Avantgarde Acoustic Duo loudspeaker Measurements

Sidebar 3: Measurements

The importance of the Avantgarde Acoustic Duo's sensitivity result puts this parameter at the top of the list of lab tests—I measured 103dB/2.83V/m, possibly the highest yet for a Stereophile review. Moreover, the impedance rating is a genuine 8 ohms, which doesn't compromise the sensitivity value.

The implications are substantial, and in no small way help define this design's engineering value. 103dB means that a loud program level of 96dBA is attainable at the listening position with input of just 0.5W. The Duo is wholly compatible with 3W-and-up SE amplifiers, while an average 10W peak of SE power will produce a genuinely high 108dBA in a typical room.

Suppose a 250Wpc Conrad-Johnson Premier Eight A had been on your amplifier wish list; with the Duo, the Premier Eleven A's 60Wpc would be more than ample, the combination capable achieving a stunning 115dBA maximum sound level. If a Cary CAD-805C is your dream partner for a conventional speaker of healthy sensitivity, Cary's 10W 300SE will be ample, saving thousands of dollars and perhaps providing an even better match.

On the other hand, the Duo's wide dynamic range means that it can drive larger rooms with aplomb, approaching the peak sound level of the Wilson X-1/Grand SLAMM. I found that the Duo will accept the equivalent of 100W unclipped music program of normal spectral content, which, in a good-sized listening room, will result in levels of 118dBA. It will possibly reach 110dBA in larger spaces, such as a small ballroom.

As well as being sensitive, Avantgarde's horn technology delivers genuinely high efficiency at turning electrical power into acoustic power: 20dB (100 times) greater than such low-efficiency classics as the LS3/5a or some of the older big, full-range Apogee planars. The Duo is approximately 10% efficient from 150Hz to 20kHz, compared with the diminutive LS3/5a at just 0.1%.

Fig.1 shows the impedance magnitude and phase plotted for the horns. A smoother characteristic would ideally be preferred, given the higher source impedances of single-ended tube power amplifiers. Note the relatively high minimum value of 5.8 ohms at 9kHz, the moderate variation from 8 ohms nominal of +6, –2 ohms from 100Hz to 20kHz. The magnitude peak (31 ohms) of the midrange driver is not important, merely indicating that its fundamental resonance lies at 58Hz. Peaks of high impedance are relatively harmless if the overall impedance remains fairly high, as it does in this case. At near-infrasonic frequencies the impedance finally falls to an inconsequential 9.5 ohms. There's no cheating with the Duo on impedance in order to give a falsely high sensitivity reading.


Fig.1 Avantgarde Duo horn section only, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Fig.2 shows the individual responses of the two horns, the moving-coil woofers, and the port (the frequency control was set to its maximum position for the latter two traces). A low-rate crossover is placed at approximately 200Hz, typically 6dB/octave, and the out-of-phase setting on the woofer panel correctly imparted a phase notch of some 23dB at around 170Hz. The implied transition frequency between the midrange and treble horns was a little lower than specified, at 1.7kHz. The exact frequency was obscured by the lift in the midrange response at this point.


Fig.2 Avantgarde Duo acoustic crossover on tweeter axis at 5', with the nearfield midrange, woofer, and port responses plotted below 500Hz, 300Hz, and 1kHz, respectively, with "Frequency" set to maximum.

My presentation of the axial frequency response (fig.3) is based on a greater-than-usual 5' microphone distance to help improve the driver balancing. This is because it's hard to determine know the rate of intensity decay with distance for multiple sources, each a different distance to the measuring microphone. With a fairly tall speaker like the Duo, it's also true that a median measuring position will give an artificial prominence to the central tweeter horn. So in practice, some of the elevated treble level (+3dB up to 8kHz) seen in this graph will actually be more in balance at a 3–4m listening distance.


Fig.3 Avantgarde Duo, anechoic response on tweeter axis at 5', with complex sum of the nearfield midrange, woofer, and port responses, "Frequency" set to maximum, plotted below 300Hz (top trace) and of the woofer and port responses, "Frequency" set to minimum (bottom trace).

Above 8kHz, it can be seen from fig.3 that the treble stepped up by an average of 4dB, reaching a commendably (for a horn) extended upper limit of 19kHz. The main range was remarkably smooth, as was the midrange; even in this narrow-band, high-resolution analysis, the output was marred only by the minor "glitch" in the crossover range at 1.7kHz. This aside, the speaker met ±3dB limits from 60Hz to 8kHz—a good tolerance, if still leaving room for a bit of "character" arising from the practical perceived tonal balance.

In the base, the nearfield measurement indicated the typical –6dB bass cutoff point at normal woofer settings to be 36Hz; a respectable value, if not low enough for a true subwoofer. The top trace to the left of fig.3 is the complex sum of the woofers and port nearfield outputs with the frequency control set to its maximum position; the narrow bandpass trace is the summed nearfield responses with the woofer system set to the same level but the frequency control set to its minimum position.

I noted that while the mid tonality generally tended to moderate lightness, this character became still more noticeable at greater listening distances. It sounded as if the frequency response of the mid horn varied a little with distance. To investigate this proximity effect, I charted the output of the midrange horn at distances of 1', 2', 3', 4', and 8' (fig.4). Comparison of the curves indicated that there was indeed a proximity effect, and that the response did "thin" slightly at farther distances. Some compensation could be obtained with the woofer controls.


Fig.4 Avantgarde Duo, response of midrange horn with the microphone at (from top to bottom): 1', 2', 3', 4', and 8'.

I was agreeably surprised by the uniformity and integration demonstrated by the off-axis response plotted in the vertical plane (not shown). Absolute and relative levels varied little over a 30° vertical window, the only point of interest being between 1kHz and 2.5kHz at the interchange between the mid and treble horns. On the optimum listening axis, the crossover and drive-unit phasing has been skillfully adjusted for the flattest response, while modest cancellations above and below axis reveal a true power maximum at around 1.7kHz, this associated with the mid horn.

In the lateral plane (fig.5), very little change occurred up to and including 30° off-axis—just some smooth attenuation of the upper-treble range, which remained uniformly extended nonetheless. By 45° off-axis the result was still extraordinarily good except for a moderate bump at 1.7kHz, which also appears in the in-room response. What a surprise to find a horn speaker with such a wide and well-controlled directivity.


Fig.5 Avantgarde Duo, horizontal response family, from back to front: differences in response 45°–15° off-axis; reference response; differences in response 15°–45° off-axis.

The Duo's response in the listening room was fascinating, and I had looked forward with interest to seeing the effects of these directional horns on the overall energy response (fig.6). Remarkably, the Duo offered a substantially uniform energy output, the in-room response meeting ±3dB limits from 36Hz to 17kHz (including the woofer). There are some expected minor lumps and bumps, but the interface to the room can be seen to be very well controlled by these carefully designed acoustic devices.


Fig.6 Avantgarde Duo, spatially averaged, 1/3-octave in-room response.

Two critical points can be made here. The 4.5dB bump at 1.7kHz was indeed pervasive. A function of the low-pass cutoff of the mid horn, it did occasionally pop up in the Duo's sound depending on music program. Second, the power response in the treble above 8kHz was a little too good for comfort; a gentle tapering-off might well be preferred. This feature could well explain why the upper treble sometimes sounded more forward and "ahead" of the mid region, although the subjective magnitude of this effect also depends on the matching electronics. Some SE designs have a degree of softness in their high range that is in fact complemented by this horn's characteristic power response.

Good output in-room was seen to below 40Hz, though it is fair to say that, by audiophile standards, the Duo's woofer is not a true subwoofer, for which extension to 25Hz or 20Hz would be expected. In practice, some scope is available for adjusting the low-pass crossover frequency and the woofer level to provide greater effective and subjective extension. Though enthusiasts could easily add more extended active subwoofers as required, when I tried the big REL Stentor it had insufficient voltage sensitivity for a good match.

Summing up, the in-room response was wide and remarkably uniform in balance, a testament to the Duo's good off-axis energy response.

Theory tells us that a high-sensitivity speaker should have low distortion and greater freedom from dynamic compression. With the aid of ear defenders (!), I was able to explore the Duo's distortion performance at genuinely high sound-pressure levels (shown in Table 1). Only the second and third harmonics are shown; the rest were below measurement limits and were inconsequential.

In the bass, a single speaker channel overloads above 105dB at 1m for frequencies below 50Hz in equivalent anechoic conditions. This is, in fact, a good result for a direct-radiating woofer system. In practice, with room-boundary reinforcement and on typical program, the woofer section is capable of keeping up with the main horn system up to the 110–115dB limit.

Taking the bass-distortion results first, these were quite reasonable: for example, at 100dB at 35Hz and 50Hz, the distortion will be barely audible in practice. At normal 90dB levels, the results were fine for both 35 and 50Hz.

Handsomely delivering the promised high linearity, the midrange horn distortion was astonishingly low, even at a measured 110dB test level. Even at 100 and 90dB SPLs, the third harmonic was unmeasurable, and the second was well below 0.1%—this rivals a good push-pull electrostatic speaker and even some amplifiers! Even at 100dB, the third harmonic remained negligible, while the second was still excellent at just 0.12%. Very fine results were also obtained for the tweeter, if not in the same class as the mid system. At 90dB, the second harmonic was 0.13%, with third at a negligible 0.03%. By 100dB, the second had risen to a tolerable 1%. By 110dB it hadn't broken loose, still holding to a satisfactory 2% of second harmonic. At this sound level, natural distortion in the ear at the equivalent room SPL is likely to exceed this figure! With that low third harmonic content, what a match this Duo speaker would make with similarly gifted triode amplifiers!

I noted some differences in arrival time from each part of the system; for example, the treble was early, arriving 0.6ms ahead of the mid (see the step response, shown in fig.7). Nevertheless, this is a good signature, especially for a horn, and there was no evidence of significant infrasonic ringing from the metal tweeter diaphragm. Looking at the broad Energy-Time Curve (fig.8), it looks good enough down to –25dB and 1ms elapsed. Some artifacts then hang on for a while.


Fig.7 Avantgarde Duo, step response on tweeter axis at 5' (5ms time window, 30kHz bandwidth).


Fig.8 Avantgarde Duo, Energy-Time Curve on tweeter axis at 5' (unfiltered, unwindowed).

Turning to the "waterfall" cumulative spectral-decay presentation, my usual 10dB/division graph (not shown) emphasized longer-term, frequency-resolved decay and showed a complex field. Overlapping resonances could be seen in the 1.7kHz crossover range, plus some "clutter" at high frequencies that might well impair upper-range transparency. A horn throat-resonance "ridge" could be seen around 17kHz, but otherwise the treble decay was pretty good. In fig.9, a graph weighted to resolve resonance-decay speed by using 5dB/division resolution in conjunction with a 0.1ms filter setting, the Duo's early treble decay is especially good, while the upper-midrange problem can be clearly seen. To investigate this further, I ran a waterfall plot for the midrange horn system alone (fig.10) and got a clear presentation of the long-lived 1.7kHz phenomenon.


Fig.9 Avantgarde Duo, cumulative spectral-decay plot (0.1ms risetime).


Fig.10 Avantgarde Duo, midrange horn, cumulative spectral-decay plot (0.1ms risetime).

The bass box was well constructed, with no significant resonances in its working range save from the slotted port. As for the horns, those curved ABS sections seemed rigid and well damped, with no consequential structural resonance modes.—Martin Colloms

Table 1 Avantgarde Duo: Harmonic Distortion

SPL(1m)Low Bass 35HzBass 50HzMidrange 1kHzTreble 5kHz
100dB3.40% .30%0.60%1.50%0.06%0v.low1.05%00.08%

* Full overload/clipping in woofer system

Avantgarde Acoustic GmbH
US distributor: American Sound of Canada Inc.
Richmond Hill, Ontario L4E 3M7

dc_bruce's picture

driven by an all-tube BAT electronics chain, including phonostage. The bass was a mess, perhaps because the person who set the system up didn't take a lot of time to dial the bass system in. On the other hand, the rendering of Ella Fitzgerald's voice was simply extraordinary in its realism and palpability. So much so, that I forgot to even listen for audiophile things like imaging, soundstage depth and so on.

Even in today's dollars, these are out of my reach financially; and, obviously they visually dominate any normal-sized room. But, if I did have the room and the $$$, these sure would be tempting, especially if a good digital LF room correction system were applied to the box woofer.

FWIW, I have heard Klipschorns in multiple settings, driven by a variety of electronics; and they never tempted me in the least.

Anton's picture

Modern "high end" is a full decimal place to right in the wrong direction.

I know, shut up and get back to my Silver Bullets in my single wide.

thatguy's picture

I wonder as the price goes up what the point is where the high price actually becomes a good factor for a certain type of well off shopper.
I'm sure there are those that don't want it if it is at all affordable. The crazy high price adds to the uniqueness and lowers the chance that anyone that visits them will have the same.

dc_bruce's picture

By "today's dollars," I meant the purchase price reported in Collums' review, as adjusted. $80,000? That's truly insane.

Glotz's picture

LMAO... You funny.

I just bought a case of PBR and I'll be over in 10 minutes!

partain's picture

What is meant by a "dry" chosen alignment of subwoofer and room location ?

tonykaz's picture

Some Bicycle Box cardboard, a razor knife, a glue gun and some Scans Drivers will provide anyone with super performance at Schiit Level Prices.

Avant is selling Artistic and Active Sculpture Art to wealthy or near wealthy Socials.

apparently only comparable to Wilsons.

This reads like prospective Marketing copy.

Tony in Venice

JHL's picture

...there you go again.

Does the inherent corruption of material currency ever weigh just too heavily on the socially deeply, deeply virtuous? Must be exhausting.

tonykaz's picture

Are we discussing DIY or something else ?

What is "inherent corruption of material currency" , are you referring to $80,000 Horn Loudspeakers?

Tony in Venice.

JHL's picture

...what we're "discussing". I thought the context was the review but apparently it's really the unsavory dollar and how it comports with your next social formulations on acceptability.

tonykaz's picture

Are you objecting to an opinion?

Nearly everything is this Publication is solidly Opinion Based. ( Mr. JA1's measurements are not )

Of course you have your opinions, don't you? Maybe you aren't allowed to voice your points of view, except to mention mine.

I don't mind you making acidic comments, they would probably be useful if they were probative.

Every single reader of Stereophile has thier own unique opinions and points of view, based on their personal Life experiences .

Bon Vivant

Tony in Venice

JHL's picture

...on the nature of opinions before.

First, they are not a right because opinions and rights are in different domains. There's no conflation of opinion and *speech* insofar that the opinion, as I once said, that I can walk into a speeding bus unscathed is obvious nonsense. I may claim a right to say so, but it remains rubbish.

Second, the right to speech is a construct of structural contract in the west and obviously has no bearing on the comments section in Stereophile - different domains here again. Whether you can make claims about audio tantamount to walking into busses and defend them as tacit rights is also crap.

Third, therefore these abstracts, as you stretch them about, don't give you cover. We're not talking about rights and we're not restricting yours. You making vague references to oppression as some *further* moral slight is just as illogical.

The heart of your inferences hereabouts are commonly that a large price tag carries all this social baggage that somehow involuntarily induces your negative value judgements on others as if such were some cosmic pronouncement. Some universal truth.

You can hold and say whatever dumb opinion you wish but that doesn't validate it. Speaker parts in bicycle boxes is only a tortured commentary on the manufacturer in the review above in your own odd compulsion. The rest of us don't care. I know I don't.

Which is my opinion. Here's another opinion. Let the reviewer have his.

tonykaz's picture

I'm reviewing the reviewers. ( journalists ) ( I love Stereophile and admire the writers and thier work, for the most part )


you don't have the right to limit others points of view. Do you?

Of course, I can understand that you have anxieties that you need to present but I have no obligation to honour you or negatively respond to you trolling and I won't !

You certainly write an interesting complaint.

Mr.HR suggests that the people that buy this super expensive stuff don't actually read Stereophile. Do you own Avantguard ? or horns in general ? Hmm.

I've owned horns ( klisph ) and built horns.

Tony in Venice

JHL's picture

Reviewing the reviewers - if that's what you're doing when you divine the supposed intersection of price, social class, and the character of others - has nothing to do with this "right" you've just introduced.

As I just noted, obviously rights lie elsewhere entirely. Rights also fail your formulation when you challenge mine, such as they are, to note your using these threads to comment like you do. How has anyone abused a right somehow "limited" your remarks? I began with *there you go again* because there you go again.

This isn't trolling, Tony. It's challenging the *anxious* compulsion to use a hifi magazine to launch off-topic critiques not supported by evidence.

Social warriors overlook the affronts they commit correcting a world whose imagined affronts they can't take.

AJ's picture

Some Bicycle Box cardboard, a razor knife, a glue gun and some Scans Drivers will provide anyone with super performance at Schiit Level Prices.

Send me the plans so I can build it Tony, I have all those ingredients. I have tons of Amazon cardboard these days! Is that of audiophile quality like "Bicycle box"?



tonykaz's picture

Bike Shops have plenty of large double thickness Boxes for FREE.

Experiment with design, copy Avant but with 8 sides or copy Klipsch.

The Horn importance comes from it's inherent 9 db gain, driver coupling to the horn can be easily worked out.

Large Bicycle Boxes are nearly inert but they can be doubled up with glue-on added mass patches in places where you discover sympathetic resonances.

It's great fun and you may become one of the great loudspeaker manufacturers.

The drivers are the singing voice, the horn is mechanical amplification.

Tony in Venice

ps. you might also invest in a bicycle and become completely Solar Powered ( like we were 200 years ago ), my bike gets 10miles to the Cheeseburger.

Cooking Man's picture

I owned a pair of Duo’s from 2001 to 2017. These had the next generation subs from Martin’s review pair. If I recall correctly the Duos (or was it the Unos?) were Stereophile Speaker of the Year one year in the early noughties.
I loved the Duo’s for their slam,dynamics,visceral presence and sheer joie de vivre.
However as the years rolled by their Achilles heal become more and more troubling. In my system in my room with my music the crossover between the glorious midrange horn and the treble horn was the problem. I tried numerous amps over the years (Quicksilver,Pass Aleph,Mark Levinson,Graff,First Watt and latterly Mactone), performed the factory Omega upgrade and around 5 years ago spent a small fortune on a custom cross over with silver Dueland components. All enhanced the great qualities BUT only served to emphasise the Achilles heal.
I endlessly fiddled with positioning,tilt,footers,cables blah blah. Sometimes that was fun. Increasingly it was frustrating. In the end I threw in the towel. I borrowed in home loan a pair of active ATC SCM100’s and they never went back.
I did love the Duo’s but at times I hated them and in the end a clean break divorce was the best outcome. I miss their seat of the pants thrill ride of aural delight. I don’t missed the screeching treble,vicious sibilants and endless head scratching.

Jack L's picture


So you cook up the wrong way ! Sorry you went thru so much frustration before you finally gave up the huge horns.

In the demo of the Trio+huge Basshorn in Vantgarde showroom years back, my quality-oriented ears just could not handle the huge
yet clinical sound, I had to call for time-out after only 10 minutes. So from sublime to ridiculous, we swiched to a pair of
BBC monitor Rogers type compact standspeakers, nothing else chanaged.

I enjoyed the demo for 2 hours!

ATC England supplies 5-star ranked loudspeakers for recording studios & for home. Are your classical or tower or professional model ?? Excellent choice, my friend !

Listening is believing

Jack L

Cooking Man's picture

Thanks Jack but please don’t get me wrong. I had many years of pleasure with the Duo’s. As the review noted in some areas they were revelatory. And still would be. I believe my room at 19x20x9 feet was too small to allow the horns to integrate better. I couldn’t sit more than 12 feet from them. That why I said,my room,my music and,I should have added,my ears. All speakers in a room are compromised-it’s just a question of where those compromises and weaknesses are and how they affect the emotional response of the listener.
I bought the Classic ATC SCM100’s and a pair of Track Audio Reference stands. Hooked up to subsequently bought CH Precision L1/Xi and Ypsilon VPS100 phono stage my music delivers thrills and enormous pleasure. I’m very fortunate and grateful to be able to own such extraordinary equipment.

Jack L's picture



A 'horn' loudspeaker is built of a compressor driver with narrow beam-like directivity & a horn-shaped diffuser to spread out the dispersion.

Sitting too close we listen mainly the beaming from the centre compressor driver, sharp like cutting our heads off: sorta torture!

Sitting too far away we will virtually perceive much larger than life-size imaging of the instruments & the performers. An oboe will look like a bass bassoon. An soprano's mouth will look larger than her head !!

Two audio fans invited me to check out their Alec/JBL horns on 15" woofer boxes. Frankly, I was not impressed at all: plenty of sound, pretty shy in music delicacy & emotional feeling

Listening is believing

Jack L

denesdr's picture

That's not true at all. A compression driver have very wide dispersion on it's own and mainly the attached horn/waveguide sets the desired radiation pattern/directivity. True that there are sharp sounding compression drivers but there are right sounding compression drivers aswell, as with any type of tweeters.