Breaking all the rules.  Again.  The Extremis 6.8™ from Adire Audio.  The ultimate 8 Ohm midrange/midwoofer driver.  Wide bandwidth, high output, amazing linearity in a video shielded package.


The Extremis 6.8 is our entry in the high end of midwoofers.  Designed to take on - and beat - drivers like the legendary ScanSpeak 8545, Seas Excel offerings, and other high-end European drivers the Extremis 6.8 simply takes everything to another level.

Start with flexible T/S parameters.  Parameters that allow use in common monitor boxes.  The T/S parameters of the Extremis 6.8 lend themselves to vented boxes in the 14 to 28 liter range, and with tunings in the 30-40 Hz range.  Extension to 30 Hz anechoic promises in-room output down to the 20-25 Hz range!  Our favorite application: 21 liters, tuned to 33 Hz with a 2" flared port of 8" length.  Clean and flat anechoic response to 35 Hz, and in-room extension to the low 20s.

And the Extremis 6.8 backs up that extension, too.  The first midwoofer ever with over 1 inch of peak-to-peak LINEAR excursion, the Extremis 6.8 can out-perform many 10" drivers in terms of clean linear output down deep.  The linear output of the Extremis 6.8 is a whole new realm - output levels equal to two (or more!) of other company's high end midwoofers, from a single driver.

How does this motor achieve all this performance?  Simple - it uses our XBL2 motor structure.  This completely unique and patent-pending motor structure allows for extremely high levels of linear stroke without loss of BL, excessive moving mass, or other problems associated with long stroke.  With this motor technology, you can have it all - wide bandwidth, low moving mass, and high stroke.  Here's a graph of the BL curve of this amazing driver:


So the bottom end is there - what about the top end?  The first indication that things are right is the extremely low inductance - just 0.13 mH!  The inductive roll-off of this driver is up beyond 8 kHz!  So the extension potential is there...  And we used as much of that potential as possible with a carefully selected exponential flared polypropylene cone, matching smooth-mount polypropylene dustcap, and butyl surround combination with hand-applied edge damping treatment on the rear.  The frequency response is free from large breakup modes that plague stiff cones (like metal or honeycomb Kevlar™) - just clean even response to 5 kHz, where the driver has a very mild dip in response as it transitions to a controlled roll-off.  The impedance is extremely smooth and flat, making crossovers in the typical 1-3 kHz range very simple:

Frequency Response (click to enlarge). 
The driver is ±3 dB up to 4 kHz.  There aren't any breakup peaks, only a single dip, making
self-excitation of breakup modes inaudible

Impedance (click to enlarge).  The impedance is extremely high and well behaved,
with a minimum of 7.2 Ohms.  The inductive rise is gentle and doesn't start until 4.5 kHz.

Here's a few more plots showing the outstanding performance of this driver:


THD: measured with the Extremis 6.8 mounted in a 1 cubic foot sealed box, anechoic (nearest boundary 10 feet away), 94 dB SPL @ 1 meter drive level (1 Pascal of pressure averaged from 500 Hz to 2 kHz).  Measurement from 50 Hz to 10 kHz (96 kHz sampling rate allows capture of the first 9 harmonics over the entire frequency range).  Red trace is THD, green trace is 2nd harmonic.  Note that the THD is below 1% from 70 Hz and up, and is well below 0.5% THD above 100 Hz.  In the suggested 21 liter, 33 Hz tuned box, THD is below 3% from 40 Hz and up, at a 94 dB SPL output level.  Also of interest is that the THD is almost completely composed of the second harmonic, meaning that subjectively this driver will be clean and smooth.


Spectral Contamination: measured with the Extremis 6.8 mounted in a 1 cubic foot sealed box, anechoic (nearest boundary 10 feet away), 94 dB SPL @ 1 meter drive level (1 Pascal of pressure averaged from 500 Hz to 2 kHz).  20 simultaneous tones spaced evenly from 50 Hz to 10 kHz.  The tones trace the frequency response of the driver; the remaining "stuff" down below the spikes is all the distortions, buzzes, and extraneous noises of the driver.  A theoretically perfect driver would have just the 20 spikes.  In this case, the Extremis 6 is amazingly free of higher level spurious signals.  It starts with nearly 40 dB of signal over the spectral contamination (1% spectral contamination level) and falls quickly.  It is more than 50 dB down above 500 Hz - that's less than 0.3% spectral contamination at a 94 dB SPL drive level!


Waterfall plot: measured with the Extremis 6.8 mounted in a 1 cubic foot sealed box, anechoic (nearest boundary 10 feet away), 94 dB SPL @ 1 meter drive level (1 Pascal of pressure averaged from 500 Hz to 2 kHz).  Waterfall measured from 300 Hz to 20 kHz, with 0.125 msec steps (8 steps per millisecond).  30 dB decay window shown.  The driver is down 20 dB or more within 1.5 milliseconds, which is an extremely fast decay (within the latent time of fusion for the human ear).  By 3 milliseconds, the driver is more than 25 dB down, and by 5 millseconds (not shown) the driver is more than 40 dB down.


This driver is a natural for crossing over at the 3-4 kHz range, and when used with a 2nd order electrical network at 3.5 kHz, a 6th order roll-off is achieved, meaning the transition to the tweeter is quick and comb filtering is kept to an absolute minimum.  The short voice coil, aluminum former (required for something with this much linear stroke), and full copper sleeving all help to widen the bandwidth of this driver.

The impedance is also very flat and free from ripples - as shown on the impedance graph, you can see the impedance doesn't double over minimum until the 7 kHz range!  And the very slow impedance rise from 150 Hz to 8 kHz is deliberate - the result of a thick plating of copper on the pole piece.  A full 1mm thick copper sleeve is used on the pole of this motor, cutting the inductance above 500 Hz by a tremendous amount.

The Extremis 6.8 has something the other high end drivers don't though: video shielding.  The Extremis 6.8 uses a neodymium magnet system which is inherently low in stray flux.  In fact, the stray field just 2" from the motor is less than 100 mG (the Earth's magnetic field is 5 times larger!).  This driver can be placed within inches of a CRT or magnetically sensitive equipment without a problem.  Try that with the other high end offerings, and you'll need to invest in good degaussing coils!

The motor is also extremely compact.  Thanks to the large 35mm diameter disc of high-grade neo, the motor is just 70mm in diameter - less than 3"!  And the height is very short for the stroke provided - just a scant 42mm (less than 1 ¾"), which is only possible with our XBL² technology.  The motor is acoustically hidden from the back of the cone, resulting in the most open, free-flowing, reflection free motor structure available anywhere.  The cone never "sees" the motorEver.

Contributing to this open structure is a quality cast aluminum basket.  This basket is a full 4mm thick at the mounting flange, for an extremely solid and stable mounting platform.  The moving assembly is held in perfect alignment.  And while there are just four thin basket arms, each basket arm is heavily ribbed for reinforcement, so that the lightweight and compact motor simply can't move, twist, or vibrate.

The suspension is just as advanced - a custom pressed linen progressive roll spider with flat attachment, the spider allows up to 15mm of motion from the driver, but maintains near-perfect linearity over a 26mm range of motionCombined with a thin, linear shallow roll rubber surround with a custom edge termination, the result is a suspension ideally matched to the capabilities of this motor.

The T/S parameters are as follows:

Fs 30 Hz BL 9 N/A
Qms 2.54 Xmax 13mm one way
Qes .383 Sd 140 cm2
Qts .332 Vd 364 cm³
Vas 33.2 liters Le 0.13 mH
SPL 86 dB  @ 1W, 1m Re 7.02 Ohms
Power 100W RMS/300W Peak Price On Application

The Extremis 6.8.  Wide bandwidth, high output, amazing linearity.  Finally you CAN have it all!

Exploded motor view

Motor animation

Motor cutaway

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Extremis 6.8 is Trademark Adire Audio.