Everything Cees Ruijtenberg signed under development credits attracts my attention, and that goes back to his products signed under Metrum Acoustics and Sonnet Digital Audio. His latest venture with Lion Kwaaijtaal delivers another very interesting high-end audio product, designed to be an ultra-high-performance, high-end monitor. The Acelec Model One passive speakers come with some original solutions and a careful choice of materials, refined in some years of research and development and proven in sound. 

As you can read further, the Acelec Model One monitor speakers are no different from the rest of Ruijtenberg products when it comes to performance.

Like many of you, I have a soft spot for 2-way monitor speakers, as they can disappear into the room and, when done correctly, offer a genuine believability and life-like aural smoothness that is much harder to achieve with multi-driver speakers.

So, Ruijtenberg didn’t have to ask me twice about the review and soon the Acelec Model One passive monitors were on their way to Mono and Stereo. 


Acelec is a sub-brand of Sonnet Digital Audio (SDA), which has a long history of innovation in the world of acoustics and electronic design. In the audio industry today, SDA is best known for its research and development of DAC chips. In the field of Electrostatic Speaker, its experience dates back to 1989, and broader electronic applications have developed over time. During this time, various products have been created for acoustic systems, always based on solid natural laws, but in recent years digital signal processing has played a significant role. Ongoing research continues, with improvements in acoustic systems being sought in all areas, and this has been utilized in the current Acelec loudspeaker product.

First electrostatic speaker project from 1990
From a digital point of view, acoustic problems seem easy to solve, but in practice, this is only partially true. Problems can occur, for example, when cheap materials are used and cannot be compensated for in the digital domain. Therefore, a good analog design is still the best basis on which all digital corrections can take place. Ruijtenberg has decided to look back and return to the roots and introduce new lines of high-performance products.
Project from 208
Sonnet Digital Audio already focuses on making complete, matched sound systems, but on a broader level, their knowledge, latest research, and ongoing developments are being used to create new OEM components. These will and already have many applications – specifically for home and professional use. 
Lion Kwaaijtaal and Cees Ruijtenberg worked together on the Acelec monitor. Lion Kwaaijtaal was responsible for the drawings and milling of the panels, while Ruijtenberg supervised and designed the complete product from the beginning. 
Each Acelec speaker embodies the advanced Acelec speaker technology, which results in a very high level of music reproduction through the implementation of advanced technology.


Church bells fall under the general heading of tuned percussion instruments. A well-tuned bell gives a distinct sense of pitch when rung. This is because some of the lower natural frequencies have been carefully tuned to fall within harmonically related ratios. However, the role of these natural frequencies in determining the sound of a bell is not the issue here.
Oddly enough, with speakers, we’re not interested in the behavior of a sound church bell. It’s more interesting to observe what happens when a church bell is old and cracked? It will lose its beautiful sound because the unity in the bronze has disappeared – caused by the impact of the bell clapper over many years. This correlates with a speaker cabinet. Traditionally, for example, when using wood or multiplex variations of wood, the six panels of a cabinet are glued together. This creates a unity in the construction due to the hardness of the glue, making it more resonant than the individual panels. 
Although they have a reasonable amount of internal damping, enclosure constructions made of engineered wood or MDF are simply not stiff enough. As a result, the acoustic energy created inside will leak out over time. Time-dependent events in the music (e.g. percussive instruments) are blurred over time due to the lack of rigidity of the cabinet and the possible ringing – due to the uniformity of the materials used.
Ruijtenberg and Kwaaijtaal have taken a different approach to address this. The acoustic properties of Acelec speakers are based on the stiffness of 15mm bituminized aluminum panels. These are not uniform in nature as the panels are bonded with rubber type adhesives. This approach, and the use of the best available transducers, allows Acelec designs to explore the most honest and colorless representation – of the electrical signal equivalents.
15 mm bituminized aluminum as used in both Model One and Model Two will result in very short vabriation behavior
Model One speakers 15mm bituminized aluminum panels considerably helps with a very short vibration response thus not getting in the way of purist sound reproduction as intended from the start. 


Model One was designed from the ground up to rival what’s out there in the market and establish the norms for the most transparent conventional loudspeaker. Intensive research has been conducted over the past two years to explore new methods and materials. These are now being incorporated into the Acelec monitors. 
Model One is a premium speaker that offers absolute performance and follows a minimalist, refined design with its compact cabinet that follows the purist concept. 
The shape of the chassis has been devised to deliver a detailed and powerful sound reproduction normally only associated with much larger, floor standing speakers.
The technical criteria were also as simple as can be. Extreme rigidity through the use of bituminized aluminum and internal damping of the cabinet to avoid any sound effects. As a result, this approach avoids any time smearing, especially during fast and powerful transients. 
And the third, highly important design goal was the ability of universal placement into the everyday living/listening environment. Performance shouldn’t be compromised by proximity to room boundaries, such as walls and corners. During the testing Model One speakers have proven to behave exactly like this, adaptively,  and without the common issues with placement. 
In addition, the design needed to behave efficiently while acting as a simple load for a wide range of amplifiers – both sensitive tube and powerful solid-state amps. A clever implementation of the crossover in conjunction with carefully selected drivers and utmost rigid speaker enclosure allows such universality.
In terms of efficiency, tweeters and woofers differ significantly. While the woofer operates at about 84dB/watt, the tweeter does the job 8dB louder. With such attenuation, the tweeter can handle more power. Consequentially the overlap point to a lower frequency was moved to 1.8 kHz. 
To get a good phase response, first-order filters with a slope of 6dB/octave are wanted. This has the disadvantage of generally sending too much power to the tweeter. The advantage, however, is that due to this low crossover frequency the wavelength is relatively long. 
This results in good matching between the two drivers without strange radiation patterns, as they act as one unit. For this particular challenge, so-called dual-slope filters were used. At the beginning (around the crossover frequency) they act as first-order filters, but further away from this crossover point, they act as second-order or 12dB/octave filters. 
The result is a very good phase response, but also (due to the second slope) this protects the tweeter and reduces the unwanted side effects of the woofer. 
Furthermore, the first breakup mode of the woofer is bypassed, which normally causes a small bump around 800 Hz with this size. The result is extremely smooth frequency response. 
As already mentioned, the impedance of the entire system was deliberately flattened to work well with tube amplifiers.


There is something mysteriously eerie about the way the Acelec Model One speakers “Aqua Marine” from Isao Suzuki Quartet album Blow Up make my listening room fill up. Yes, this is one of the iconic Three Blind Mice Records releases that still holds up fantastically in time and space, despite being recorded in 1973. 
Model One speakers’ capability to revive the almost ghastly atmosphere with a sonic expansiveness that was almost thoughtful as far bigger floor-standing speakers was quite extraordinary. The sheer lightness of electric piano tones and shimmering metallic percussion notes were rendered with a sense of feathery float, filling the listening room with an impressively wide vertical and horizontal augmentation. 
The “Blow-Up” barrage rhythm and spot-on pulsating bass speed were also furnished with a refreshing tautness, that “Blow-Up” calls for to materialize at full. Not something that is usually the strong suit at this price tag. A big thumb up!


The impressive sonic depth and musical flow continued with Maarten Van Der Walk – “Touch Siltech” far-out sonic voyage. It is always interesting to scrutinize how a particular speaker behaves when it comes to representing Van Der Walk’s percussion championship. 
The Acelec monitors have punctuating the sharp attacks with pinpoint accuracy, without flattening or rounding them off at any point. It was also inciting to observe the level of acoustic point density generated by the Model Ones.  Everything was projected forward into the listening environment with a far better than just a required and necessary sonic frame. An achievement is solely worthy of high praise.

The impressive qualities of the Model One speakers also came to the fore at Malcom Araold Overtures – Reference Recording The London Philharmonic Orchestra.
This recording is a real tour de force and can easily drive speakers into dynamic congestion. The Acelec monitor speakers presented Malcom Araold Overtures boldly, nimbly, and melodically. 
Many speakers, even larger and far more expensive ones, can act stoically with this splendid Reference Recording album. The efficacious Model One core not only provided a unique togetherness but allowed London Philharmonic Orchestra’s orchestral spirit to capture the essence of the music with a one of the kind interconnectedness. 
This was especially evident in “A Sussex Overture, Op. 31” where the momentum was crafted with non-faux, sense-making qualities, and in the absence of aural blind alleys that are all too familiar with across too many loudspeakers on the market regardless of the price. 
This was especially evident in “A Sussex Overture, Op. 31” where the momentum was shaped without faux pas, with sense-making qualities, and in an absence of sonic dead ends, as one knows all too well with too many speakers on the market, regardless of price.
The needed speed, clarity, and unproblematic stacking of notes were validated with an excellent result on Wu Man And The Master Musicians From The Silk Route – Borderlines.
Acelec Model One Speakers kithless sound rendering shaped Wu Man’s musical ethos with a unique freebooting quality, without a hint and smidge of tonal coloration. 
Notes and tones’ overflow were reproduced with noble clarion. The Acelec monitors provided gratifying and immediate sonic excursions with uniquely nested aural convexity.


It may not seem like it, but the most important thing about the Model One is the speaker cabinet. It all starts here with the Acelec monitors, and the rest of the paraphernalia follows. 

The review sample came with a stylish, matching, and modified version of the Monitor Audio speaker stands. In due course, Acelec will launch their speaker stands, but for now, they are working on a new active subwoofer that will also host the Model One monitors. 

I have put a lot of time and effort into getting my listening room ready, and at this stage, it can accommodate a variety of speakers and show what they are made of. 

The Model One enclosure has proven to allow remarkable sonic performance even in my nearly 50 m2 reference listening room, where the Acelec monitors more or less show their true potential at 27db noise floor, 0.47s total RT60 time, and properly addressed room modes, Model One were operating far, far outside their given proportions. 

I also heard the Model One in two smaller listening rooms, but for safety’s sake, I wanted to give them the freedom to show their mojo in my main studio. And the results were pretty amazing. I am no stranger to active, smaller pro audio speakers, where you can achieve a certain level of impressive performance even with a small, self-powered studio monitor. But, the Acelec monitors are passive, and as such performing boldly in a larger listening environment certainly is a great challenge. 

The biggest mistake associated with any speaker setup is underpowering them. I have shown many people how, to the surprise of most, some of the common acoustic problems can disappear to the most extraordinary degree when the speakers are driven properly. With an appropriate, suitable, and sufficiently powerful amplifier, the drivers will be locked and loaded and show the true feeling of power, size, and disappearance. And Acelec speakers really loved the pure and unaltered power control to fully bloom.

There are different schools of thought when it comes to speaker cabinet materials. Acelec follows their philosophy, which was explained in one of the previous sections with the analogy of a cracked church bell. All of the Model One’s parts are glued together with very soft glue, creating a stiff, quiet chassis that cannot ring. A simple tap on one of the Acelec monitor panels results in a dull knock that immediately dies away. There is no resonance effect. No ringing. 

The secret of the case is that a lot of glue isolates the individual panels so that there is no unit resonance. This greatly reduces the usual ringing associated with normal enclosures. The sidewall accelerometer energy measurement shows that a singular bituminized panel reduces sound by up to 18 dB!

Not much is revealed about the Model One’s crossover, but it is carefully tuned and optimized with select components and advanced circuitry to achieve a truly balanced and smooth frequency response. Splitting the frequency ranges is strictly avoided, so the idea of bi-wiring and bi-amping is not pursued, which seemed neither sensible nor optional to the designer. 

Although these monitors were undoubtedly designed for use in smaller rooms, they can also assert their qualities in medium-sized and, to a certain extent, even large listening rooms without any problems. The Acelec speakers feature a classic bass-reflex design with an adapting bass reflex port. Foam plugs inserted into the back of the bass reflex port reduce room-induced boost in the low-frequency range, thus providing a clearer and tighter sound when the speaker is placed near rear walls.

In all three listening rooms where I tried the Acelec Monitor One, the speaker placement ended up in an isosceles triangle, which allows for straightforward positioning in different places. 

The Acelec Model One speakers acted as a full-blooded David against Goliath (in a form of floor stander) with their impressive transparency, subtlety, overriding credibility factor, and, above all, very engaging nature. 

While I wouldn’t call Model One loudspeakers cable depending, they’ve shown to respond easily and instantly to the different high-end audio cables. This calls for careful overall system integration and in my evaluation Acelec monitors found a happy synergy with full loom Nordost and Tellurium Q interlinks. 

The Acelec monitors are highly enjoyable speakers that defy their size and sonic expectations. Model One deserves to be highlighted for all the reasons I have mentioned throughout the first part of the review and in the first part of the review I’ve wholeheartedly granted the Mono & Stereo Best of 2020 Award. 

It’s rare to combine so many sonic qualities in such a compact package, but Acelec’s passive monitors are remarkable in conveying holographic three-dimensionality that is never overshadowed by other qualities. The transparency, fast music reproduction, and top-notch coherence just go along with Model One fantastic vertical and horizontal expanse. 

For being true to the source, for its disappearing act, unreserved dynamism and perhaps most importantly to get it so right in the first take the Acelec Model One speakers are more than deserving to be bestowed with the 2021 Mono and Stereo Upper Echelon Product Award. 

Matej Isak


  • Price per pair (Standard finish Silver or Black)  Euro 6500 including 21% VAT.
  • Optional (sand filled) loudspeaker stand Euro 600 including 21% VAT.


  • System Format  2-way 
  • Housing  15 mm bituminized aluminum 
  • Bass Alignment  Low Q  alignment Vented port system.
  • Frequency Response (-6 dB)   45 Hz – 35 kHz  Max. deviation ± 2.5 dB
  • Sensitivity (1W@1M)  84 dB
  • Nominal Impedance  8 ohms
  • Minimum Impedance  5 ohms at 375 Hz
  • Maximum SPL  110 dBA (Pair)
  • Power Handling (RMS)  120 W
  • Recommended Amplifier Requirements 25 – 100 W
  • Crossover Frequency  1.8 kHz
  • Drive Unit Complement 15cm bass / mid transducer,  18 mmAir Motion Transformer
  • Cabinet Dimensions (H x W x D) 285 x 195 x 300 mm 
  • Finish  Silver or black finish. Special versions or colors on request
  • Weight 17 kg


Sonnet Digital Audio BV

Daviottenweg 9-11,

5222 BH ‘s-Hertogenbosch

The Netherlands



Phone sales office:  +31(0)36-7856259