Coaxial / Concentric Driver

What a coaxial or concentric driver is

A coaxial loudspeaker puts two drivers on a common axis so that they radiate from the same point or line. A concentric or coincident-source design also lines up their acoustic centers. ELAC describes a concentric driver as a tweeter mounted directly at the center of the bass or midrange cone, at the apex where the dust cap would normally sit, on top of the woofer's pole piece. Fyne Audio draws the line more sharply: its IsoFlare is a coaxial driver that places the tweeter at the acoustic centre of the midrange or bass driver, "not just on the same axis." In practice the terms are used loosely, and the acoustic-center alignment is what separates a good implementation from a simple tweeter-on-a-post design.

A short history

The Altec Lansing Duplex 601 coaxial driver appeared in 1943, and the improved Altec 604, with a 15 inch (380 mm) woofer and a compression driver on a horn behind it, followed in 1945 and became the recording studio standard in the U.S. The Tannoy Dual Concentric was first shown in England in 1947. It also used a 15 inch woofer and compression driver, but the woofer cone served as the final horn flare, so the pattern was radially symmetric. Both put the high-frequency driver behind the cone, so highs arrived slightly later than lows. In the late 1970s, Ed Long's patented Time Alignment crossover was applied to the Altec 604 in the UREI 813 studio monitor to fix that. Wikipedia lists the weaknesses of these early designs as intermodulation distortion, because the bass cone modulates the highs, and diffraction around the central horn.

In 1988 KEF introduced the Uni-Q, a coincident-source driver in which a 25 mm (1 inch) tweeter sat at the neck of a 200 mm (8 inch) woofer. It was possible because neodymium-iron-boron magnets had become small and strong enough to fit inside the voice coil. KEF now counts 12 generations of the driver, with the second generation moving to a 160 mm (6.5 inch) cone in 1989, the fifth using a 25 mm titanium dome in 2001, the eighth adding the Tangerine Waveguide around a 115 mm (4.5 inch) unit in 2007, the twelfth adding a tweeter gap damper in 2018, and a 2020 version adding Metamaterial Absorption Technology, a maze that KEF says absorbs 99% of the unwanted sound from the rear of the tweeter.

Why it helps: off-axis and crossover behavior

In a conventional two-way, the tweeter and woofer sit at different heights on the baffle. KEF states that in conventional loudspeakers, mid and high frequencies radiating from different locations interfere with each other, and that Uni-Q avoids this by dispersing from a single point source. ELAC says the shared origin gives better off-axis response, imaging, and less dependence on room acoustics. Fyne adds that a mathematically designed waveguide loads the tweeter, which improves dispersion and integration with the bass cone. These are manufacturer statements; the independent data below show how far they hold.

What measurements show

audioXpress measured the KEF LS50 Meta with a Klippel Near-Field Scanner at Warkwyn, collecting 1,800 points around the speaker at 18 points per octave from 20 Hz to 20 kHz, referenced to 2.83 V at 1 m. On axis the response held within about 2 dB from 135 Hz to 3 kHz (on a 5 dB scale), rose about 3 dB at 3.5 kHz, and had a dip spanning 1.75 octaves from 1 kHz to 3.5 kHz at 1 to 2 dB. Off-axis response at 15 degrees held together admirably and at 30 degrees held together very well. The radiation pattern is good up to 30 degrees off-axis, to the 6 dB-down point. Because the driver is symmetric, the vertical map is almost identical to the horizontal one, with an asymmetric loss between 100 and 180 Hz at -100 degrees. In the CEA 2034 listening window (nine curves within 10 degrees vertically and 30 degrees horizontally) the 2 dB dip stayed, and the sound-power directivity index confirmed higher directivity from 1 kHz up.

Coaxial vs a conventional two-way, and the trade-offs

A coaxial driver keeps the radiating point the same for woofer and tweeter. The cost, as the Altec and Tannoy history shows, is interaction: the bass cone moves behind or around the tweeter and can modulate it, and sound from a horn can diffract and reflect off the cone. Modern designs reduce these effects with a waveguide, the cone neck control KEF introduced in 2014, the tweeter gap damper and, in pro audio, DSP such as Fulcrum Acoustic's Temporal Equalization. A conventional two-way avoids these coupling issues, but KEF argues it leaves a sweet spot and a mid-to-high frequency interference problem that a point source avoids.

Where concentric drivers are used

KEF says the Uni-Q array sits at the heart of almost all its speakers. Its history page lists the R11 flagship (2018) with a 125 mm (5 inch) 12th-generation array, the Q300 bookshelf (2010) with a 165 mm (6.5 inch) aluminium cone and the LS50 Meta (2020). KEF's earlier KHT3005SE home theater system used the eighth-generation driver in its compact satellites. Fyne Audio's F500SP uses a 6 inch IsoFlare, and its Vintage Fifteen uses a 15 inch one. Tannoy has built Dual Concentric home speakers since the early 1950s, and Wikipedia also names TAD, Cabasse with its 4-way Concentric QC-55 in the La Sphere, Hsu and Vandersteen among home coaxial makers. ELAC says its concentric driver is found in select models. ELAC's claim of uniform sound across a wide listening area is the practical draw for a center channel, where dialog has to stay consistent for seats off to the side. The related article on controlled-directivity waveguides covers the horn-loaded alternative, and the one on compression drivers covers the driver type used in the original Altec and Tannoy coaxials.