Floor Loudspeaker Reviews

Stereophile has favorably reviewed three loudspeakers from Swiss manufacturer Stenheim. Art Dudley reviewed the Alumine standmount in April 2012 and the Alumine Five floorstander in March 2018. More recently, Herb Reichert reviewed Stenheim's three-and-a-half-way Alumine Three tower in October 2021. My assignment for this month's issue was to review the company's Alumine Two. Five floorstander, which, at $23,500/pair, fits neatly between the Alumine Two ($13,500/pair) and Three ($36,500/pair).

Stenheim was founded by four engineers who previously had worked at Goldmund. A couple of years after the introduction of the company's first loudspeaker, the Alumine Two standmount, in 2011, Stenheim was acquired by Jean-Pascal Panchard, who had been an engineer at Nagra and operated an audio store. Panchard is now the CEO and chief designer for Stenheim.

The Alumine Two.Five ...
... is an elegant-looking, slim tower standing 38" high on spiked feet. In common with Stenheim's other Alumine speakers, the enclosure is formed from precision-machined aluminum panels, with internal bracing for a rigid structure. The three drive units—a 1" tweeter and two 6.5" woofers—are mounted vertically in line on the front baffle, slightly offset from the centerline. The woofers, supplied by French company PHL, feature a half-roll surround and a 5" cellulose-fiber cone with a proprietary coating. The woofers are reflex-loaded with a slightly flared port 3" in diameter, mounted close to the bottom of the front baffle, offset on the other side of the centerline. The soft-dome tweeter, sourced from Norwegian manufacturer SEAS, uses a substantial half-roll surround and is acoustically loaded with a shallow waveguide. Electrical connection is via a single pair of WBT binding posts. The crossover uses high-quality components including Mundorf capacitors.

Panchard explained in a Zoom conversation that the tweeter is the same one used in the Alumine Two. The Alumine Two.Five's crossover frequency is around 2.5kHz, with a topology close to Linkwitz-Riley to give phase coherence between the midrange and high frequencies. "We work a lot on the crossover, on the choice of the components, to get the best balance possible, to have a natural sound," Panchard said.

The loudspeaker's published specifications describe the Alumine Two.Five as a "two-way" design. However, the name implies that it is actually a 2.5-way speaker, where only the upper woofer's output extends all the way up to cross over to the tweeter. I asked Panchard for clarification.

"Yes, it's a two-and-a-half way," he replied. "We have both drivers working in parallel until around 120, 125Hz, then only one is playing in the midrange; ... we didn't want to have a three-way. We love the Alumine Two. It was our first loudspeaker. Iconic. We wanted to keep the qualities of the Alumine Two, but with a second bass driver to reinforce the lowest frequencies."

About those lows: "Hofmann's Iron Law" dictates that for a given enclosure volume, you can have high sensitivity or extended low frequencies but not both. Yet a constant feature of Jean-Pascal Panchard's Stenheim designs has been a high specified sensitivity. The Alumine Two.Five is rated at 93dB (footnote 1). According to Panchard, the loudspeaker is placed directly on the floor for the sensitivity measurement and therefore radiates into half-space. "The half-space value is then the minimum sensitivity a user can get in a well-damped monitoring room," Panchard told Herb Reichert when Herb reviewed Stenheim's Alumine Three.

"I like the highest efficiency or sensitivity," Panchard told me. "We get this excitement coming from the high efficiency, a very dynamic and microdynamic, very detailed, lively sound. Of course, you can have much higher efficiency, but then you need to have horns, or much bigger speakers, or you can sacrifice the lowest frequencies. That's always a challenge: to get the highest efficiency with a full bandwidth. So we try to reach the highest possible efficiency with a reasonably sized enclosure but also to keep the sound very natural."

I asked Panchard what his goal was for the woofers; was the alignment maximally flat or was it overdamped?

He said that it was something between those two possibilities, since his goal was to optimally reproduce transients. A tuning that gave a higher sensitivity would have slowed down the bass. "So of course we don't reach 20Hz, but we wanted the low frequencies to sound rich and give the same impression in terms of speed and coherency as the midrange."

Setting up
I used my Roon Nucleus+ server to feed audio data over my network to a Roon Ready MBL N31 CD player/DAC. This was connected first to the combination of the Ayre KX-8 preamplifier that I reviewed in the May 2025 issue and an Ayre VX-8 power amplifier, then directly to the VX-8, and finally to a pair of Parasound JCA100 Tribute monoblock amplifiers.

The first issue I had to deal with was which loudspeaker to use as the left and which for the right. The drive units are mounted asymmetrically on the front baffle, mirror imaged on the pair of speakers. The manual doesn't specify which edges of the baffle should be facing each other and which should face the sidewalls. (With hindsight, serial numbers on the rear panels ending with "L" or "R" should have given me a clue.) I experimented both with the drivers on the outside edges and on the inside edges and found that the stereo imaging was more stable and precise with the tweeters and woofers on the inside edges. It came as a relief, therefore, when I asked Jean-Pascal Panchard which was optimal and he responded that unless you have a very narrow room or need to place the Alumine Two.Fives very close to the sidewalls, Stenheim recommends positioning the speakers with the drivers on the inside edges.

With that question resolved, the next issue was where I should place the speakers. The manual suggests that for the best results, the speakers should be at least 50cm (~20") away from the back and sidewalls and should not be positioned equidistant from those walls. "It is always recommended to leave a greater distance between the speaker and the back wall than between the speaker and the sidewall," the manual says.

After some experimentation, I ended up with the Alumine Two.Fives' front baffles 74" from the wall behind the speakers, the right-hand woofers 50" from the books that line that speaker's closest sidewall, and the left-hand woofers 35" from the LPs that line that speaker's sidewall. The low frequencies sounded a little lightweight with the speakers this far from the wall behind them, but it wasn't possible to move them closer to that wall due to the short flight of stairs behind the right-hand speaker that leads to the vestibule.

I subsequently moved the speakers 6" closer to the listening chair, which optimized the reproduction of soundstage depth without significantly affecting the level of the low frequencies.

With the Alumine Two.Fives in these new positions, the 1/3-octave warble tones on my Editor's Choice CD (footnote 2) were reproduced cleanly with full weight down to the 63Hz band. The 50Hz and 40Hz tones were lower in level but still audible. I could just hear the 32Hz tone, which was boosted by my room's lowest-frequency mode, but I couldn't hear the 25Hz and 20Hz tones. The half-step–spaced tonebursts on Editor's Choice spoke cleanly and evenly down to 63Hz, though there was some emphasis between 2kHz and 4kHz. I heard no audible wind noise from the port with the lowest-frequency tones, and the tonebursts and warble tones sounded clean, with no "doubling" (second-harmonic distortion). With the Stenheim speakers toed in to the listening position and the tweeters just below the height of my ears at 36" from the floor, the dual-mono pink noise track on Editor's Choice was reproduced as a stable central image. The high frequencies sounded smooth and the midrange uncolored. The noise sounded hollow if I stood up. Moving my head from side to side, I heard very little comb filtering.

Enough listening to test tones; on to music.

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Footnote 1: At the rated (nominal) impedance of 8 ohms, the two common units—dB/W/1m and dB/2.83V/1m—give the same numerical result.

Footnote 2: While this CD is out of print, the test-tone files can be downloaded free of charge here.