Acoustic Energy AE1 loudspeaker:
Page 2 In addition, a part of the Cordesman case against minimonitors was that they are said to operate in severe overload at anything approaching realistic levels. Yet, if you glance at the specifications at the head of this report, you will see that Acoustic Energy is honest enough to talk about the compression, or to be exact, the AE1's lack of compression. (Not just minimonitors, but nearly all conventional dynamic speakers suffer from compression effects: the harder they're driven, the hotter the voice-coil gets, increasing its resistance and changing the tuning of the crossover filters. Whether the inevitable heating will produce audible aberrations or not depends on the degree of over-engineering the drive-unit designer has managed to incorporate at a given price (footnote 1). What's going on here? Has this new British company managed to pack a quart, perhaps even a half-gallon, into a pint-sized package?

The answer is yes and no—Love that equivocation!—or, rather, no and yes.

The AE1 is a small box and suffers from the inevitable consequences: low sensitivity and restricted LF extension. However, the drive-units employed have very high power handling, and are said to be linear over a wide dynamic range. The woofer, in particular, designed by Acoustic Energy's Phil Jones and assembled by the British Elac company from components manufactured by Acoustic Energy, is intriguing in that its cone is fabricated from metal rather than the ubiquitous paper or plastic. To be accurate, the cone (straight-sided rather than flared) is spun from aluminum alloy, with a graded thickness across its profile, and is then hard-anodized black on both sides. The anodized layers take up almost a third of the cone thickness, so the cone should really be regarded as a sandwich of alumina (aluminum oxide) and aluminum, which should be both stiffer and better damped than a pure metal.

(The anodized layer is a very tough substance—when I used to teach science, one of my favorite lessons would be to get the children to anodize pieces of aluminum rod, then hold the rod in a burner flame. The metal would melt, but would be held in a bag formed by the oxidized film.)

A straight-sided, cone-shaped dust-cap is added, the cone then being attached with a thermally conductive glue to a 1½" aluminum voice-coil former. The voice-coil itself is made from edgewound copper ribbon, and the whole is said to be capable of a 20mm peak-peak excursion without the coil moving from the linear region of the magnet gap. And when the voice-coil gets hot from those large excursions, the cone acts as a heatsink with a fairly large time constant, keeping the coil temperature relatively uniform.

It has been leveled at metal-diaphragm drivers that well, yes, they do act like perfect pistons until such frequency as they do go off—and then they go off like a bell. The AE1's cone is said to be well-behaved in its passband, any problems being of high Q, and thus not easily excited, and at a frequency where the drive from the crossover filter is well down in level. In addition, the half-roll surround is said to terminate the cone in such a manner that the resonances will be appropriately damped.

The idea of metal-cone drive-units is not new—GEC's Hugh Brittain designed a well-respected driver over 20 years ago in the UK; again in the UK, Ted Jordan designed a metal-cone full-range unit for Jordan-Watts, then went on to produce first his classic 50mm unit, and recently a woofer used by Elite Townshend; in the US, the metal-cone drivers from Hartke are in much demand by bass guitarists. Acoustic Energy's driver is different enough, however, that they have applied for a US patent (No.133,020).

The bass loading is reflex via two ports on the front panel, each 1¼" in diameter and 5 3/8" deep and profiled to reduce wind noise. The tweeter is a version of the Monitor Audio magnesium-alloy dome unit, which also appears in the Musical Fidelity speakers, and is related to the unit used in the Epos ES14. Manufactured by Elac and using an edgewound aluminum-ribbon voice-coil and ferrofluid damping, this drive-unit and its variants have rapidly gained a name as some of the most neutral HF units around. The crossover, which is mounted on the rear of the recessed terminal panel—good-quality five-way binding posts are fitted—acts at 3kHz and consists of a third-order Butterworth, 18dB/octave high-pass leg and a 24dB/octave low-pass leg with a phase-compensating network. The low-pass section uses iron-dust-cored inductors, the high-pass air-cored coils. Both use low-loss polyester capacitors. (I must admit that I raised an eyebrow at this, polyesters, in my opinion, not being as sonically neutral as, say, polypropylenes.)

Eschewing the stiff-but-light approach pioneered by Celestion, which aims to push the inevitable panel resonances above the critical midband, Phil Jones has gone for more of a brute-force approach. The cabinet is made from 22mm medium-density fiberboard, then a "high-density lining" is poured in and allowed to set asymmetrically so that the internal surfaces are not parallel. This both damps the cabinet walls and spreads out the air-space resonances in frequency. Finally, the cabinets have the corners rounded and are finished in an attractive rough black finish.

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Footnote 1: There are other reasons for compression, the most common being saturation of the ferrite core of a crossover inductor. But as we all use loudspeakers where the designer has not cut corners by using under-specified inductor cores, I won't mention it.—John Atkinson