Stand Loudspeaker Reviews

I'm a sucker for materials, whether it's finishes for loudspeakers and other audio equipment, a shoe's fine, supple leather, a crisp cotton shirt, or a cozy cashmere scarf.

Apart from their inherent sensuousness, materials can make a difference in the sonics of audio components, especially loudspeakers.

Exotic wood enclosures are old hat. Carbon fiber isn't exotic anymore. Glass seems an odd choice for a loudspeaker enclosure—but that's the choice made by Perfect8 speakers, which I encountered at T.H.E. Show in Newport Beach a few years back. And then there are the Jörn speakers from Denmark, which are made of iron; America's OMA uses iron, too, in some of their designs. Fischer & Fischer uses enclosures made of slate

Acora Acoustics is the first company I'm aware of that makes speaker enclosures out of granite. And not just any granite: They use a particular kind of granite, sourced from Africa. Ever since he was 16, Valerio Cora, the founder and self-taught designer of Acora Acoustics, has designed speakers with enclosures made from unusual materials including various types of rock.

Countertops, sure, but speakers? Why granite? Because granite is extremely hard, dense, rigid, and well-damped. All those properties are desirable for speaker enclosures.

The downside of granite is that it is hard to work with. Granite is so hard that only diamond cutting tools can be used—that or water jets, which are far less precise. CNC machines go slowly when cutting granite, and the tools wear quickly (footnote 1).

Cora first tried to manufacture a granite speaker some 20 years ago, but manufacturing consistency proved impossible. The machining technology just didn't exist yet. "I decided to just wait until machinery caught up," Cora told me by phone from Acora's headquarters in the Scarborough area of Toronto.

Cora said that manufacturing developments within the last 5 or 6 years have made it practical to CNC-machine granite with the consistency and tolerances that speaker enclosures require. Tolerances of 15–20 thousandths of an inch are achievable now, about a tenth the standard for counter-tops (footnote 2). "You can't have a quarter-inch tolerance when you start putting a driver into a front panel," he said. "It needs to be put in within millimeters."

The two-way Acora SRB standmount loudspeaker is a bass-reflex design, with an oval port carved out of the cabinet's back panel. The SRB's drivers are flush-mounted to the granite front baffle, which slopes bottom to top almost 5 degrees to help the sound from the drivers reach listeners' ears at the same time.

Acora loudspeakers are made in Canada from a specific black granite from Zimbabwe called Nero Assoluto (Italian for "absolute black"). Cora says it's the strongest of all granites, with the smallest, most homogenous crystals, and is least likely to warp. To make the boxes, sheets of granite are joined with an acrylic epoxy.

Granite speakers are durable. You don't have to worry much about scratching these cabinets. If you slap the top, you'll hurt your hand a little; the energy bounces back.

"It's not absorbing any of the energy into the cabinet," Cora told me. "It's fully pushing that energy out and coupling it to the air, which is what makes it so much faster and so much more dynamic" than the same drivers and crossover would be mounted in a cabinet made from a different material.

Cora chose this granite for its sonic properties, but it also has an aesthetic appeal. It comes in any color, as long as it's black. The SRBs make a visual statement, especially when used with the matching granite stands.

Two types of stands are available. Both have a granite base and top, below and above a V-shaped stainless-steel frame. The upgraded stand has granite panels that cover both sides of the frame and a larger granite base that extends forward a couple more inches. It's much heavier.

Beyond the rock
The SRB crossover is a fourth-order variant, Cora told me: "I am very much a believer in a high-order crossover because I don't like driver interaction. I don't like hearing a midrange and a tweeter in the same frequency span, because now you have two points of source, which are always going to smear the image and cause off-axis problems, etc." The drivers cross over at 3kHz.

"We go pretty crazy with the crossovers," he continued. "Every single little component is aligned so it has minimum interference with the next one. The capacitors are stacked where they need to be stacked so that we're not doing capacitors at 90 degrees, which causes interference. All the bigger capacitors are bypassed with Teflon caps." "Bypassing" big capacitors with smaller-valued capacitors is said to allow energy to flow out of the caps more rapidly, improving sonics.

Acora's specified SRB sensitivity is 86.5dB/W/m. (If the SRB is in fact a nominal 8 ohm load, as the specifications claim, then the sensitivity is the same in units of dB/2.83V/m, Stereophile's preferred units.) "There's 7±2 ohms across most of the audio band," Cora said. The impedance increases at port frequencies. "I could have suppressed that with additional crossover parts, but I think it causes more issues than it solves for me to kill the resonance at the peaks."

The crossovers were created to work with the SRB's granite cabinet and the Scan-Speak drivers Cora tweaks to fit those crossovers more seamlessly. The 5.5" woofer has a two-layer paper cone with a rubber surround. The 1" tweeter uses a beryllium dome that Cora said is flat up to 20kHz on-axis.

I was wary of the beryllium at first, fearing the sonic characteristics that have given metal tweeters a bad name in some circles.

"I worked incredibly hard to get rid of that 'metallic sound'," Cora said. There's a phase plug in the center. A honeycomb-patterned grille covers the dome, serving both as a waveguide, helping with upper-midrange dispersion, and as a guard required by North American law: Beryllium is dangerous if damaged and inhaled.

The cabinet has no vibration-reducing internal bracing; it's granite after all. The 2cm-thick walls are coated on the inside with a thin layer of a proprietary material—it seemed slightly sticky/jellylike—for a little bit of damping internal air-space resonances.

Setup
I first heard Acora Acoustics speakers at the Florida Audio Expo in February 2019, my last audio-related foray before the pandemic inhibited travel and human contact. I spent most of my Acora time listening to a different Acora loudspeaker, a two-way floorstander called the SRC-1, but I was intrigued by the little two-way that seemed to match the room it was in—an average-sized hotel room—better than the bigger speaker did. Its sound was detailed, open, and room-filling—big for the speaker's size.

Here, I'm obligated to mention that I listened to two pairs of SRB speakers. The first pair that arrived (and were set up by Acora Director of Sales & Marketing Scott Sefton) had a technical issue: The tweeters had been wired backward, which impacted phase and, hence, frequency response; John Atkinson discovered the problem during his measurements. I had noticed some peculiarities but figured the granite and my lively room were factors. Acora's eagerness to ship them expeditiously for this review, combined with some confusion from just moving to a new factory, meant that a standard testing step was skipped. Acora promptly dispatched a new pair directly to John Atkinson for measurement. After they checked out, he shipped them to me for listening. My listening observations will focus on the new, corrected pair.

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Footnote 1: Random interesting granite-related fact: Because of its density, dimensional stability, and good internal damping, granite is often used as a base for high-precision CNC machines. ("CNC" stands for "computer numerical control." These are automatic, computer-controlled machines used in manufacturing.)

Footnote 2: ...though it's still quite crude compared to what can be done with metal.