Acuhorn 15 Loudspeakers

Wojtek Unterschuetz rarely shares information on a new product in Acuhorn's range, but when he does he always has something truly interesting to offer. Like when I first saw and heard the tall, narrow, featuring a single driver fullrange speakers, nero125 (review in Polish). It was an unusual design in every respect, because it was made of waxed plywood, and featured no crossover. Beautiful smell stayed in my room for a long time after nero returned to Acuhorn. This was a design clearly referring to the classical techniques of sound reproduction from the 1930s and 1940s, ie. to the relationship between high-efficiency speakers and tube amplifiers. The sensitivity of nero125 was even higher than 94dB and just a few watts were enough to make them sing.

The subsequent models proposed by the company confirmed the direction it took from the start. Strive for perfection was so strong that after a while Wojtek developed his own full-range driver, made with utmost precision, which was used in, released in 2012, Superleggera giovane85. Please take a look at the photographs, which clearly show how complex this design was, despite the fact that after all, there was only a single driver used in it, and thus there was no crossover, and the wiring length was reduced to a minimum.

Somewhat earlier, in 2013, the also the first amplifier was designed, conceived as a partner for high-speed horns by Acuhorn. In order to somehow separate the electronics from the basic scope of activities it carried a logo nowe audio (the original spelling). Exterior of these devices is special because they are very low profile and relatively small devices, even those that use tubes. This was possible by eliminating output transformers—this type of design is called OTL (Output Transformer-Less). The mono3.5 amplifier reviewed by us was sort of an exercise of logic—it helped with breaking free from the routine and showed me how different approaches to the same issues might be.

The latest Acuhorn's product, loudspeakers model 15, are still different. Number comes from the diameter of a woofer used for this model—it is a 15" (381 mm) monster made by Italian company FaitalPRO operating in this particular case in a bass-reflex enclosure, supported by another interesting driver—a ribbon tweeter by American company HiVi. Both use neodymium magnets. A similar solution used for these drivers is also used by JBL and I'd say it is this brand that would be the closest to the idea behind Acuhorn's "15", although—let me add—only in general terms.

It is not the drivers that attract attention when one unpacks these loudspeakers but rather the proportions of their cabinets. This is a very large loudspeakers with a wide front baffle, but limited depth. When in packaging these look a bit like IKEA desks prepared for shipment. Their cabinets were covered with natural oiled veneer and their smell is almost as intense as the one I remembered nero125 by.

The crossover frequency is set at 1 kHz, and it is a simple, 1st order crossover. I'd say it is quite low frequency for a tweeter, but at the same time very high for a such a large diameter woofer. This simplifies the design, eliminating many of the problems associated with the more complex crossover, but also suggests a reduction in the frequency range below crossover point. The principle says that the higher the frequency band reproduced by the driver, the radiation pattern is more directional (a phenomenon called "beaming"). And the greater the diameter of the driver, the lower the frequency where narrowing of the beam begins. At 381 mm at the point of crossover there is only a small radiation energy. As alway, however—it's sort of a quid pro quo.

Model 15 a offers a high efficiency (96dB) and flat impedance of 4 ohn, but Acuhorn is able to prepare an 8 ohm custom version. The company declares that bass goes as far down as to 30Hz and at 30W the SPL of 116dB is achieved, which translates to a very loud presentation.

TEST METHOD

Model 15 will most likely be working in most setups with low-power amplifiers, because that is the common paradigm in which high efficiency = low power amplifier (preferably tube based). For years though, I've been testing such loudspeakers driving them with powerful amplifiers, because such setup allows me to better assess loudspeakers' quality rather than their interaction with an amplifier.

I am not the only one who believes this is a good idea as the famous JBL company also usually drives their horn speakers with large woofers using powerful solid-state amplifiers. The Avantgarde Acoustic prefers such setups too. They've never offered any tube amplifiers, but they do make solid-state ones (more HERE).

The model 15's designer seem to agree since one can read in the manual that, a great performance can be achieved also with high-power amplifiers, as long as one is careful when operating the volume control". The latter is quite important as the HiVi driver's nominal power is just 30W (60W peak).

I placed the 15 in exactly the same spots as my Harbeth M40.1 and most other loudspeakers I tested before such as: JBL, Avantgarde Acoustic and Polish Horns FP10 (review in Polish). They must be toed in so that the tweeter radiates directly at listener, because only then woofer is given a chance to use its optimal band. No spikes or anti-vibration feet are included in the package and therefore I placed them directly on the floor.

A few simple words…

WOJCIECH UNTERSCHUETZ

Owner, designer

Acuhorn has been a high-efficiency speakers manufacturer, or so called: "Single driver speakers", since 2003. Our workshop has developed its proprietary solution for audio processing in an Acuhorn acoustic box, consistent for all of our speakers. It has introduced a new standard for the presentation of the soundstage and low frequencies. So far, we presented three lines of loudspeakers, starting with the classic in 2003 with neodymium driver on a metal frame. In 2007 we introduced the improved audio series featuring a heavy, CNC machined design. The latest line—superleggera—from 2012, features a wooden frame integrated with the box, and the diaphragm is mounted directly on the wood.

I came up with the idea for the Acuhorn 15 already in 2004, when I visited the High End Show in Munich. Back then the Stereoplay magazine presented a very interesting comparison titled: "Superboxen" focused on large loudspeakers. One could have noticed that the speakers with large bass woofers, for example, Focal-JMlab Grande Utopia BE, were able to deliver a different quality of low frequency sounds. The following year in Munich the Stereoplay editor, Wolfram Eifert, repeated such demonstration of a 15-inch speakers using B&W 801D and JBL K2 S9800 loudspeakers.

At the same time, remembering very well the sound of full-range Acuhorn speakers, I found the Ascendo listening room with their top level model, System M, where I heard the sound closest to my nero125's. It featured the RT2H-A Isodynamic Ribbon Tweeter by Swan Speaker (you can find more on loudspeakers with this driver HERE - ed.). And so I carried this impression for 10 years in my memory, up to the moment when I decided to implement this idea in a speaker made for my friend. It's a great man who needs a big sound stage and powerful bass. This was to be a combination of the 15-inch JBL driver sound with a high-end ribbon tweeter's, combined in a purist Acuhorn way. An acoustically tuned cabinet is the basis of this project and it is combined with the purest possible signal path.

SOUND

Recordings used for the test (a selection)

Il Canto D'Oreo, perf. Il Trionfo del Tempo, Et’Cetera KTC 4030, CD (2009)
Dubois, Les Sept Paroles du Christ, perf. Chœur Radio Ville-Marie, cond. Simon Fournier, Fidelio Audio FACD008, CD (2002)
Ed Sheeran, X, Warner Music UK/Warner Music Japan WPCR-15730, CD (2014); review HERE
Guttenberg Brothers, #One, 59music, CD (2016)
Holst, The Planets, Op. 32, Telarc/First Impression Music FIM UHD 058, CD (1986/2011)
Laurie Anderson, Homeland, Nonesuch 524055-2, CD + DVD (2010); review HERE
Michael Jackson, Xscape, Epic 3053662, CD (2014)
Suzanne Vega, Nine Objects of Desire, A&M Records 540 583 2, CD (1996)
The Beatles, Revolver, Apple/USM Japan UICY76972, SHM-CD (1966/2014)
Thelonious Monk, Tijuana Moods, RCA Records/ORG Music ORGM-174-3, SACD/CD (1962/2015)
Tsuyoshi Yamamoto, Autumn in Seattle, First Impression Music FIM UHD 043, CD (2001/2011)
Jan Garbarek, I Took Up The Runes, ECM Records ECM 1419, "ECM Touchstones", CD (1990/2008)
Jan Garbarek, The Hilliard Ensemble, Officium, ECM Records ECM 1525, "ECM New Series", CD (1994)

We make our purchase decisions using our eyes, even if we do not want to admit it. Also when we think that a simple "black box" is enough for us—such look tells us that the manufacturer "invested all the money in the components", leaving aesthetic on the side as a non-important factor. Therefore, I am more than certain that the Acuhorn loudspeakers each time have to undergo a severe pre-selection based on their appearance before even a customer actually assesses their sound—the design is very characteristic and one surely either loves it or hates it from the start. Model 15 is so large that it can dominate a room. Fortunately the cabinet is quite shallow so these loudspeakers can be placed really close to the rear wall and there is no need to place them in the middle of the room.

Acuhorns dominate the space also in a different way—they deliver a particularly rich, dense presentation and the way they render soundstage is similar to what you get with headphones. This is not the first time I've heard such sound, and usually it is associated with the type of design, which to some extent liberates us from the influence of room's acoustics, transferring us to the acoustics of the recording. I do not know if I'm right, but that's how I understand it—Acuhorns build dense, large "bubble" and we seem to be glued to it. They present full, rich transitions between the instruments.

This impression is created due to the robust front of the soundstage, where you see large bodies of instruments of realistic dimensions. They enlarge phantom images, ie. the main elements on the stage are slightly larger than in reality. But it is not a flaw, because a microphone set very close to the source of sound also enlarges it—Acuhorns "notice" that immediately and present it accordingly. So it is rather a feature that should be kept in mind, because it is not a perfectly "neutral" sound. It didn't bother me, because the presentation is not pushed forward towards listener—if it was it could become too offensive.

In turn, the lower midrange is slightly withdrawn, which results in vocals that are not properly rich. Suzanne Vega was nicely shown, she was not a just a "point" in a space but a real, palpable image, but also the lower registers of her voice were somewhat tempered. I could hear the same effect on The Beatles Revolver album, which is not particularly rich with low frequencies. Acuhorns set its tonal balance pretty high, slightly emphasizing the little difference between the realization of this type, and one in which a full bandwidth is presented. The point is not that it is a "light" sound, because it is exactly the opposite. But I would like to point out to you that the designer's choices always have different consequences.

And so, despite the large woofer and bass-reflex cabinet, the model 15 is not trying to blow us away with a bass, these are not stage speakers and they do not pretend to be ones. I mean they do not try to impose too much of their own character, resulting from their large size, on the reproduced recordings, they do not pump some crazy amount of air. They are not totally transparent in this, and the selectivity, leading edge, creating three-dimensional images, are not among their biggest strengths, especially in the bass and lower midrange area.

Also the tweeter is incredibly interesting. It has a sweet, vibrant tone and it does not emphasize the attack, which is a problem of most ribbon tweeters. It is incredibly detailed and delivers a lot of information. It does this without imposing its presence, without brightening the sound. And yet, when the recording consists a lot of noise, say on Anderson's vocal, or on analog recording of Autumn in Seattle by Tsuyoshi Yamamoto, one can hear it better with Acuhorn than with Harbeth M40.1. And only the best speakers such as: YG Acoustics Carmel 2 and Audiomachina Pure NSE (review in Polish) go deeper into it, presenting this information in even more orderly fashion.

With this method of treble reproduction, "Fifteen" becomes a loudspeaker delivering warm, sweet sound, with a good bass extension. The latter is not clearly textured and is not very resolving, and yet we feel as if it was really well controlled. Listening to the Laurie Anderson's Homeland while focusing on this aspect revealed that these loudspeaker even tamed some part of the bass range, not to go too far while prolonging its slow damping. Combination of all these small element results in a sound of a large scale, momentum and size. The momentum is a particularly important element here.

High efficiency loudspeakers, primarily horns, are capable of delivering incredibly fast sound, so that an impression of the "real" sound is created. And indeed—a compression is one of the major problems of classic designs and one of the most important differences between live performance and a recording. Listening to, for example, live drums and then comparing them with the recording one immediately feels the difference, in large part due to the differences in attack speed. We are much more tolerant to uneven frequency response and other distortions.

I can understand what drives many manufacturers to eliminate the compression, because in this way they actually get closer to a live performance. The realization of this postulate has its consequences though, primarily a leaner sound. An attack is fine, also dynamics, punch, but behind them there is not quite as much happening. Perhaps that is why Acuhorn 15 adapt so well to a system even if they are its least expensive component. It's because they do not cause leaner sound, do not deliver a "single-note" performance. They do not try to attract attention to themselves, because it is actually focused on the major phantom images present in the foreground and on a multitude of things that are happening in the back of the stage.

The latter is very stable, and the loudspeakers actually disappear from the equation, one can not "hear" them as such. The high tones played by instruments are a sole exception here as these are set exactly in the speakers, thus one can hear them directly from the speakers rather than from behind them. But this is not an intrusive attraction of listener's attention but rather just a reminder of where the loudspeakers actually sit. However, this is nothing more but a distraction, because there is much more happening in the center and we never get the impression that the sound really comes from the speakers. Like other successful designs with a wide front baffle, also these speakers prove that the tendency to use as wide front baffles as possible is not the only choice allowing designers to produce satisfactory results. I would even say that the idea behind Acuhorn 15 is much more interesting and delivers more natural results.

Summary

There is quite a large group among music lovers who prefer loudspeakers with large bass drivers; there are even special clubs for them (in Asia), which allow only members who own loudspeakers with twin 300mm woofers (and bigger) only. Having lived a long time with Harbeth M40.1 that meet the first of these conditions, having also a chance to listen in my room to some wonderful examples of large woofers implementation, eg. in Tannoy , JBL and Trenner & Friedl speakers, I can easily be accused of certain bias.

And most likely rightly so. But the music reproduced in this way sounds, for me, more natural. Large driver not only affects the bandwidth and real bass reproduction, but above all, the scale of sound, its dynamics, momentum, soundstage size and the quality of the treble. It all adds up to fullness, richness, color and naturalness of the sound that I appreciate the most.

Acuhorn 15 are the cheapest loudspeakers of this whole group, they do have their own problems as the part of the midrange that is laid back, not the best selectivity and not quite as resolving lower bass. But at the same time they present the better side of every recordings, without any treble roll off and without pretending that these are not even needed. These are really fast, dynamic loudspeakers that deliver a warm, pleasant sound, which fills even large rooms with more refined sound compared to what stage speakers have to offer.

Design

Measuring 1050 x 570 x 240 mm (H x W x D) model named ‘15’ is a two-way, large, floorstanding speaker with a ported cabinet and quite large internal volume of 100 l. The front baffle is very wide, much wider than the one of Harbeth M40.1. After all it has to fit a 381 mm (15" – the model is named after this diameter) low- midwoofer. But this was not the only reason—designer wanted to have a wide front baffle reminding of open baffle designs. The cabinets is finished with natural veneer and oiled. The rear panel features a large diameter port and a pair of solid speaker posts. These do not fit large spades, like the ones used in my Tara Labs Omega Onyx, so I had to use banana connectors.

The 15FH500 woofer was made by Italian company FaitalPRO. It features a paper cone and rigid suspension, made of coated textile material. Its basket looks like a part of a jet engine—and that was the point: it had to be as rigid, and at the same time the arms had to produce the least interference with the flow of air from the rear side of the membrane. Although historically the first transducers of this type used Alnico magnets and later Ferrite ones, here they used a powerful neodymium magnet.

Acuhorn 15 Loudspeakers

A similar magnet (Neodymium and Barium Ferrite) was used for a ribbon tweeter MdRT2H-A. It was made by the American company HiVi and it uses a Kapton diaphragm covered with aluminum ribbon. Metal covers 90% of the membrane surface and it is polarized with magnets placed on both sides. This solution is called Isodynamic Ribbon Tweeter.

The 1st order crossover is very simple and contains Mundorf polypropylene capacitors.

One can not ignore these loudspeakers in the room as they become an important part of the room's equipment room.

Specifications (according to manufacturer)