markets.
And yet, if we look at AMTs with a magnifying glass and ultimate sound quality in mind, we can clearly see some drawbacks. A typical problem for high-output applications is limited power since the conductors are very close to each other and there is no metal in proximity to draw heat away.
Recent designs use a polyimide (Kapton) diaphragm to mitigate this to some extent, but the early onset of power compression is still typical. Even though the Kapton could handle higher temperatures, the folded accordion quickly gets heated and then its shape distorts generating intemal stresses and distortion.
Designs that aim at higher power and a lower crossover point use larger diaphragms. The width of such a diaphragm could be wider than 1" and/or longer than 3", which results in high-frequency beaming with an extremely narrow vertical coverage above about 8 kHz. This puts a listener in
a very restricted sweet spot.
The folded diaphragm has an accordion pattern and its vibrations are naturally nonlinear. While lateral displacements of the fold walls are more or less linear (at least at lower levels), the radiating area of the exit of each fold changes (“breathes”), which is inherently a nonlinear effect. When
folds are moving inward, “squeezing” air, the exit area grows smaller, and when folds move apart, the radiating area gets larger. Thus, we can see a clear modulation effect. Higher levels would require larger displacements where air in the narrow folds becomes nonlinear and viscous, leading to higher losses/compression and intermodulation.
Yet another design feature is that the magnetic field from bar magnets or steel poles, which are located across conductors, is only partially crossing the conductors. Approximately 50% of the conductor is closed by magnets and about 50% has an opening for sound to exit, where
there is no magnetic flux or it is very much reduced to a stray field. This results in an uneven driving force distribution and leads to various forms of distortion.
Yet another problem is that the way an ATM diaphragm is folded and kept in position does not reliably provide the same form factor and position for the folds and the conductors, which increases deviation of parameters from unit to unit.
And yet, those drivers are becoming very popular, which tells us that our “hearing apparatus" could be very forgiving to certain types of distortion, especially if they take place mostly at higher levels and are absent at lower levels as is the case with AMTs.
Planar ribbon drivers (ribbon, planar—magnetic, and isodynamic)—These designs have the least problems within their thin-film transducer family and over the years have proven to be the most universal, combining superior sound quality, the widest application range, reasonable
costs, and reliability that grants them use in various professional markets.
There are several major/visible brand names currently operational that are the best representatives of a wide application range of this technology. Alcons (formerly Stage Accompany engineers), Dolby’s speaker division (formerly SLS Audio), and Vive Audio (a Christie Digital subsidiary in
commercial cinema born out of an acquisition of BG Radia) in the pro market; Wisdom Audio in the high-performance residential; Coastal Source and Ambisonic Systems in the outdoor market; and Radian Audio Engineering (an OEM driver supplier to both markets).
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@Chris
I own a Wisdom adrenaline-75 dipole system which is 20 years old. I still use the two subwoofers and electronic crossover from 75 hz and down, but the planar line sources are in storage. I thought that the planars were probably not as good as some dynamic speakers that took their place.
I was curious if you thought the planar line sources could stand up to what is available today. I’m guilty of audio nervosa, but who isn’t, given the rapid pace of audio development today. -Would appreciate your opinion.
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Well, I’m very familiar with that system because the engineer of the driver (david grabener) worked for my father and I later worked for the company that made it for Wisdom, at the time (Bohlender Graebener).
The subs are still great and are some of the TC Sounds woofers. I might consider a modern crossover DSP system for them.
Yes, the planars aren’t as good as some modern speakers but a lot of it was a design issue. Tommy Bohlender (who founded Wisdom) wasn’t an engineer and made some odd design choices. The issue with the planars is that they shouldn’t be playing to 80 Hz. They have very high distortion and essentially no output down there. They play comfortably to about 250-300 Hz or so. They also have very little output above 8-10khz and need some active EQ there.
I think that the system could be re/integrated at a higher crossover point (with those woofers or dropping a woofer in that box with a better low midrange performance) and DSP the system and get a good deal more performance out of it. You could use something like DEQX or a Minidsp SHD However, that’s a bit of a DIY project.
That planar was using ceramic magnets and is only about 84-85 db and can sound congested at higher levels (versus a newer neo design that has much higher dynamic performance) and had a fairly large cavity resonance around 6Khz because of the depth of the front magnets, but is still an interesting speaker.
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Thanks for your thoughts, Chris. The crossover point is 115 hz. You confirmed what I thought. The planar line sources will stay in storage. I have Reference 3A Grand Veenas in their place, running full range and bypassing that crossover. If I could find a buyer for the Wisdom system, it would be gone.
Jawdropping respect for your knowledge and will to explain so detailed and even with brand examples, thanks a lot!
I got accustomed to planars and dynamic speakers with ribbons, at the end AMT, for so long, that I had to smile when reading Fremer‘s last rave review about the 300k Wilson Cronosonic. He wrote:
————
„What exactly is it that has kept me for so long in the Wilson camp? It isn’t that the company’s speakers are without flaws. All speakers have flaws; you learn to ignore or listen around them. For a few Wilson iterations, I had to listen around the in- verted titanium-dome tweeter, which was fast but kind of edgy. The right associated gear helped smooth things over.“
—————
This is exactly what I always heard as a weak point of Wilson‘s, the relatively rough and edgy tweeter and the strong dependence on the right amplification to be able to live with certain characteristics (for the price).
What another great review of Fremer about those exceptional speakers. No one else except him would (n all respect) name such truths in such a review (most wouldn’t even notice the characteristic). Top reviewer.
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Thanks Chris. Really great reading.
In trying to find the answer to my original question about differences between ribbon and AMT tweeters I first found what they have in common:
-
Both use very thin diaphragm made from polymer film/aluminum foil laminate (or just pure aluminum foil ribbon)
-
Both have diaphragm uniformly driven more or less by either electrostatic (based on forces acting on electric charge in electric field) or electro-magnetic (using Lorentz force on a current—carrying conductor in a magnetic field} motor system
And differences:
-
Ribbon uses thin corrugated aluminium foil strip suspended in a relatively weak magnetic field and require an impedance matching transformer
-
AMT use a laminated diaphragm with polymer substrate instead of thin aluminium foil and do not require a matching transformer
Some questions from me on differences:
-
AMT diaphragm is folded but ribbon is not , is that correct? Therefore is it correct to write that AMT diaphragm can be described as squeezing air but ribbon cannot ?
-
AMT: the magnetic field from bar magnets or steel poles, which are located across conductors, is only partially crossing the conductors. But for ribbon tweeters, the magnetic field fully crosses conductors? What are these conductors? Any pics to show this?
As you can see my questions about differences between AMT and ribbon is really mostly about construction, ie what parts are required / different between AMT and ribbon
Wow! I bet you weren’t expecting such a detailed answer.
Wow, Chris. Thank you for taking the time to explain all this.
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A stunning amount of information, well presented.
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I think that Chris did answer many of your questions and I am flabbergasted by the useful information we got.
Manufacturers like Hedd, Mundorf (AMT), Piega (flat ribbon) are proudly demonstrating how their systems work, if it is the construction of these drivers you are looking for, Google searches might even reveal many more sources.
I am just curious why you are so focused on these. Every method has its drawbacks and the only way you are going to find out what is best for you is try speakers at home in your own room. You might be very surprised that perhaps none of these two drivers are the best solution for your room and ears.
Ever since I became interested in good sound reproduction again I have heard so many different good speakers. The variety utilized any driver concept mentioned above, they all sounded very good, they reproduced beautiful music. With today’s technology your room and speaker positioning matters more than the construction of the drivers.
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Actually I can’t find the answers to these questions. If you can find them can you help?
5. AMT diaphragm is folded but ribbon is not , is that correct? Therefore is it correct to write that AMT diaphragm can be described as squeezing air but ribbon cannot ?
6. AMT: the magnetic field from bar magnets or steel poles, which are located across conductors, is only partially crossing the conductors. But for ribbon tweeters, the magnetic field fully crosses conductors? What are these conductors? Any pics to show this?
As you can see my questions about differences between AMT and ribbon is really mostly about construction, ie what parts are required / different between AMT and ribbon
Yes we got a lot of great reading for sure, as I wrote to Chris already. A lot of info was not specific to this particular thread but a lot was very relevant obviously, especially the similarities between AMT and ribbon tweeters but less on differences.
I’m more interested, than “focused”, since I own ESS Heil AMTs and have always been curious. I actually wrote that earlier in the thread… The reason for my curiosity is because I shared earlier in the thread, that marketing makes it confusing sometimes. See my earlier example.
If this doesn’t interest you, you don’t need to participate in this thread right?. You are free to comment on threads that do interest you.
But if it interests me, I believe I am free to ask the questions to @Paul or @Chris_Brunhaver , whether it interests you or not.
Best regards
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Also @Chris_Brunhaver I thought you might find this interesting although you and Igor Levitsky probably already know this:
While discussion of dome tweeters isn’t relevant to my specific AMT vs ribbon query, an interesting thing I read by Dr Klaus Heinz (ADAM Audio and HEDD Audio founder), is that Dr Oskar Heil (inventer of Heil AMT) told him this (see highlighted):
In the history of science these is often multiple people solving the same problem or developing the same technology independently. It is as if there is a world-wide petri dish of facts, knowledge, need, etc. ready to grow the new idea. I have always found this intriguing.
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For those interested, a cool interview by Darko with Dr Klaus Heinz.
And he’s sitting next to the ESS Heil tweeter I have too, even though he makes his own AMT 
Chris did write that AMTs are folded.
If you like Oscar Heil speakers, another person who worked with Oscar Heil is Herr, Karl Heinz Sonder, he founded Abacus in Nordenham Germany. His son Hanno has studied physics (including acoustics and electronics) and is also extremely knowledgeable.
Abacus dedicated the 2 top of the line speakers Oscara 212 and 210 to Oscar Heil, hence the speakers names.
These would have been my preferred speakers by far. To my opinion absolutely superior technology, I love the sound of their AMT driver speakers and active technology. Their products are really made in Germany, all enclosures, all PCBs and electronics except of the components (that are only manufactured in Asia), even most drivers, the ones they use are all European.
But, unfortunately my wife doesn’t like the looks of the Oscara speakers. So we look and listen to many different options. I cannot dispute that there are better looking speakers out there. There are many options in that price range.