June / July 2009

World Premiere
Acuhorn Nero 125 Loudspeaker
Does every speaker sound good with some music, yet not so well with others?
Review By Jules Coleman

  Full range single driver loudspeakers often display a coherence, presence, immediacy and dynamism that precious few multi-driver speakers approximate, and fewer still can match. Such speakers, like the Acuhorn Nero 125 loudspeaker, typically mate well with low power amplifiers — often but not necessarily built around single-ended directly heated triodes — that exhibit many of the very same virtues. More than a few audiophiles have found the synergies irresistible and vow a life-long (by audiophile standards) allegiance, forswearing all others. I’ve been down that road before myself — many times — and I understand the pull on one’s heartstrings a system built around a single-driver full range loudspeaker mated to low-powered tube amplification can have. Here we will see if the Acuhorn Nero 125 loudspeaker is match initiated by cupid if not quite one made in heaven.

There is more than one way to load a full-range driver into a loudspeaker cabinet. Two of these are more common than others: open baffle and back-loading. The former is favored for example by Keith Aschenbrenner as represented in his stunning Rondo and SoloVox speakers. Many of my friends in the audio community have tried their hand at open baffle designs that feature the legendary Altec 604 driver. For many, the box of a 'box’ speaker is an impediment to persuasive musical reproduction. What better solution is there than to do without the box — or to do away with as much of it as possible?

Open Baffle Explained
The phrase 'open baffle’ is a bit misleading because it covers a range of different approaches. All open baffle designs share the absence of a 'completed enclosure.’ Some are one sided only, a driver mounted on a 'front’ baffle with no sides or back to the cabinet; some are three sided and some are nearly four-sided but all remain incompletely enclosed. Common to many open-baffle designs is a much wider front baffle than is common in 'audiophile’ approved, modern speakers. The wider front baffle is essential to extending bass response; and in my experience whatever the driver compliment, wider front baffles contribute to better and more natural bass response. In fact, the best bass I have heard (not the deepest but the best)— quick, textured, and well pitch-defined — has been produced by speakers featuring wide front baffles.

The move to narrow profile speakers is another of many accommodations in audio equipment that has more to do with cost savings and the domestication of audio reproduction than with the musical quality and integrity of the listening experience. With the change to narrow front baffles has come the emphasis on what I think of as visual aspects of music reproduction including especially 'imaging’ and 'soundstaging,’ features of the experience that are of subsidiary interest at best and are more often than not distractions.

Whereas the open baffle design has its champions, the most popular implementation of the full range driver is (some or other variant of) the back-loaded horn. There are a number of variations of the design philosophy but they share a similar strategy. Several different kinds of drivers can produce output from nearly 20 Hz to 20 kHz. Typically that output falls off greatly below 80 Hz or so. Back loading is a way of capturing the back wave of the driver in a chamber (sometimes two — as in the case of the Acuhorn Nero 125) and sending that output through a horn — usually a folded horn of a non insubstantial length — the point of which is to amplify the output in the crucial range where the driver’s output falls off increasingly more steeply. The output that the driver puts out going forward into the room is reinforced by the rear wave output that has been amplified by the horn. The aim is to thereby create sufficient output at all relevant frequencies to get a single driver working in a cabinet to produce the full frequency range or close to it. If it works the speaker will produce the energy and musical information from one driver that otherwise requires at least two drivers and a crossover to accomplish. In theory, when such a design is successfully implemented, one will experience full range musical output without the distractions of multiple drivers made of different materials and crossovers that eat watts like candy.

In effect the open-baffle and back-loading employ polar-opposite approaches. The former eschews as much cabinet as possible and lets the back wave energy escape, though sometimes as in the Rondo and SoloVox designs the back wave is channeled out with a purpose. In contrast, back-loading requires an enclosure (the horn is enclosed in the cabinet and the cabinet is essential to the design). There is all the difference in the world between having the back wave escape and capturing and using it to work with the front wave to create overall musical output.

About Full Range Drivers
Full range drivers have been around for a long time and are produced by many manufacturers, the most well known of which are probably Lowther and Fostex. Other notable full range drivers include AER, Reps, Tesla, Feastrex and PHY. There are others of course but these are among the best known and are in any event the ones with which I am most familiar.

PHY drivers are typically employed in open baffle designs and I have not personally heard them in a back loaded horn. I have not yet heard any of the many Feastrex drivers though I am keen to hear their field coil driver and am very soon to do so. I have heard any number of Fostex drivers — including representative samples from the older and newer Sigma series — implemented in a number of cabinet designs from the little Hornshoppe Horn to the Jericho horn design that was quite the rage for a while in Germany. I have heard and owned several speakers employing Lowther and AER drivers, the latter in both front (e.g. Oris horns) and back (Beauhorns, Rethm, Medallion, to name a few) loaded cabinets.

All full-range drivers with which I am familiar have distinctive characteristics. The Lowthers are extremely resolute, immediate, vivid, dynamic, and agile if a bit on the lighter side; they are notoriously peaky in the presence region. Many a Lowther lover puts up with this problem — which is worse in some of its drivers than in others — finding the benefits to more than cover the costs; others take comfort in denial, deaf, as it were, to the problem. Still, every review of a back loaded horn featuring a Lowther driver begins by talking about the steps the designer has taken — often imaginative, sometimes heroic — to tame the notorious Lowther peak. This should give you a sense of the extent of the problem. To my ears the most successful implementation of a Lowther driver has been as an extended midrange driver in various Horning loudspeakers.

I have had nearly as much experience with AER and Fostex drivers, including several from the much-praised older Sigma series, as I have had with Lowther drivers. In my experience AER drivers are smoother and better constructed than the Lowthers; the smoothness comes at a slight cost in overall information definition and dynamics. The Fostex drivers are less informative still and considerably less open, transparent and beguiling. They are an all-purpose unit that while short on the nasties in back-loaded configurations are also a bit short on the magic.

When I first began experimenting with full range drivers in back-loaded horn configurations, the vast majority of the designs employed 8-inch drivers. I recall smaller units in the Hornshoppe Horn and in a smaller Rethm speaker. One of the most interesting recent developments has been the increased use of even smaller drivers running full range. In any back-loaded horn, the vast majority of the musical information still comes from the front wave. Thus, even when the smaller 5 and 6 inch drivers are better behaved than their 8 inch and larger counterparts, it is hard to imagine how they might move enough air (regardless of the cabinet size and horn design) to render a persuasive musical experience in a normal sized listening room.

Horns can be used to amplify output or to increase efficiency (or both). In the back-loaded horn, the emphasis is on increasing output. The design goal is more complex and demanding, however. It is not enough to increase output where the driver naturally falls off. One must do so in a musically persuasive way. The design needs to amplify output while getting the tone, timbre and timing right and capturing the dynamics, conveying the weight and authority of live music.

This is not just a problem of designing a great horn. It is also a matter of choosing the driver that is the optimal match for the horn/cabinet design one has settled on. One simply cannot build a cabinet with a folded horn enclosure and expect it to work optimally with any driver one happens to place in it.  The driver and horn have to work in consort with one another and a driver that is the right choice in one cabinet need not be as good a choice in another. A good designer is someone who finds the right combination and keeps working the two elements, making adjustments to each. I feel the way about drivers and cabinets the way I feel about cartridges/tonearms/tables and phono sections. They are really one unit and have to be thought of in that way. Similarly, the driver/horn/cabinet has to be thought of as a single unit in the music reproduction chain.

If it were easy successfully to implement a full range driver in a back-loaded horn, more designers would abandon multi driver designs with their complex, power eating crossovers and their drivers of different materials and resonant properties in favor of the full range driver approach. In fact, of course it is not easy at all. It is natural to think of the single-driver back-loaded horn approach as a different way of securing the same results other designs achieve by using more than one driver in conjunction with various crossovers.  It may be natural to think this way but it is a bit misleading.

That is because someone pursuing the single driver horn loading approach is not looking to replicate the sound or presentation of a multi-driver speaker. His goal is not to produce the same result in a different way. He is looking to present music in a different way, highlighting certain features of musical performance over others. The single driver design will not play as loud as will the typical multi-driver loudspeaker. It will not typically be thunderous in the lowest frequency range. It cannot possibly extend as high into the stratosphere. But then neither does the Quad 57 ESL and there has rarely been a speaker as musically convincing since.

On the other hand the single driver full range speaker is unhampered by a need to make two or more drivers, usually of different materials, work coherently together and it has no crossover to serve as roadblock to the music. The designer is choosing to emphasize immediacy, vividness, presence and a distinctive kind visceral engagement that once experienced is not easily forgotten. Single driver loudspeakers induce immersion in the musical experience.

The design involves trade-offs of course, but the better way to look at it is that the designers pursuing this approach are offering a different interpretation of what is musically important to a performance and are inviting the listener to experience the performance differently. There may be an absolute sound in the sense that there is a way in which the music actually sounds when played live or in the recording studio, but there are many different ways of approaching and hearing it: of experiencing and responding to it. The very same object can be photographed from different angles and in different light at different times of the day. There is an object that is what it is independent of our seeing or experiencing it. Still, seeing it under different light, at different times of day, at different times during the year, and so on not only allows us to experience it differently, but to more fully understand it, to know more about it, and to grasp features of it we might not be otherwise able to.

There are many legitimate ways to accurately to represent the same event. The single-driver back-loaded horn is one of them. To many its virtues far outstrip its costs, and it provides a window into the musical event that I have time and again found almost irresistible. And yet every time I have reneged on my vow never to return to a multi-driver loudspeaker again. In time I came to see myself wavering back and forth somewhere between the 'nightmare and the noble dream.’ I have made it clear I hope why one might be seduced, but why, once seduced would one ultimately abandon?

Are There Problems?
The characteristic limitations of the design can be hard to live with long term depending on the kind of music one listens to. The unparalleled dynamics in the midrange are rarely matched in the bass or the higher frequencies. The invariably truncated top end sometimes leaves one hankering for more harmonic information than is available. These limitations are not decisive at least for me. They just mean that even if the speaker works according to plan it won’t be for everybody. No big deal. No speaker is.

The real problem with back-loaded horns, however, is that they display a variety of characteristics that get in the way of the speaker’s ability to produce a persuasive representation of music: once heard, moreover, they are impossible to ignore. Back-loading creates phase anomalies that deeply undercut the claim to sonic coherence. Back-loaded bass response can be surprisingly deep, but the deeper it gets the less well pitch-defined it is. Moreover, the deeper the bass it is the more one note-like it becomes. Even more important, the lower registers seem to lag behind the rest of the music interfering with musical timing and flow. The high frequencies are often lacking in harmonic structure, heft and density.

So what you have is really a midrange speaker that is being stretched beyond its optimal bounds to secure some desirable gains, but to one or another degree, the stretch is audible and disconcerting: neither the bass nor the upper frequency responses have anything like the character of the midrange. It is only a matter of time before the listener is aware of these discontinuities and once he or she is, the claim to coherence is rendered fraudulent or at least misleading. Too much information is missing for many listeners, and what you get is not what in theory you were willing to pay the price of admission for.

At least that has been my experience: not once, but nearly a dozen times. And yet I have kept coming back for more. Why? Listening to nearly a dozen designs that have fallen short of the ideal has had what one might think of as the perverse effect of making me want to hear the most recent effort. No doubt a sensible person would have given up by now. I make no claims to being sensible, however.

The truth is that one cannot help but be intrigued by credible efforts to overcome the limitations of the design philosophy. This is not just a curiosity or a scientific project. There is something so alluring about the way such speakers produce the music that they do produce well that one just can’t help but want more of it. And so I never tire of seeing what progress has been made, and to what extent the gap between the nightmare and the noble dream has been narrowed.

Acuhorn's Solution
This past year I was alerted to two relative new comers in the back-loaded horn sweepstakes: Acuhorn and Maxxhorn. Neither was previously familiar to me, and I was intrigued by both because the main claim in both cases is that the designer has made significant improvements in cabinet/horn design, not with the purpose of ameliorating driver idiosyncrasies but with aim of attacking the problems to which I have alluded above. I was able to obtain representative models from both Acuhorn and Maxxhorn. Indeed, in the next speaker discussion I will be reporting on my experience with two Maxxhorn models, one of which features the much-ballyhooed Feastrex field coil driver. In the remainder of this essay I report on my findings with the quite wonderful Acuhorn Nero 125 (Improved) loudspeaker.

Acuhorn is a relative newcomer, little known to American audiophile audiences.  Acuhorn has enjoyed some success abroad and two of its designs have won speaker of the year awards from credible journals. Acuhorn makes four loudspeakers: two in each of two ranges. The Nero 125 and the Rosso Superiore 175 are both designed for 'modern music.’ The difference is that the Nero employs one driver and the Russo Superiore employs two in dipole configuration. The dimensions of the speakers reflect this difference. The other two speakers in the line-up are designed for 'classical music’ and once again one of these employs only one driver and the other, larger speaker employs two also in dipole configuration.

I have only heard the Nero 125 and so cannot comment either on the efficacy of the dipole design or on the manufacturer’s claim that one series is suited for modern music while the other shines on classical music. I did not restrict myself to modern music when listening to the Nero 125 and the speaker accommodated itself admirably well with music of all types — with one exception that I will discuss below.

The Acuhorn Nero 125 is a one-way floorstanding loudspeaker 20cm wide, 50cm deep and 125cm tall. The cabinet is made of solid wood as the designer eschews both plywood and MDF alternatives. A good deal of the speaker’s expense is the result of this construction decision as well as the quite special Acuhorn TSR 200 driver. Less than five inches in diameter, the driver is a paper cone featuring an all aluminum driver body and a neodymium magnet. Impedance is a surprising 4 ohms and sensitivity is reported as 96dB. All internal wiring is silver Siltech G6 and terminals are Nextgen WBTs. The speakers sit on a black integrated anodized base measuring 20 x 47 cm.

I used the Acuhorn in both of my systems: the reference system featuring all Shindo equipment including the 300B Ltd monoblock amplifiers; the NYC apartment system where they were driven by the 18 watt EL84 based Shindo Montile as well by a Sound Quest SV 84 integrated amp also using the EL84 tube and also producing roughly 18 watts. Both listening rooms are above average in size with the Connecticut room being especially large at 30x18x9. The Acuhorn performed best in both rooms when placed far away from back and sidewalls. In both listening rooms I preferred the speakers no more than 7-feet from one another. The manufacturer emphasizes the speaker’s imaging capabilities and encourages listeners to set the speakers up further apart to improve soundstaging. As readers know, I am not drawn to soundstaging and do not view the soundstage of a speaker to be an important attribute — and certainly not a musically significant feature. So I set the speakers up with aim of optimizing balance, coherence, density and authority. I found that as the speakers are moved further apart, the sound becomes ethereal and whatever increased spaciousness in the soundstage one achieves comes at a price that I, for one, am unwilling to pay. Keeping the speakers within the specified distance helped reinforce bass output and gave the speakers their weightiest and most authoritative foundation.

The speaker performed admirably in both set-ups and to its credit reflected the vast differences in associated equipment. In my home, I listen almost exclusively to vinyl whereas in my apartment I listen almost exclusively to CD. The analog front end in my home is the Shindo Garrard 301 and my current digital front end in NYC is the Raysonic 128. The Raysonic is a fine player and is a high value performer, but it cannot hold a candle to the Shindo analog set up. It isn’t supposed to and the fact that it does not is hardly cause for consternation or disappointment.

Though the Nero 125 is not nearly as sensitive a loudspeaker or as easy a load as other single driver loudspeakers I have owned, it proved to be no problem for any of the amplifiers I had on hand. The sound was considerably more powerful, richer and resolute in the reference system than in the second system. I viewed this very positively. The speaker revealed differences in ancillary equipment and while it was very much at home with push pull amplification and a modest digital front end, it shone in a system featuring state of the art electronics and front end.

I resist the common wisdom that what is wonderful about high sensitivity loudspeakers is that you can mate them with any low power amplification you can cook up in yours or your friend’s garage (or basement). The theory is that any low power amplifier (because of its simplicity) will sound better than any more complicated design and so all it needs is a highly sensitive loudspeaker to shine.  In fact, however, a well-designed high sensitivity loudspeaker can be extremely revealing of differences in everything from front ends to amplification. The best can show you just how extraordinary really wonderful some electronics are. The better your electronics are, the better high-sensitivity loudspeakers will sound — up to their limitations of course. The Nero 125 is both highly resolving and highly revealing plus always enjoyable and engaging.

The Nero 125 is described as the Improved model, but it is basically a different loudspeaker than the original, so substantial are the changes to it. Of these, the most important is a change in the back chamber arrangement. Where there was but one before there are now two. This is a major change in a design of this sort and the effect presumably is to improve midrange tonality and balance, while improving as well upper bass and lower midrange performance. I have not heard the earlier award winning design so I cannot make a useful comparison. What I can do is report on what I heard.

One reason I love the sound of Shindo is that it gets the timing of music correct and reproduced sound has a musical flow and dynamic realism that is uncanny and is not approached by other equipment with which I am familiar. There is a rightness, wholeness and integrity to the way music is reproduced that is enthralling. The Shindo electronics put a heavy burden on associated equipment to measure up. For example, the otherwise extremely capable Clearaudio Reference turntable was revealed to keep time in the fashion of a marching band. Back-loaded horns are especially vulnerable to the standard that Shindo sets because as a general matter they can sometimes reproduce bass that seems like it is behind the beat. In addition, in their efforts to extend bass response to the deepest regions, the upper bass and lower midrange are compromised. Instead of coherent loudspeaker the net result is discontinuity and the absence of the promised coherence.

So how did the Nero 125 perform? In a word: excellent. Prior to the Acuhorn’s stay in my system, the best sounding single-driver full range back-loaded horn I have had any experience with was the Beauhorn. Every Beauhorn I heard employed a Lowther driver but in each case it did a remarkable job of taming the Lowther peak. But the real trick of the Beauhorn was that it did not try to do too much. The Beauhorns I have heard have virtually no bass to speak of. The downside of course is that the sound is lightweight and everyone who has kept Beauhorns for a long period of time has mated them to subwoofers, and not just to fill in the lowest octaves — but to give the speaker any discernible bottom end.

The reason I mention the Beauhorn as a success is that in my experience the vast majority of back-loaded horns have tried to get too much bass out of the design. In an effort to prove that the speaker is full range, designers try to get more bass out of the speaker than is sensible. In every speaker I have heard there is a sacrifice between extension and musical character of the bass. There is deep bass but it is one-notish and not particularly musical. In addition, the deep bass is produced at the expense of a rich, dynamic, musically convincing upper bass and lower midrange. In trying to do too much, the designs produce too little of what counts: continuity, integrity and in a word, music.

The Nero 125 is unusual in several respects. Using a smaller driver, it too tries to plumb the depths, and it succeeds more than it has a right to. More importantly, it does not buy its bass extension at the cost of a gaping hole where the lower midrange and upper bass should be. The Nero 125 is gloriously coherent from the upper midrange to the upper bass. There is a consistency of dynamics and resolution that is natural and seductive. The basic presentation is balanced within its range. There is no artificial vividness or immediacy that comes from a spotlit presence region. As a result, the speaker is much easier to listen to over a broad range of different kinds of music. I listened to lots of acoustic and electric jazz, blues, chamber music and pop rock, all to very good effect with great enjoyment.

One of the other extraordinary features of the Nero 125 is that it has very little distortion. This means that the sound comes across in a relaxed fashion that is easy on the ears. The sound is not quite as distortion free as from a field coil, but by comparison to every Lowther I have heard the Acuhorn TSR 2000 driver is a revelation.

The upper frequencies of the Acuhorn were similarly well balanced but not particularly extended. This is no surprise, and it did not disappoint me. Whereas the fashion nowadays is to produce a speaker with a tweeter capable of output at 30 kHz or higher, I have enjoyed music most in speakers that do not reach much above 16 kHz or plumb depths much below 35 to 40 Hz. The key is not how much is reproduced, but how well it is reproduced — and how balanced the overall presentation is. In this regard as well, the Acuhorn shown. The Nero 125 is an exceptionally well balanced loudspeaker.

It is also a musical loudspeaker. It is common to distinguish between speakers that play music and those that are like tools for revealing the various parts of a musical performance. Acuhorn falls on the musical side. You can follow the parts if you like, but the speaker does not take the music apart. It presents the music as an organic whole: continuity, integrity and balance. It is very informative, but not in the way that some other speakers that emphasize the leading edge of notes are.

But what about the timing? Better than most, but not perfect. The bass is very good and very well pitched-defined, but I think the Nero 125 tries to do a bit too much in plumbing the depths. The deepest notes are just a bit sluggish by comparison to the rest of the bottom end and the speaker as a whole. If I had one suggestion, it would be to forget about the very bottom octave. The deepest bass notes don’t have the weight and authority that they would with a multi-driver speaker or with the aid of a subwoofer. Audiophiles who are drawn to speakers like the Acuhorn are not organ freaks or electronica aficionados. Why bother?

Conclusion
In my experience, one cannot help but listen to music that sounds good on one’s speakers. Every speaker is flattered by some music and flattened by others. I own a pair of rebuilt Quad 57 ESLs and it has become apparent to me that when I listen to them I listen to music that flatters them. No Led Zeppelin and no Who Live at Leeds, yet plenty of jazz including Getz and Gilberto. To its credit, there is very little I don’t find myself willing and often anxious to play on the Acuhorn. The speaker thrives on solo piano, but it excels on pretty much everything else as well.

Well not quite everything else. There is just no way that a single driver loudspeaker can sort out large scale choral music or lay the foundation for large orchestral pieces. There is only so much one can legitimately ask from a single driver speaker.

The Acuhorn Nero 125 is not inexpensive. But its price is warranted by its construction, and more so by its performance. Between around 40 Hz and 15 kHz the Acuhorn Nero 125 is a smashing success. It is beautifully balanced, dynamic, and immediate without being artificially hyped up, relaxed, involving and downright seductive. It is unquestionably among the best back-loaded horn loudspeaker I have heard.  If full range back-loaded horns appeal to you, then you owe it to yourself to listen seriously to the Acuhorn Nero 125. It does so much so well. An excellent speaker, very highly recommended.

Specifications
Type: Single driver loudspeaker with wooden two acoustic chambers type
Driver: Neodymium TSR200
Impedance: 4 Ohm
Sensitivity: 96dB/W/m
Internal Cabling: Siltech silver G6
Loudspeaker Terminals: Nextgen WBT-0710 Ag
Weight 66 lbs each
Dimensions: 20 x 50 x 125 (WxDxH in cm)
Price: $9900

Company Information
Acuhorn
Kartuska 245
Gdansk 80-125
Poland

Voice: 0048 601 622 528
E-mail: info@acuhorn.pl
Website: www.Acuhorn.pl