Acoustic Research M4 Speaker System Review

The new Acoustic Research Holographic Imaging Series of loudspeakers was designed to create a more natural and believable sound stage, or stereo image, than conventional speakers do. To achieve this effect, all six models in the series have the driver that handles the musical midrange at the top of the cabinet, angled upward at about 30 degrees. The tweeter is just below that driver, offset a few inches to the outside, so that the tweeters are spaced slightly farther apart. According to AR, the upward-firing drivers provide improved hall-simulating midrange reverberation through ceiling reflections. The output from the tweeters slightly favors the inward direction, toward the listening area, creating a clear center stage in the sound field.

Three speakers in the series are designed for installation on shelves or stands, and three are floor-standing models. The M4, the smallest of the floor-standing speakers, is a three-way system in a slim, compact cabinet that measures 7-3/4 inches wide, 10-1/8 inches deep, and 28-1/8 inches high. The cabinet leans slightly backward to tilt the tweeter radiation a few degrees, making the overall depth from top rear to bottom forward edge about 15-1/4 inches. Each speaker weighs about 23 pounds.

The upper half of the cabinet is a sealed acoustic-suspension enclosure for the 6-inch bass/midrange driver, whose carbon and mica-filled polypropylene cone has a 32-Hz free-air resonance. The driver’s response is allowed to roll off naturally at low frequencies. At 3,600 Hz there is a crossover, with 12-dB-per-octave slopes, to the 3/4-inch soft-dome tweeter, whose voice coil is cooled by ferro-fluid.

The lower half of the cabinet contains a separate sealed enclosure for the 6-inch low-bass woofer, which is mounted upside-down at the cabinet’s bottom, with its basket and magnet external to the sealed cavity. This arrangement is claimed to provide more effective cooling of the voice coil when it’s operating at high levels, since air can flow over the metallic parts of the driver. The sealed cavity, like the tweeter’s enclosure, is filled with sound-absorbing material.

Although the woofer is the same size as the midrange driver, it has a relatively massive paper cone with a free-air resonance of 28 Hz, and a series inductor rolls off its output above 100 Hz. The output below 100 Hz emerges from two rectangular ports on the sides of the enclosure. The two cone drivers have different low-frequency resonance, or “Q,” characteristics, as well as different resonant frequencies, so that their outputs combine to produce a strong, uniform response down to 50 Hz.

The manufacturer’s specifications for the AR M4 include an anechoic frequency response on-axis of 50 to 20,000 Hz ±3 dB and a sensitivity of 88 dB sound-pressure level (SPL) at 1 meter with an input of 2.83 volts. The speaker is recommended for use with amplifiers designed to drive loads of 4 to 8 ohms and with power ratings of 20 to 150 watts.

The AR M4 is finished in charcoal-gray vinyl. It is sold only in pairs, shipped in a single box weighing 53 pounds. Price: $600 a pair.

Lab Tests

The averaged room response of the AR M4 speakers was among the flattest we have ever measured. ±1.5 dB from 200 to 20,000 Hz. We measured the responses of the two 6-inch drivers with close microphone spacing. The upper one (bass/midrange) had a well-damped resonance and a response variation of ± 1 dB from 65 to 400 Hz. Its output dropped at 12 dB per octave below 65 Hz. The lower driver (bass) had a higher-Q resonance, peaking at 70 Hz and falling off at 6 dB per octave at higher frequencies and 12 dB per octave below 70 Hz.

Combining the two responses modified the bass performance below 200 Hz, yielding an output variation of ± 2 dB from 200 to 58 Hz. Response was down 4.5 dB at 50 Hz. This bass-response curve spliced easily to the room response, creating a composite response of ±3.5 dB from 50 to 20,000 Hz, essentially as rated.

Quasi-anechoic FFT frequency-response measurements yielded equally good figures, approximately ±2.5 dB from about 500 to 24,000 Hz. The speaker’s horizontal dispersion was excellent, although—as we expected from its asymmetrical tweeter placement—there were slight differences between the response curves at right and left 45-degree angles from the forward axis.

The group-delay variation was extremely low, less than 0.1 millisecond overall in the tweeter range, from 21,000 Hz to 5,000 Hz, and reached a maximum of only ±0.2 millisecond from 5,000 to 180 Hz. The narrow cabinet apparently helped to keep diffraction effects to a minimum.

System impedance was considerably lower than suggested by a brochure describing the speaker series. It reached a minimum of 3.5 ohms at 20 and 200 Hz, with maxima of 8 ohms at 70 Hz and 8.5 ohms at 2,000 Hz. The phase angle of the impedance function did not exceed ± 30 degrees over the rated frequency range of the speaker, however, and it should not present a difficult load to any amplifier designed to operate into 4 ohms.

The sensitivity of the M4 was 89 dB SPL, slightly better than rated. Its distortion with a 3.2-volt drive level (corresponding to a 90-dB SPL in our sensitivity measurement) was less than 1 percent from 75 to 700 Hz, rising to 3.8 percent at 50 Hz and a maximum of 7 percent at 30 Hz.

The M4’s woofers were able to handle 100-Hz pulse power inputs of 105 watts (into a 4.1-ohm impedance) before they emitted audible sounds of distress. At 1,000 and 10,000 Hz, the amplifier clipped at about 1,200 watts before the speaker reached its limits.

Comments

The measured performance of the AR M4 met its ratings easily (within normal measurement tolerances). It is highly unlikely that a speaker with the characteristics of the M4 would sound anything less than good. In this case, the sound was sufficiently better than merely “good” to make us sit up and take notice.

The M4 produced perhaps the most natural music stage in our room that we have heard from any conventional or unaided speaker. We are not comparing it with some larger, and much more expensive, speakers that employ interaural crosstalk cancellation to enhance their spatial characteristics, or with systems using electronic means of achieving “holographic” effects, which can be pleasing but are very different from the sound of the M4’s.

Putting it as simply as possible, the M4 speakers rarely appeared to be the source of the sound we heard. The music came from that end of the room, with much of the depth, width, and height that we associate with a live performance, but never seemed to emerge from those small black boxes.

Although on rare occasions we have heard similar effects from other speakers, they were considerably more expensive than the M4 and required careful placement for best results. The AR M4 is surprisingly noncritical in respect to placement. The speakers’ overall frequency balance was excellent, sounding consistent with their measured performance. The highs were crisp and the middle frequencies fully detailed, without any stridency or harshness. The bass was surprising in its solidity and ease, qualities that were unexpected from a speaker whose response drops rapidly below 50 Hz. It gave the impression of being much deeper than it actually was, yet with no artificial boom.

The AR M4 is one of the best values in today’s speaker market. These days so many products seem to be equal to each other that it is refreshing to find one that is strikingly “more equal” than most of its competition.

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