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The Coaxial Conundrum

Sceptre S6 (2)

These CoActual 2-way Active Studio Monitors Solve The Problems of Traditional Coaxial Designs.

Coaxial designs offer the advantages of a single point source for a consistent acoustic center and a symmetrical dispersion pattern. But designing coaxial systems without acoustic anomalies has been extremely expensive—until now.  The Sceptre S6 and S8 by Presonus are the first I’ve heard that surpass both KEF and Tannoy when it comes to pin-point stereo imaging and high SPL from multiple angles of axis.  They cross over from the Pro AV Studio category to the Audiophile category with this performance, its jaw-dropping.

​Solving the problems involved in coaxial designs requires massive amounts of DSP and subtle, sophisticated transducer design, which is why such systems have, in the past, been limited to very high-end systems with external processors.

However, Fulcrum Acoustic’s amazing, cutting-edge technology has allowed Presonus to overcome the usual design problems of coaxial systems while taking advantage of coaxial’s unique properties. Thanks to custom transducers and TQ algorithms designed by Fulcrum’s Dave Gunness, Sceptre CoActual monitors deliver clarity and coherence that has previously only been available in ultra-high-end systems.

Multi-way, non-coaxial speaker designs have long been the way to go in speaker applications. But they suffer from the changing relationship between the listener and the speaker Sceptre Asymmetricalelements: When you move around in the coverage area, the sound is inconsistent. Even more troublesome, the crossover point between the drivers can sometimes be audible.

​Sceptre Symmetrical
Symmetrical dispersion pattern from a single point source.Sceptre Symmetrical

Coaxial systems solve this by having the drivers on the same axis, thus providing a single point source for a consistent acoustic center. This results in symmetry of response on both the horizontal and vertical axis, at any given angle. The crossover transition is seamless (inaudible) at all angles. (By “symmetry,” we mean that whatever response is observed at a given angle with respect to the axis, the same response will be observed at that angle in the opposite direction. The loudspeaker’s behavior is “mirrored” about its axis.)

Each Sceptre monitor contains a DSP chip that enables the use of Fulcrum Acoustic’s TQ algorithms. The result is speakers that deliver the full benefits of coaxial design.temporal eq

​No driver is perfect, nor can a driver be made perfect with hardware alone; there are always compromises in physical systems. To overcome these driver compromises, many speaker manufacturers apply DSP in the form of a combination of multiband EQs but the processing is more or less an afterthought—it’s not accounted for in the design of the transducer. This sorta works—more or less.

A few high-end loudspeaker companies have developed sophisticated solutions with active DSP that is co-designed with the speaker, but these are expensive solutions. And only one company has done it with coaxial designs: Fulcrum Acoustic.

So instead of taking the crude, old-skool approach, we partnered with Fulcrum Acoustic and licensed its high-end TQ™ Temporal Equalization, which, up until now, could only be found in very high-end speaker systems.

With TQ, the basic speaker and enclosure designs meet the fundamental physical requirements for accurate, clear sound—characteristics that cannot be obtained with DSP. The TQ algorithms deal with the remaining issues using multiple fully addressable, fairly large Finite Impulse Response (FIR) filters to eliminate horn reflections and to correct linear time and amplitude anomalies. The hardware and TQ settings are designed to work together from the outset, rather than taking an existing speaker system and trying to correct it with DSP.

Sceptre starts out with a transducer that solves many basic coaxial problems through its physical configuration and is also designed so that DSP can be used to correct other issues. In the 8- and 6.25-inch CoActual™ transducer, magnet structures are in close proximity for extremely smooth off-axis response. The purpose-built high-frequency horn contributes to frequency-pattern control and keeps high-frequency energy off of the woofer cone. The woofer’s larger radiating surface works with the HF horn to improve bottom operating-range directional control.

Then Fulcrum’s Temporal EQ™ (TQ) DSP algorithms are applied. It starts with the standard complement of Infinite Impulse Response (IIR) highpass, lowpass, and parametric filters, plus delay. To this is added fully addressable, fairly large Finite Impulse Response (FIR) filters that implement more detailed frequency-response adjustments; and the precise temporal (time domain) filters that are responsible for the most remarkable TQ benefits. As explained in Fulcrum Acoustic’s white paper, “Loudspeakers tuned with TQ provide a crisper stereo image, greater soundstage depth, more separation between the components of a complex mix, increased resistance to feedback, more seamless transitions between distributed loudspeakers, and a less fatiguing listening experience at very high SPLs.”

It seems pretty incredible that a computer—a custom digital signal processor—can eliminate physical horn reflections. But it’s possible when the resonances happen consistently. Knowing in advance how the loudspeaker will respond to a particular signal, it is possible to calculate a special new signal that not only avoids exciting natural resonances, but also actively kills these resonances before they become audible!

Collaborating with Fulcrum Acoustic co-founder, Dave Gunness, PreSonus software designers incorporated custom TQ algorithms into the onboard DSP. The result is a studio monitor with remarkably clear, coherent, detailed sound at a breakthrough price.

These speakers use a nontraditional, asymmetrical crossover scheme and some very tricky time-based processing to increase the output capability and overcome any weaknesses of the drivers.

This scheme has never been implemented before – largely because it is not possible without digital processing, but also because nobody thought of a way to do it, and it takes some pretty crafty phase manipulations to get everything to work.

They come in two sizes, the S6 at $599.95 each and S8 at $749.95 each, which in audiophile dollars is really quite affordable for the level of performance they deliver compared to KEF X300A’s, Tannoys, and Genelecs let alone some of the more expensive desktop nearfield monitors now on the market.

Bob Rapoport

Award-winning sales and marketing exec, consultant and product launch specialist to manufacturers of high performance audio / video products for consumer and professional applications. Holder of 2 patents for analog matrix surround decoding, winner of 12 CES Innovations Design & Engineering awards, recognized in the Who’s Who of American Inventors, and the leading advocate for adoption of the Blu-ray audio standard for music and movies; currently the CEO of Essence For Hi Res Audio manufacturing the HDACC Upsampling 24/192K DAC Preamp, DPA-440 Class D Power Amp, and online marketer in the USA for Acoustic Energy, Ceratec, Core Power Technologies, REVO, Verisonix Headphones, and ARCHT One.

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Bob Rapoport, CEO
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