Products: Acuhorn Nero 125 Loudspeakers
Manufacturer: Acuhorn - Poland
Cost, approx (very): 4000 Euro (YMMV)
Reviewer: Geoff Husband - TNT France
Reviewed: May 2009

Introduction

For the last 6 years my own system has been fronted by a pair of single-driver, full-range horns - the Loth-x Polaris (no longer made). These large speakers have sat in the system and loved unreservedly. Those who say horns are hideously coloured or band-width limited, or have no bass have never heard properly designed horns. Likewise they are unlikely to have become besotted by the coherence of a speaker that runs without a crossover, doesn't have to integrate two wildly different driver designs and requires a watt to have a party. Such is the world of really good horns - once sampled going back is very difficult.

But there are downsides - even for an addict like me, and one of the biggies is just that - they are big...

The limiting factor is that horn speakers need a big mouth to drop to low frequencies. My own Polaris are huge and yet even then their bass drops like a "buzzard shot with an Uzi* " below 60Hz - a level you'd expect from a compact standmount.

There are small horns out there but generally they are weedy, thin sounding things that struggle without a sub and to my mind generally push the concept too far. Often better to use other tricks like Voigt Pipes and folded pipe configurations and put up with their own quirks.

The Acuhorn looks like a scaled-down version of my Polaris except that the horn fires from the rear - the question is how small can you successfully downsize the horn concept before they become too unbalanced? The horn mouth of my own Polaris measures 30x48 cms and as mentioned about this gives bass down to about 60 Hz, the 125's measures 16 x 58 cms, considerably smaller, and with less space above for complex folding.

So though the Acuhorns looked attractive and have garnered praise elsewhere I approached them with a little scepticism...

Construction

Basic cabinet construction is to a very high quality. The material used is special compressed plywood and a wide choice of veneers are available. Finish is faultless as you'd expect (but don't always get) at the price. The horn exits at the back, which in theory allows for corner/wall loading. It's an utterly conventional looking, and room friendly tall, thin box (W 20 x D 50 x H 125 cm) very much in the modern mode.

The custom driver, labelled the TSR 200 (200mm I assume) sports an unusual but beautifully made CNC'd, square alloy chassis (see pics) and small (but powerful) Neodymium magnet. These rare-earth magnets are far more powerful than the normal ferric type and are seen on many quality drivers nowadays as the smaller magnet makes it easier to concentrate the magnetic flux, and diffraction effects from the magnet are less. The cone material is a kind of pulped fibre as used in many full-range drivers.

Binding posts are quality WBT silver-plated affairs and (praise be) mounted at floor level so avoiding the dangling-wires syndrome. Internal wiring is Siltech G6 silver cable.

This all adds up to a pleasingly proportioned, medium/compact floorstander with quality materials used throughout. At 30 kgs it's substantial without being a back-breaker...

The interesting bit isn't the outside, but rather what is inside, because the Acuhorn isn't made like most horns - in fact you could look at it as being a hybrid ported-horn. Look at the diagram and you'll see that instead of the small compression chamber of most back-loaded horns there are two relatively large, tuned boxes. These mid/bass chambers must load the driver at lower frequencies and somehow blend their outputs with the forward radiation of the driver and the rear horn.

In Use

With a claimed efficiency of 96 dbl the 125 would seem ideal for low-powered valve amps and so I wired them up to my Opera Cyber 300 PSE monoblocks - hardly fleapowered at 18 watts per channel and so in theory an ideal match. The efficiency claim seemed to be reasonable as the subjective loudness matched the Opera M15 horn hybrids which also claim 96 dbl - it's rare that this happens as most manufacturers cheat:-)

At this point it became apparent that I had work to do. In my last speaker review I said that some speakers dropped straight into my system and matched from the first instant - primarily because they have a similar balance to my own. In this case the opposite was true, and that the Acuhorn's were giving a very different view of events and that would need to be both tamed, and adjusted to personally.

This state of affairs wasn't a huge surprise as speakers are the most characterful component in any audio chain, and the differences between one and another are often of the grossest nature, without that necessarily being an indication that speaker A is fundamentally superior to speaker B...

I started with the speakers as the manufacturer recommended - 0.6 m from side and 0.9m from rear walls. This makes them close to a free-space design, and surprised me somewhat as I'd assumed that the rear firing horn would appreciate some wall, or even corner loading - the manufacturer says not.

They were set up in my new music room, which is a big room - 5m x 6.5m but with the ceiling going right up to the apex of the roof some 7m up - a mezzanine extends the volume well back from the rear wall below. It's a big volume to fill.

Normally I can pick a speakers response just by ear - i.e. I can guess pretty much what the in-room response will be, but in this case I was a bit lost. The treble was shy, and there wasn't much in the way of deep bass (not a surprise), but the midband seemed to run pretty flat - not peaky at all. But the overall feeling was a little lacking in dynamics and excitement.

To see what was going on I pulled out my own very crude measuring gear to see what I was hearing from the listing position. I don't publish these results because any in-room result is so alarmingly uneven that anyone not used to my room would take them to show a truly dreadful speaker - in all cases. In fact only by looking at lots of such plots in my room can you really see what is room-induced and what comes from the speaker.

Anyway the result was interesting - in fact in one respect very surprising... The little 125's produced a clean undistorted tone at 25 Hz. OK it was 15 dbl down on the midband, but I had expected either nothing, or the flapping of a cone operating in free-air as the cabinet failed to load it. In comparison the massive M15's with their 15" bass driver, do produce more volume (down 12 dbl) at this point, but it's of the flappy, out-of-control nature and not really much good for music. By 40 Hz (around the bottom note of a bass guitar) output was really clean and only down 6 dbl. This is not a result I'd expect from a small horn and is considerably better than my Polaris, which must have 3 times the internal volume.

Bass then shelved nicely from 50 - 100 Hz before a fairly precipitous 12 dbl suck-out. This is partially due to room effects, but I suspect does reflect a drop in output - perhaps where the bass 'chamber' crosses over to the horn-section as it were? And whilst we're on the subject, the driver showed very little movement during these tests - this means that here we have a well-designed speaker loading the driver properly.

The midband was a little uneven as you'd expect from such a design but nothing hideous, but for a dip around 400 Hz. What you didn't get, and often do get with a full-range driver like this - was a big hump around 3000 Hz

Overall a pretty impressive result, but two traits did show up that explained the character I'd heard. First the bass level, though commendably even from 40 - 100 Hz was 3 dbl down on the broad sweep of the midband. The bass is there, and it appears clean, but it is a little recessed. At the other end of the scale there was no denying that compared to the other speakers to hand, the treble rolled off early but smoothly from about 3 kHz.

The overall result was a presentation that sounded just a bit too band-width limited and lacking in dynamics for my taste. A bit too smooth to be honest, and although that is something that many listeners might appreciate it's just not what I'm used to or want in hi-fi. More to the point - the 125's are supposedly designed for rock rather than classical music - that's certainly not how they seemed at the moment.

In a way speakers like this - that promise a lot but then fail to deliver - are more interesting to review. Given that they are obviously a serious design and have some fine engineering in them there must be more to come and so now comes weeks of trying new things to get them to sing.

Tuning the 125

The first and most obvious tuning technique is to move the speakers around. The manufacturer recommends the speakers fire straight up the listening room with no toe-in at all. This is critical with a full-range driver as the "whizzer" cone produces very directional output. Toeing the speakers into the listening position upped the midrange output considerably, to the point where when aimed right at the listener it sounded more peaky than was really healthy.

This I'd expected. In my home I run my Polaris very heavily crossed over with the imaginary crossing point well in front of the listening position. The reason for this is that as well as reducing the midrange "shout" you can pull a rather clever trick unique to such full-range drivers.

As explained above the output in the upper-mid and treble falls off rapidly off-axis. Now if the speakers cross over well in front of the listener then if the listener moves to the left, the treble output of the left-hand speaker will inevitably fall as you go increasingly off-axis, but the right-hand speaker's output will rise as you go increasingly ON-axis. As it's largely the treble that gives positional information the image between the speakers remains stationary as you move off centre - you can even "look" into the soundstage from the side. In fact this can be quite fun with the right recording as you really get the feeling of the performers remaining in front of the speakers in 3D as you move around. The parallel positioning recommended by Acuhorn does the complete opposite. If you sit in the hot-spot everything is fine. Move 1m to the left and the image moves even further to the left with you, and soon collapses into the left-hand speaker.

Using this crossed positioning the Acuhorns managed to pull the same trick of the solid image regardless of listening position, and though it could be argued that the hot-spot was less good, I did prefer the effect. In the end I settled for the speakers crossing 1m in front of me - my choice - my compromise. YMMV.

This still left the bass a little shy, and here the problem could be reduced by wall and corner loading. The snag is that there is a price to pay in that the good imaging qualities reduce in sympathy. Here again a compromise will be personal - in my case the speakers moved a little further in the corners.

Which left the last bit of tuning and that is the recessed treble. I'm aware that this is personal and my 49 year-old ears are far from golden, but I found that the addition of the JohnBlue supertweeter gave a wholly positive result. Set with its crossover at 12 Khz and with medium output it managed to add sparkle and attack to percussive sounds without sounding harsh. It also seemed to allow the soundstage to breath a little more freely.

Of course all this is well-and-good, but the single most fundamental problem is that the 125's are just not designed to drive such large volume of air, probably not far off 200 m2! What the hulking M15's managed with ease left the 125's just sounding lost and undynamic. The cure was of course to move the whole kit-and-caboodle into a room more suited to the speakers.

And so now to my living room - about 80 m2 so still big, and very well damped. As you'd expect the result was a complete change in presentation The bass now didn't feel so recessed and the dynamics were lifted to a perfectly acceptable level, though a long way behind the Polaris that usually reside there - an area in which they reign supreme. The treble was still a little shy for my taste but the well damped nature allowed me to listen a little more on-axis than before which lifted the presentation.

Normally at this point, having got the best balance I could I'd settle down for serious listening, but something rather put a "spanner in the works"...

My amps decided to eat a 274 rectifier. Muttering oaths about unreliable valves I was forced to replace 6000 Euro worth of SE 300b amps with the Lehmann Stamp I had here which cost around a 10th the price.

Imagine my surprise when I found that I preferred the little 20 Watt digital amp to my normal monoblocks. The difference wasn't huge, but the presentation took on a bit more energy and the bass tightened noticeably- what's going on?

A look at the impedance graph gives a clue, it drops to 3 ohms right in the mid bass where many valve amps are struggling. It's not 'bad' but not perfect for valves - however even knowing this I was surprised at the effect.

But it did open a potential can-of-worms. If a 600 Euro transistor amp was outperforming a similar power 6000 Euro SE amp then what would a more powerful transistor amp do?

And so the review got delayed as I organised the loan of another amp (I gave up "muscle" long ago) in the massive form of a Jungson Ja-99d. This 100 Watt class A behemoth was probably going over-the-top but I had to give it a try.

The result was beyond my expectations - the sledgehammer output of the Chinese amp seemed to really get to grips with the bass. There's nothing you can measure, but the attack doubled and drove music along with a pace that before had been sadly lacking. The shy treble now gained an edge that meant I could live without the supertweeter (even though I still preferred it). So my inevitable conclusion has to be that though the Acuhorn does OK with a valve amp, it really needs the control of a powerful transistor amp to take it in hand. Strange, but in my system - true...

Sound quality

Having spent an inordinate time trying to get the best from the Acuhorn I finally settled on them in my living room driven by the 99d amp - for the purpose of the critical listening I dispensed with the supertweeter as it's hardly a fair comparison otherwise.

Despite the aims of the designer the 125 wouldn't be my first choice as a "rockin" speaker - ultimately its bass just isn't hard and fast enough, and the slightly shy treble just tones things down. Its forte is a smooth and even mid-band that shows the coherence typical of the full-range driver breed. Given smaller scale music, female vocal, jazz, acoustic ensemble, it was very pleasant to sit in front of - at levels suited to a mature domestic environment it sounded at ease and uncompressed. In these conditions the soundstaging is excellent, showing good width and well above average depth. The images are also gifted with enough body to seem real; again something my Polaris fail to do at low level. Small horns tend to be strong medicine, not the 125s...

In fact if I were to pick out it's strongest suite, it was at relatively low levels - not background, but where you can talk without shouting, often speakers simply don't wake-up at these levels and my Polaris are a case in point. Here the 125s managed to still draw you into the music in a natural and unforced way. I know that there are many listeners out there that listen in precisely this way, out of choice or for domestic harmony. In my case if I don't have to raise my voice it isn't loud enough:-)

One minor weakness was one shared with many speakers based around a driver with a "whizzer" and that was a slight softening on sibilants such that the various vocalists who fronted Steely Dan all seemed to develop a slight lisp. My own Polaris share this characteristic if you listen very hard but it was a little more pronounced on the 125s.

Conclusion

Anyone reading this review will see that the Acuhorns aren't really my taste in speakers - I like big, bold, dynamic speakers, meaty enough to fill big rooms with 90+ dbl - the Acuhorns are none of these, being refined and smooth. As speakers are generally such a varied bunch, that response probably applies to 90% of the speakers I hear (why I've kept my Polaris for 6 years...). But putting my personal prejudices apart, the Acuhorns make a good case for themselves. Build and finish is excellent and with a custom driver this is obviously not some ill-conceived product knocked up is someone's back shed. In the right environment - a smallish room and a moderate levels it has some fine abilities. My gut feeling is that even my listening room is too large for it to dovetail naturally, I think in a more typical "British" living room of say 5 x 4 m, and with less demand on headroom, the result would be much better integrated. I don't think you need a 100 Watt class A amp to make them sing, but I would lean towards transistor amps with decent current ability to get the best from the speakers.

For those of you which fit this description the 125's are going to be worth an audition, certainly they avoid the acid-etched sound and extremely limited bandwidth of some small horns. That they are domestically acceptable just makes that choice all the easier.

*Scott Faller's phrase but too good not to use