MartinLogan Aerius Floor standing speakers

Martin-Logan is well known to serious audiophiles as a manufacturer of high-quality electrostatic loudspeakers. Wide-range electrostatic speakers I are typically large and expensive-until the recent introduction of the Aerius, Martin-Logan speakers ranged in price from $3,000 to $60,000 and in weight from 85 pounds to almost a ton. The Aerius makes the unique qualities of an electrostatic speaker practical and affordable for a larger number of audio enthusiasts.

Unlike conventional dynamic loudspeakers, electrostatic speakers do not use a cone or similar element, driven by a voice coil, to create a pressure wave in the air. Instead, a thin plastic sheet, treated during manufacture to give it an electrically conductive coating, is mounted between two perforated metal surfaces supported by a rigid frame. A high-level audio voltage is connected between the perforated plates, creating a strong, variable electrostatic field that acts uniformly on the entire surface of the plastic diaphragm, which carries a fixed electrostatic charge. Changes in the audio signal cause the diaphragm to move accordingly, creating a sound wave in the air.

It is possible for an electrostatic speaker to generate all audio frequencies from the low bass to the highest treble. Among other problems, however, such full-range speakers tend to be very large and costly. A more practical solution, found in most of the Martin-Logan systems, is to use a conventional dynamic woofer for the bass and cross over to the electrostatic diaphragm at the lowest frequency consistent with its dimensions.

The Aerius's 9-inch-wide electrostatic element is curved to form a 30-degree section of the surface of an imaginary cylinder about a yard in diameter, giving the speaker a 30-degree horizontal dispersion. The transparent diaphragm, sandwiched between two perforated plates, is a dipole radiator, generating equal (and opposite-phase) outputs to the front and rear. It also provides a fair view of the area behind the speaker.

The electrostatic element, operating from 500 to 20,000 Hz, is about 3 feet high and occupies the upper two-thirds of the Aerius. Below it is a forward-facing 8-inch cone woofer, in a sealed enclosure, that handles the frequencies below 500 Hz. The crossover network has 12-dB-per-octave slopes. The woofer's response is rated as ± 3 dB down to 40 Hz.

The woofer's enclosure serves as a base for the electrostatic radiator. It also contains the crossover network and the power supply for the electrostatic diaphragm, which uses only a few watts and is normally left energized at all times. A perforated metal grille covers the front of the speaker.

Specifications for the Aerius include a system frequency response of 40 to 20,000 Hz ±3 dB, nominal impedance of 4 ohms (with a minimum greater than 2 ohms at 20,000 Hz), and sensitivity of 89 dB sound-pressure level (SPL) at 1 meter with a 2.83-volt driving signal. The speaker is recommended for use with amplifiers delivering from 60 to 200 watts per channel.

Our test samples were biwirable, with separate binding posts for the bass and electrostatic portions, normally connected in parallel by jumpers. The speaker is also available with a single pair of binding posts for a conventional single-cable connection to the amplifier. The insulated multiway posts accept bare wires, lugs, or banana plugs.

The averaged room response of the Martin-Logan Aerius was very flat, within 3.5 dB overall from 70 to 20,000 Hz except for a dip of about 4 dB at about 10,000 Hz. The close-miked woofer response was flat within ±1 dB from 50 to 150 Hz, falling at 12 dB per octave below that range and with a gentle drop-off from 150 to 500 Hz, where the crossover began cutting off its response.

Splicing the woofer response to the room measurement produced a composite frequency response that correlated very well with what we heard from the Aerius. Except for the high-frequency dip (which appeared to some degree in virtually every response measurement we made), the system response was flat within ±2.5 dB from 45 to 20,000 Hz and fell off at 12 dB per octave below 45 Hz.

The directivity of the electrostatic element was rather unusual. Although the response curves measured on-axis and 45 degrees off-axis began to diverge above 1,000 Hz, the difference between them varied only slightly from 1,500 to 10,000 Hz, remaining between 4 and 6 dB over that range. At higher frequencies the gap between the two response curves opened to a maximum of only 7 dB at 20,000 Hz. In other words, although the level of the midrange and treble was reduced somewhat at moderately off-axis listening angles, their relative balance was affected only minimally.

Quasi-anechoic MLS frequency-response measurements showed typical variations of several decibels between 300 and 10,000 Hz, as well as a slightly reduced output between 9,000 and 12,000 Hz. These effects were generally similar to the variations observed in room response.

Impedance was an almost constant 6 ohms from 80 to 500 Hz. The woofer resonated at 43 Hz, where the impedance rose to 20 ohms, and there was another 20-ohm maximum at about 1,200 Hz, above which the impedance fell off smoothly to a minimum of 2.4 ohms at 20,000 Hz.

The Aerius's measured sensitivity, 85.5 dB SPL at 1 meter with a 2.83-volt input, was slightly lower than rated (the manufacturer makes this measurement at a 3-meter distance, which was impractical in our situation), but it was within the normal range for most home speakers. The woofer distortion at a 4.73-volt level (equivalent to a 90-dB SPL) was surprisingly low, remaining between 0.6 and 1.2 percent from 500 Hz down to below 50 Hz and rising to 3 percent at 40 Hz, 6.5 percent at 30 Hz, and only 10 percent at 20 Hz.

The Aerius also showed that its small woofer can play in the big leagues when it comes to power-handling ability. Unlike most cone speakers, whose voice-coil movement often limits their low-bass performance, this one was undisturbed by an 1,100-watt single-cycle burst at 100 Hz, which was the clipping point of our amplifier into the speaker's 5.7-ohm impedance at that frequency. The electrostatic radiator also took everything the amplifier could deliver, including a 2,100-watt burst of 10,000 Hz into 2.7 ohms, without damage or undue distress.

As always, the proof of a speaker's performance is in the listening, and here the Martin-Logan Aerius acquitted itself handsomely. We placed the speakers as recommended, about 4 feet from the side walls, 4 feet from the wall behind them, and 7 feet apart, angled slightly inward. As our measurements would suggest, the frequency response was audibly smooth and extended. Like any large-area radiator, the electrostatic element generated sound that could be enjoyed at almost any distance.

The stereo image was accurate and stable, and the bidirectional pattern of the output produced a startling effect as we walked toward the plane of the speakers and on behind them. From the front, the soundstage extended across the room somewhere behind the speakers. When we reached the plane of the speakers, the apparent sound source suddenly shifted to a greater distance behind them, typically well beyond the wall only a few feet away. The back radiation of a dipole can easily be reflected from the wall behind it, normally reaching listeners in front of the speakers after a delay of 8 to 10 milliseconds. This delay can add an element of spaciousness to the overall sound that I find one of the most appealing qualities of a dipole radiator. A sound-absorbent wall surface can reduce or eliminate this effect if desired, however.

To me, the Aerius's most surprising quality was its bass performance. The bass enclosure is all but invisible behind the slim front panel, but the 8-inch woofer delivered a clean, deep bass output with very solid fundamental content at 40 Hz and even a perceptible fundamental at 30 Hz. Although the Aerius, like almost any full-range speaker, can benefit at times from a true subwoofer, you don't really need one with this system unless you are a bass freak.

Summing up, the Aerius is a very successful combination of an 8-inch acoustic-suspension woofer and a moderate-size electrostatic element, with the best qualities of each type of driver. It won't visually dominate even a small room, yet it should easily fill any home listening room with first-class sound. And it won't overtax the budget of almost anyone who is serious about good sound.

Finally, although this has nothing to do with the speaker's sound, the user's manual is unique in my experience. Not only does it tell you all you have to know about installing and using the Aerius, but it explains in reasonable (and not-too-technical) detail how and why the speaker works. I also found its two-page "History of Loudspeaker Development" to be both objective and good reading. Consider it a bonus with this fine speaker.

MartinLogan Aerius Floor standing speakers photo