HES, High-Efficiency Speakers
Copyright 2008 © Troels Gravesen

Efficiency is inversely proportional to the moving mass and proportional to the square of the product of cone area and BL.

The number of (triode) valve amplifier producers today compared to two decades ago is phenomenal and there's a steadily increasing interest in high-efficiency speakers, thus this page on my experiences of high-efficiency speakers and some comments to what I've picked up from the web.

Why bother about 95-100 dB/1 watt speakers when we have almost unlimited power available from solid state or digital amps? Why do people build two watt single ended triode valve amps with subsequent need for extremely efficient speakers? Is it pure nostalgia or do these systems have something to offer conventional hifi does not?
I guess it has to be heard to be believed, and not only that, you have to kind of "discover" the triode sound. Some will, some won't and taste cannot be argued, but not all will think flea-powered triode amps and high-efficiency speakers are the best since sliced bread.
The need for extremely low moving mass calls for serious compromises in terms of possible cone break-up and possible linear and un-linear distortion as I'll elaborate on later.

There are not a whole lot of places to go and hear low-wattage SET amplification combined with good high-efficiency speakers. And even if we do find such a place, there's a fair chance we may leave, saying: "Is that it?" I've heard 50,000 USD triode systems that really didn't impress me. Silver trafos and everything to make the best of sounds and yet it didn't convince me at all. And I heard a system consisting of a cheap Chinese triode amp connected to pair of mock-up HES speakers that forever changed my ways of thinking hifi.

Art Dudley/Stereophile has these comments:
"Bear in mind that even the hard-core single-ended-triode movement wasn't conceived as just a new branch of high-end audio: It was intended as a whole new tree, by hobbyists who considered the old tree to be very, very sick. To the SET pioneers, audio had gone from being an interesting way of enjoying recorded music to a turgid and fetishistic exercise in irrelevance in which wealthy men with too much time on their hands try to outdo each other in identifying arcane sound effects in a handful of guru-approved recordings.
Also remember that the high-end audio establishment didn't take long to dismiss the SET guys as a bunch of self-conscious hipsters less concerned with fidelity than with the coolness factor: gracelessly aging punk wannabes whose willingness to live with grossly colored , narrow range sound reproduction is rivalled only by their disdain for any model of amplifier or speaker that might be owned by more than two other people.
The fact is, there's an archetypal high-end audio sound and an archetypal SET sound. At its best, high-end audio sound is impressive open and clear, with deep bass, shimmering highs, and a flair for thrilling spatial effects: At its worst, it's lifeless , boring, constricted, undramatic, uninvolving, and incapable of any suggestion of flow in the music.
At its best, SET sound has the kind of punch and drama that can startle you out of your pants - plus real musical drive, momentum, presence, and tone; at its worst, it has no bass, no treble, and a level of coloration and sheer fuzz that could drive you out of your house after a few songs".

"The punch and the drama" - is what we're always looking for and sometimes SET + HES can deliver this kind of aural nirvana. I've been sitting next to people experiencing SET + HES for the first time and saying: "Holy crap, this sounds like music!" I've had people in listening to the OB7 and saying: " - whauu...this sounds like a live performance!" So, what do these people do after returning home? They start yet another project based on an 85 dB heavy alu cone driver. Either the prospect of bringing two 100-200 litre cabs into their living room is too scary or the last 30 years of hifi-tradition is so engraved into our way of thinking hifi that we resign and turn to well-known technology.
In some cases I'm sure the prospect of huge cabs and glowing valve amps is simply not the way they want their system to look. We may have fixed ideas of what our system is going to look like and that's what it's going to be. Period!
From what I pick up meeting hifi people, most gear is bought on looks rather on sound. It has to at least look expensive and project potency. The drivers even have to look sexy! Very much like buying a fancy sports car, except that huge and expensive hifi systems do not attract (young) girls! On the contrary! Most likely it will only produce sympathy for the poor spouse that has to live with these weird looking monsters.
Expensive hifi gear is show-off to your (young) male friends and whoever is going to follow presentations on countless discussion forums, where people display pictures of their systems and are being congratulated every time they've made a new purchase.
Old farts - like myself - no longer care too much about visual appearance, although I do like to make nice products from time to time. The satisfaction and pride of good craftmanship is - hopefully - a never ending issue.

A lot of things can be found on the web on high-efficiency speakers and what these speakers can do - and not so much about what they cannot do.
So, can high-efficiency speakers do things low-efficiency speakers simply cannot? Well, try a 12-15", 96-100 dB/1W bass driver on an open baffle - or in a huge vented cabinet - and hear what it does to a kick-drum and make your own judgement.
Take an 8" Supravox, place it on an open baffle or in a TQWT, mate it with a proper 95 dB tweeter, do a proper crossover (the tough part) - and hear what it can do. Hearing treble from a good fullrange driver can be very special despite serious beaming and it may change our way of thinking treble as we're so intoxicated from 1" domes and how treble usually sounds.
Somewhere I said we cannot accelerate heavy cones as fast as a light-weight e.g. paper cone and I immediately had a mail from an engineer who told me I was seriously wrong - and he was right. Acceleration has nothing to do with mass, only force. Problem is we don't have magnet systems that will accelerate a 200 grams subwoofer cone and kick butt like an old 15" ALTEC driver in a 200 litre vented cabinet. It's two worlds. Subwoofer drivers are supposed to say ooomph... in small cabs and being often used to reproduce electronic music we really can't tell what it should sound like. Electronic music can sound like anything, but it's not the kind of bass performance I like to hear from an up-right bass. Acoustic bass shouldn't say oomph and I don't even think a Fender bass sounds the way it should from a conventional heavy-cone subwoofer.

Aiming at really, really high-efficiency, there are prices to be paid like in any extreme direction. 2-3 watts from a 2A3 SET triode is not a whole lot of power. Some manufacturers claim their 2 watt SET amps to perform well with 90 dB (minimum) speakers. Yes, yes, if you never ever play at anything more than - to my volume setting - well below average listening levels. I know what it's like to sit 5th or 10th row in a club and listen to a jazz quartet. It's damned loud and I want my ultimate speaker to be able to recreate some of that drama. This doesn't mean I drive my speakers to 95 dB average level - far from - but I may play my speakers a bit louder compared to the average listener judged from the visitors I have. Playing the 15" Tannoys at realistic levels I remember one visitor who almost fell off the chair from hearing a snare drum being hit hard. I mean, this is what it's like at a concert....
When I visit our local Symphonic Hall and all 70 musicians are on the stage it sometimes goes pretty loud too. No wonder even classical musicians are complaining about damage to their hearing. And by the way, there's not a whole lot of deep bass, even from a full orchestra with 4-6 bass players.
Now, recreating a live musical event from 2 watts take serious measures and I won't recommend anything less than 97-98 dB sensitivity, preferably 100 dB/2.8V. Some day I'll try to make a ~100 dB system from my Supravox drivers with the 215RTF in front and 2 x 215GMF in a double-tapered-quarterwave-tube. Simulation suggest a system sensitivity around 100 dB in the lower midrange. (Done: read here)
If we fall in love with a 2 watt SET amp we do not set up a 100 dB system from a 15-18 inch bass and suitable mid and tweeter drivers. We need very light-weight cone drivers, around 5-10 grams and an easy impedance load too. Flea-powered amps can't handle 4-5 inch voice coils and heavy cones. Some say they can but I don't think it's an ideal situation.

Single driver systems

Looking for high efficiency speakers on the web, we find the single-driver aficionados. These people use wide range drivers with very light-weight cones, reaching well up above 10 kHz, making the need for tweeters questionable. Sometimes only acoustic equalisation of the drivers are permitted. Crossover components are definitely no-no! Purity and simplicity at any cost?
If we want a single driver to do it all, the driver cone simply cannot be very big in order to keep mass low and maintain some treble level, and the price to pay is weak or no bass at all, unless we load it with a huge horn. A 5" driver fitted with a 3-5 grams membrane may be a suitable compromise. The Fostex construction at Nelson Pass' website may be the best compromise if it has to be "the one and only": http://www.passdiy.com/pdf/KleinHorn.pdf; I guess WAF is at an all-time low. But I'd like to hear it for sure.

For those interested in single driver speakers, the single-driver website http://fullrangedriver.com/ is a must.

If we increase driver size to 8" diameter, treble may be limited unless we use a whizzer cone and the whizzer cone introduces some new problems. I still have to see (and hear) a decent response profile from a whizzer cone driver. People play all sorts of games to tame the uneven response from these drivers and the often seen rise in amplitude at 3-8 kHz (e.g. Lowther).
The response graphs presented at manufacturer's websites are usually fake or digitally manipulated to look reasonably smooth. But response graphs tell only little about sonic performance. I've had drivers showing bad response profiles, yet didn't sound harsh or sibilant at all. All depends on what causes the response irregularities. As a rule of thumb: If a peak does not follow a serious dip in frequency response, the peak may be kind of good-natured, only representing higher level in a certain area, not tearing our ears apart as if derived from serious cone break-ups and sudden very sharp phase shifts (Again: Lowther).

There are a few exceptions to this. The vintage Altec 755C may be worth a look. http://members.myactv.net/~je245/755survey.htm
These drivers appear to have a rather smooth response all the way up to 10-11 kHz. But the price for a good pairs of these drivers is horrific. Sometimes 2-3000 US $/pair on eBay. Be sure you won't have any bass below 70 Hz from an open baffle here. These drivers have absolute cult status and pity no one makes a similar driver today. It could be made for nothing.

Here in an old Western Electric version.

PHY-PH extended range drivers

I think the French PHY-PH drivers look interesting. High efficiency drivers with lightweight paper cones - and no whizzers! These drivers are aimed at an upper limit of some 8-10 kHz and an unusual tweeter is supplied; a small 70 milligrams paper cone driven by a piezo electric device. It doesn't say much below 10 kHz. I would love to do an open baffle 3-way from their 12" drivers, the 8" for mid and possibly this piezo "thing" for treble. But this would be something like 25-30,000 DKK or ~5-6,000 US $. Quite some money. Let's take a closer look at one of these PHY-PH drivers, the H 21 LB 15. I recently found a French website showing a magazine test on drivers in this category. But first the data from the PHY-PH website (left image) - and some LspCAD simulation/crosscalc. Even with a few data we can try calculating if what the company claims can be right. Quite often we find major deviations from claimed data, usually too high sensitivity, too low Fs and too low Qt.

Click images in middle to view large.

Usually Re and membrane area is correct. Moving mass may be measured before assembling the driver. These are the easy parameters. Rms (the mechanical resistance of a driver's suspension (lossiness) in N·s/m) and Cms (compliance of the driver's suspension, in metres per newton (the reciprocal of its stiffness)) are some of the tricky ones as they are dependent on the signal applied during measurement. All drivers are highly un-linear devices and what we get is dependent on how we measure and there's no fixed rules for doing so. By manipulating cone mass, compliance, etc. in LspCAD we can usually by cross-calculation get a set of data that looks plausible. And I've often found these data to better correlate to what I measure for e.g. sensitivity. The claimed efficiency on the H 21 driver seems to be quite optimistic. 98 dB, although the website doesn't tell whether this is sensitivity or efficiency. Either way, I suggest being a bit sceptical when someone claims 98 dB from an 8" driver. At least don't expect much bass from such a driver.

Click image to the right: This is what a French magazine found. Here we read "94 dB" - and again we are not told what it is. 2.8 volts or 1 W? It does make a difference when we're dealing with a driver having an Re = 12.9 ohms. Fs is 10 Hz higher than claimed. I'm not surprised. This was what I found for the Supravox drivers as well. The frequency response doesn't quite look like what is shown at the PHY-PH website. Looks like we have a 5-6 dB peak at possibly 5-6 kHz. Hard to read the scan. The Qt is much higher than claimed. 0.83 rather than 0.59. Anyway, we have a driver that is born to be used on an open baffle. The interesting review can be found here (in French): http://petoindominique.fr/php/planhp.php

One important thing: What we often hear reported from single driver set-ups, is the revelation of minute details in the recording, things we have never heard before. BUT, if we increase a narrow frequency band some 3-4 dB - and from fullrange drivers with whizzer cones we often experience major peaks in frequency response, much more than 3-4 dB - we will always hear things we never heard before.
What we may experience here is sometimes not the ability of a certain driver to reveal details much better than other drivers, but simply a matter of amplitude (linear distortion) revealing certain parts of the recording more "transparent".
Amplitude and phase irregularities are linear distortion. The music really wasn't meant to sound like this. Taste cannot be argued, but don't tell me a particular driver can reveal details no other driver can if derived from an e.g. 5-7 dB peak in a certain area. It may overall sound good, and I won't argue against that, but it's basically a distorted signal and low-fi. To a certain degree we may learn to live with it, but a long term relationship is questionable.
More serious is non-linear distortion, harmonic, intermodulation and whatever may come from poorly constructed drivers or from the box in which the driver is placed.

One single driver set-up I think looks interesting is found at Decware: http://www.decware.com/newsite/mainmenu.htm. A tweaked Fostex driver and some interesting vented designs aimed at reducing standing waves inside the cabinet. But Steve Deckert, please show us high-resolution impedance curves from your designs.

An interesting comparison of single drivers can be found here: http://www.tnt-audio.com/shows/etf03_e.html; I wish I'd been there..

I think this "review" on a field-coil Supravox is interesting: http://www.supravox.com/feedback.html
It tells a lot about what this driver is good at and there's an interesting list of hints, cautions and shortcomings. Something we should consider carefully before engaging in 8" full range drivers. Read carefully between the lines.

I recently had an opportunity to test the all time full-range classics: Lowthers. Read here: http://www.troelsgravesen.dk/Lowther.htm
These drivers are in my "worst drivers ever had" category together with Wharfedale Super 8s. Sorry, Lowther lovers!

2-3-way high-efficiency systems

Next on the web we find people aiming at high efficiency but from multiple drivers and yes, we may even find crossover components. The Dick Olsher website is an example, http://www.blackdahlia.com/html/tip_56.html. Large 12-15" bass drivers in large vented cabs - or on open baffles - and a speedy 8" (Lowther, Fostex or even greencones) for mid + e.g. ribbon tweeters for highs. Gut feeling tells these are best driven by bi-amping and the bass section may like a sturdy solid state amp.

An interesting TQWT variant is the double Voigt horn, where all drivers are placed in conical horns (TQWT) of possible different lengths and firing into the same common middle section. The woofers can be connected in parallel or series depending on impedance of actual drivers.

The two bass drivers are fed through a huge coil, the mid no-whizzer-Lowther is having a series LCR to smooth the upper midrange/lower treble and the tweeter runs from a single cap. A fairly simple design, but finding the right drivers and doing the necessary driver tweaks has probably taken quite some time. I've heard a system like this fitted with a no-whizzer Lowther and a ribbon tweeter and it wasn't good. I brought my torture female vocals and it couldn't handle it. Serious distortion in upper midrange. Which brings me to a good case I've recently experienced :

My friend Michael bought an old pair of ported Dali 800 speakers with an unusual design. The main bass-driver being a Vifa M21 variant is accompanied by a Vifa P17 pp driver placed in its own enclosure at the bottom of the cabinet. Why would you have two bass drivers of different size tuned to quite different Fb (port resonance)? The P17 has a rather small cabinet fitted with a unusual long and narrow vent, apparently tuned to a quite low Fb. What puzzled me even more was that Michael told me he could run these speakers really high from his newly acquired 2A3 push-pull integrated valve amp, bought at DiyHiFiSupply/HongKong. This amp delivers some 2 x 8 watts and I didn't quite believe Michael until I vent down to hear for myself. The Dalis probably have some 90-91 dB sensitivity and the 2A3 push-pull amp could easily drive these speakers much louder than I would have thought possible. And by the way, this 2A3 pp is one heck of an amp. Highly recommended - with amorphous output trannies, please.

I'll borrow Michael's Dalis some day and make a report, because there's a good lesson to be learned here. This design was probably made by Hother Bak, not anyone in Danish hifi. The trick is - as is with the double Voigt above - to create an amplifier-friendly impedance profile. From own experience with PSE triodes (20 wpc) I know that some speakers having 90-92 dB sensitivity simply do not perform well with these mono amps, and some speakers having even 86-89 dB sensitivity are doing well. So, sensitivity is important if we cherish low-wattage glowing tubes, but impedance may be just as important. One thing tube amps do not like is low impedances combined with odd negative phase angles and what two individually tuned bass drivers can do is produce an overall smooth impedance like seen from the example below:

What's seen above is an example created from two drivers in my LspCAD files and it can be further improved by properly selecting drivers, box volumes and vent tunings.

27-04-2008: As promised, here's the report on Michael's Dali 800. Not exactly what we had expected, but this speaker has a very tube amp friendly impedance profile and no wonder it works well with the 8 wpc 2A3 PP amp.

It's remarkable that so few impedance plots can be found on high-efficiency commercial designs. Actually I don't know of any. How does a Lowther impedance profile look from an exponential horn? What about the Living Voice d'Appolitos? How about Beauhorns? And the Horning Hybrids? A few frequency response graphs have shown up lately, but impedance? Nobody apparently wants to speak about it - or manufacturers don't want to show it. Unfortunately few horn speakers show up in Stereophile to be dissected by John Atkinson. Impedance plots show an awful lot about how the drivers are performing in the actual cabinet, and higher up in frequency a lot about intrinsic driver performance, e.g. resonances in cones and suspensions.
Speaking about impedances and low-wattage amps we shouldn't forget that due to the usual low damping factor of e.g. SET amps we may get away with a bass-light design having an F3 of 55-60 Hz and still get a decent low-end response. Matching gear becomes very important. I suggest reading Paul Messenger's report on first-watt and transconductance amplification in the January 2007 Stereophile edition, page 23.

- size, efficiency and bass extension

No matter what we do, we have to balance these three basic parameters: The trade-offs in loudspeaker building.

Efficiency is inversely proportional to the moving mass and -
proportional to the square of the product of cone area and BL.

So, once upon a time -

- the family would gather happily around the Altec alter, and -

- women would help their husbands
building humongous cabs - or would they?

If size is not a limiting factor, we can get really low bass and very high efficiency. If size is restricted, we may have to sacrifice efficiency - or low-end extension. If an F3 of 60 Hz is fine, we can get efficient speakers from quite small speakers. Somehow I don't like the idea of supplementing a HES speaker with a mega-powered inefficient subwoofer as seen from some of the Avangarde-Acoustic speakers. 12" woofers in small cabs usually have very low efficiency and need a lot of equalisation to get the bottom end going and they are slow compared to the frontloaded midhorn they have to play up to. I'd expect the same kind of trouble here as seen from hybrid electrostats.

Post scriptum

I had a mail from a guy who had just heard a pair of vintage Pioneer speakers, actually the HPM-100. Below to the left:

The questions from the writer were: Why does this speaker provide such a pleasant sound? Why is it so easy to listen to? What does it have that most modern speakers do not have?
Well, I have never heard the HPM-100s, but this speaker is a Japanese clone of the famous JBL L100 Century (speaker to the right above). The JBL was a three-way system not having the supertweeters these Pioneers have.
One important thing that may be part of the "pleasant" sound and an "easy to listen to" speaker is probably the fact that a major part of the midrange is handled by a large cone. The JBL 12" bass driver really handles all of the midrange and the Pioneer probably also. Large cones = minor cone movement and reduced distortion. Now, the L100 is (was) not the most respected speaker around when it comes to neutrality, three-dimensionality and a number of other parameters we take for granted these days. The mid had major trouble due to poor filtering but I wouldn't be surprised if Pioneer had made a decent crossover for this speaker - and probably used a middriver not as notoriously difficult as the JBL LE5-2. I guess the system sensitivity is around 90 dB like the JBL L100.

A 12" driver usually has a cone area of a little more than 500 cm^2. This is more than 3 times a ScanSpeak 18W/8531G00 driver! Imagine 3 x 6½ drivers in the same cab. A 12" based system can do things none of today's 6" skinny runts can do. It seems like most diy'ers start with a 6" 2-way making deep bass from 24-33 litre and an efficiency rarely exceeding 85 dB. We're all impressed from the bass it can make - but after some time we find out it really can't play very loud and we try another 6" driver or we make a 2½-way from 2 x 6" drivers. And we're never really getting anywhere. So, if you can handle a 60-70 litre cab, find a 12" driver, mount it on the side if the speaker must be skinny and use a 6" for midrange with a point of crossover around 250-350 Hz - and now hear what happens. With a 12" making 90-92 dB and a large middriver that no longer also has to handle the bass - we have a much better starting point in making an easy-to-listen-to speaker. Generally people have way to high expectations from a 24 litre floorstander. And they're often mislead from much too optimistic specs. Very often you can subtract 2-3 dB from claimed sensitivity and 5-15 Hz from claimed low-end extension.

The speakers of the Sixties and into the Seventies often had high-efficiency paper-cone drivers - where the British constructors were heading towards heavyly coated bextrene cones with an all-time low sensitivity of 78-80 dB/2.8V and became famous for their neutral sounding speakers, e.g. Rogers, B&W, Spendor, etc.
What the British school of speakers did not have (apart from Tannoy) was maybe the speed and sparkle of some other designs - primarily from JBL, Altec and others. These manufacturers provided what was known as the "west-coast" sound = "tizz and boom". Lots of boomy bass and loads of treble.

By the way: Read interview with Joachim Gerhard (Audio Physic) on the secrets of old drivers: http://www.speakerbuilding.com/content/1039/ but don't think high mechanical Q is the only thing that makes a good driver. It must be balanced against a lot of other parameters.

11-03-2009:
Hi Troels,
I went round a friend of a friends house last night, who had about 4 valve amps and lots of vintage speakers. I was only interested in the valve amps as i have never heard one before. He had some Tannoy gold line monitors that were made in 1962, with a 10 inch dual concentric unit, hooked up to a 22 watt valve amp. I was blown away by the sound, I just couldn't believe how good the sound was that come out of these old speakers. The sound was just huge!! with fantastic clarity. I went back home to my Musical Fidelity A308 pre/power and ekta grandes and it all sounded a bit dull in comparison. How can something so old still sound so good? He seems to think its all down to the valve amps? If so, I am very impressed and will have to look into investing in a valve amp at some point in the future.
Cheers, Steve

Hi Steve,
Well, I'd say 90% speakers and 10% amps. I don't know what amps we're dealing with, but most likely you have experienced what the above HES scribbling is all about: The punch and the drama that very few modern speakers can manage. Modern drivers may excel on a number of parameters, but they were made to make deep bass from a small footprint (= high-WAF), not for a realistic reproduction of a e.g. kick-drum. If we're talking high-efficiency 10-12-15 inch drivers @ 92-98 dB/watt - we need huge cabs - or we may sacrifice the low end. I'm sure these Tannoys you've heard do not go nearly as deep as your Ekta Grandes, far from, but the other qualities are hard to ignore.
Thus my preference for valve amps and high-efficiency speakers.

High Efficiency Speakers, II.

2006 has brought the C17, Classic-3W and Poor Man's Strad, all producing more the 90 dB/2.8V and the C17 has had another round of tweaking, now running with a Monacor DT300 tweeter + waveguide. Read C17-II.
Looking for more than average sensitive 8" drivers for a new 3-way floorstander I came across the discontinued SEAS CA21REX driver. Solen in Canada had a few and I found mine in Stockholm at HiFiKit.

What I wanted was an 8" driver with paper cone, foam surround and sensitivity around 91-92 dB/2.8V and this CA21REX comes close - except for the foam. The CA21REX has a short voice coil compared to the new CA22RNX and it is not made for excessively large cone excursions and may benefit from some acoustic loading. An additional magnet will provide another single dB of sensitivity. More on this later.

Last year I ordered new foam surrounds for a pair of Dynaudio 21W54 drivers from Simply Speakers in the US. The surrounds didn't fit the Dynaudios at all - they said it didn't matter if the outer diameter was too small; my goodness, of course it matters - but these foam surrounds make a perfect match for the SEAS CA21REX drivers, although they are rather thick and may provide a higher Fs than intended. We'll see. The foam surrounds will replace the rubber surrounds of the CA21REX. For midrange I think surrounds made from foam sometimes perform better than rubber, but it depends on a lot of things. Foam has very low weight and for lightweight paper cones we may not need a heavy load of rubber (mass) at the end of a thin "sheet" of paper.
Basically the surround is supposed to make an acoustic barrier between front and rear and only provide a minor part of the total compliance, approx. 20% according to Vance Dickason. The spider, not the outer surrounds, should provide the main compliance, but this may depend on the properties of the cones, e.g. stiffness and mass. The properties of the outer surround are a major concern in designing a driver and most often, very little have been done. What we see is irregularities in the critical upper midrange frequency response due to resonances in the cone and suspension. Sometimes manufacturers will add a ring of soft glue where the membrane and the suspension meet in order to terminate the energy from the cone. Adding mass here may - hopefully - absorb the energy and leave better performance. But it's tricky business. Some of the old Vifa C17 and C20 drivers are good examples of drivers where a lightweight cone is terminated with a very thin foam suspension and the frequency response is generally smooth and provides easy crossover construction. And they sound good too...

Surfing the web I also came across Audio Note and here we find an 8" drivers with a claimed "high sensitivity", presumably around 91-92 dB. The SPKR04 looks interesting. Large magnet, long fibre paper cone, foam surround and a coated dust cap. The chassis is a non-vented old SEAS version, the same as used for the CA21REX. Silver voice coil. There's only one obstacle to this driver, the price: 310 £/ea. Serious money.

Following the Audio Note path, I also came across the review of the Audionote AN-E Lexus Signature and this is interesting reading: http://www.stereophile.com/standloudspeakers/506an/index.html. First of all the claimed sensitivity of 98 dB (!) turns out to be 92.5 dB as from John Atkinson's measurements. Some difference I should say. I'm sure the answer from Audio Note would be that this speaker should be placed in corners, giving the additional low-end decibels, at least in the bass. But it won't raise the response in the midrange and treble by 6 dB, that's for sure. However, this clearly shows there's no limit to how much producers will overrate their products. The point of crossover is claimed to be around 2 kHz and it turns out to be close to 3 kHz. See graph below.

This speaker is also available in an all-silver version for the humble amount of 69,500 £. Three quarter of a million Danish Kroner. Well, well, well......

An 8" driver running to 3 kHz? Well, all depends. Next the frequency response from this AN-E speaker is claimed to be 17 Hz to 23 kHz. Now that's something! Qvortrup suggests that the woofer's theoretical maximum sensitivity is in the neighborhood of 100dB—and the overall system sensitivity is stated as a very high 98dB....(read here) "theoretical maximum sensitivity"..... what a load of rubbish! 98 dB sensitivity and only 6 dB down at 17 Hz. Dear friends, seek no more. Go buy the AN-E and all you need is 2-3 SET watts from a good 2A3 valve.
We could try to simulate cone excursion to make this performance, but it's futile. The paper cone would have left the chassis for good from making this stunt!

The Tonegen tweeter in this kit seems a bit odd. Peaking severely at 10 kHz and falling some 8-10 dB from 10 kHz to 20 kHz. It's really not a problem getting a 92 dB tweeter these days that will perform better. I don't say the sound of the Tonegen tweeter isn't good, but....
Anyway, this doesn't mean there may not be a lesson to be learned from this speaker, in particular because it's derived from the extinct Snell E/II designed by Peter Snell more than two decades ago and Peter had a lot of good ideas.
A simple two-way from a relatively large bass driver, a 1" dome tweeter and the crossover is simplicity itself, 2nd order to the bass and 1st order to the tweeter. Now this is interesting. Why not the other way around? 6 dB to the bass and 12 dB to the tweeter... Well, one explanation may be that 12 dB to the right midbass may provide a close to 24 dB roll-off characteristic and this - sometimes - goes well with a 1st order filter to the tweeter. It's a tough job for tweeter and it needs an LCR circuit to flatten the impedance and keep response down below 1 kHz, otherwise we'll burn the voice coil quickly. Below the AN-E kit from a German magazine. I don't know how old this scan is.

The notch filter is interesting as it works exactly at 1 kHz and the two 1 ohm resistors in parallel may not only flatten the impedance notch but also take the impedance close to zero at this frequency.
In a probably later crossover version taken from the Owner's Manual, this is found (my transfer to LspCAD):

Not much different from the first one shown.

On the AN website it is clearly stated that these speakers should placed in corners to perform as specified and that's fine if you don't care about early reflections and lack of transparency. Not so clear is what the various bass drivers are. Apparently the SPKR-002 and -003 are small magnet 8" drivers and the SPKR-004 and -005 have a larger magnet, the -005 even an alnico magnet although this is not available for the time being due to supply problems of alnico magnets. My best guess is that the -002 and -003 are 89-90 dB drivers where the larger magnet may provide an additional 2 dB, thus 91-92 dB. And this in the critical midrange. It doesn't matter what a driver does at 1 kHz (old time practice for measuring a driver's sensitivity was at 1 kHz). A driver may easily have a sensitivity around 93-94 dB at 1 kHz, but it doesn't help if the sensitivity is 90 dB at 400 Hz. The lower and middle midrange (160-640 Hz) is basically where a driver's sensitivity is defined. From a 20-25 grams cone weight and a magnet size of 120 x 20 mm it's difficult to extract more than 92 dB sensitivity unless we take special precautions in designing the voice coil and magnet gap.

Just found this picture on the web. These are probably the new Audio Note drivers with new spider chassis. To the left the alnico version. Right/front, the ceramic version and at the back, the small-magnet version. Dependant on the voice coil length, impedance and cone weight, these driver may easily reach 93-94 dB sensitivity except for the small-magnet version. Some of former SEAS cones are blue (CA25FEY), so these are not necessarily hemp cones.

If we have to go higher in sensitivity, we need a much lighter membrane like seen on the Supravox 215GMF where we are down to around 12 grams moving mass. This makes some 96 dB sensitivity, but there are other problems when we do this. I've recently tried the 215GMF and I was very impressed by its sensitivity, but not so much from the serious cone break-ups at 1-2 kHz, right where it was least needed. This driver apparently needs a horn or a high-sensitivity middriver from around 500-800 Hz.

With a suitable ribbon a two-way can be made with sensitivity around 96 dB/2.8V (- and 8 ohms impedance!)
Unfortunately the 215GMF has a response like this:

The blue is the 215GMF. What happens between 1 and 2 kHz is irreparable.
The red is the 215RTF, seems like a better options, but there are other problems.

Anyway, this wasn't going to be about Supravox, rather a conventional 2-way from an 8" midbass and a dome tweeter. Back to the -

SEAS CA21REX + Dynaudio Esotar.

These drivers were set up in a 60 litres cabinet and a vent tuning of approx. 30 Hz was provided. The crossover is this:

Fairly simple crossover. Basically 1st order on the electrical side and drivers has same polarity as the tweeter being in front of the bass driver. Same as Audio Note. The CA21REX doesn't behave too well at 800-1000 Hz and a minor bump had to be removed by a notch filter.

SEAS CA21REX + Dynaudio T330D.

The SEAS CA21REX with extra 120 mm magnet.

By the way: The "DD" of the SEAS CA21REX refers to "two high conducting rings placed at each end of the motor coil. These act as eddy current brakes, firmly halting additional cone excursion as the rings enter the magnetic field". Thanks to Martin Colloms/High Performance Loudspeakers, page 79, 6th edition.

So what does it sound like? Well, better than expected I have to say. The Esotar is an excellent tweeter but it's on a tough job running from a single 6.8 uF capacitor. Placing a finger on the dome it can easily be felt that it's moving a lot due to the shallow slope of the crossover. What struck me the most when I first connected the speakers was how lean it was. The bass goes really deep from the 60 litres cab volume and you surely don't miss any low-end information. The bass is fast and transient and maybe lacking in some upper bass/lower midrange warmth. This cabinet is reasonably braced and I guess the severe resonances of the low-damped AN-E plywood box may add to the overall sound and possibly give a warmer presentation. The midrange is far better than I would have thought and dispersion doesn't seem to be a major problem and I'm sure the tweeter's 1st order filter helps here. Would I listen only to my latest vinyl - the Anthony Wilson trio/our gang - and similar stuff - then I could easily live happy ever after with this speaker. Transparency is fine and listening to a drum kit is very enjoyable. It is physically a big speaker, and it sounds like a big speaker.

Response of drivers in cabinet without crossover attached.

The CA21REX is well above 90 dB for 2.8 V, but had we used the 1 kHz rule it would have been around 94 dB. Actually the PD from SEAS says 93 dB, which is not very useful. The Esotar performs exceptionally well but is does have a very early roll-off compared to many other 1" domes. Really designed for 1st order filters. And I'm sure the Dynaudio people would tell us not to worry, this tweeter can take a lot of heat.

Changing the rubber surrounds of the CA21REX to foam surround.

Rubber to the left and foam to the right.

Rubber and foam to the left and the rear of the DT300 tweeter that will be used in these experiments.

Changing the surround was easier than anticipated. The rubber can usually be peeled off from the metal chassis and with a scalpel the rubber was cut close to the cone. Slicing off soft rubber from a soft paper cone with a scalpel is not funny at all. However, in this case it was possible to also peel off the rubber remains from the cone - very slowly and it only ripped off a very small amount of paper pulp material. So all in all, a nice job could be done.

New cabs for this experiment:

A pair of test cabs were made resembling the AN drawing above. Approx. 55 litres of volume and I chose MDF for front and rear panel, chipboard for top, bottom and sides. Mixing various materials may not be such a bad idea. The modest bracing of the AN cab was followed closely and also modest damping material was added. I admit this cab appear less resonant than I had expected. No bitumen pads were used.

TS data CA21REX-foam/magnets:

The Fs really went up from this foam surround. I had hoped for some 40 Hz, but 47 Hz is a bit on the high side. From the Supravox drivers I learned not to worry too much about the Fs as these drivers have an Fs around 65 Hz! Yet, they provide excellent bass from 80 litre cabinets.
Looking at the math, 55 litres cabinet volume is too much for the AN driver as well as the modified CA21REX driver. They both - ideally - should have smaller cabs, around 30 litres, but here we go with 55 litres and should make an F3 around 42 Hz. The modified CA21 REX wasn't run in when these data were made and hopefully Fs will go down a bit and subsequently increase Vas. Fs around 40 Hz would have been ideal. The foam surround has a thickness of 0.7 mm, where the Vifa C20 drivers use 0.5 mm foam. The Vifa surround is very narrow, where the new foam surround for the SEAS has a larger half-roll. I guess 0.6 mm foam for the SEASs would have been ideal. So what about sensitivity from these tweaks?

The crossover:

Measurements

High Efficiency Speakers, part IIb
Vifa C20WH-07-09

Looking at extinct Snell E/II and /III speakers, I came to think about the old Vifa C20, 8" driver. The C20 is found in numerous versions like the C17. The C20 has a lightweight paper cone, a narrow foam surround and the "H" provides a 90 mm magnet for a 1" voice coil, probably giving decent sensitivity. Googling the C20WH, I found a single shop where the C20 was available, probably some old stock and this is what I found in seller's storage.

Crossover version 2 and LspCAD SPL and phase prediction.

The X-over

10 ohms resistor used for by-passing the 0.56 mH coil. 10R or 12R can be used.
I suggest using 22 uF pp in the bass section and 2 x 33 uF pp for the "68" cap, however electrolytic, as seen above, will do fine.

Crossover components available from Jantzen Audio.

Measurements:

SPL of C20 "as-is" (red) and one 90 mm magnet added (blue).
Reading merged with nearfield response at 350 Hz.

Left: SPL from drivers without crossover attached. Right: SPL from drivers with crossover and summed response from system.

Impedance of final system. Smooth from 200-20,000 Hz and with the
usual bumps from a vented system, tuned to 40 Hz (72 mm (ID) x 50 mm vent).

SPL from system at 2.8V. Red = minimum phase.