Acoustic Research M.5 Bookshelf speakers

The Acoustic Research Holographic Imaging (H.I.) series of loudspeaker systems was designed, as the name implies, to provide a broad, natural soundstage with accurate imaging. With the addition of the new M.5, the H.I. series now comprises nine models, in a wide range of prices and sizes, that share the same spatial properties in their sound. The M.5, near the low end of the series, is a small two-way speaker whose 5-1/2-inch polypropylene-cone woofer operates in a vented enclosure. At 6,000 Hz, there is a crossover to a liquid-cooled 1/2-inch polypropylene-dome tweeter.

The M.5's enclosure resembles the upper sections of the larger H.I. systems. Both the front and rear panels slope backward at an angle of about 12 degrees. The woofer occupies the upper portion of the front panel, and the tweeter is located near the bottom, offset toward one side. The woofer's port is on the rear panel below the recessed spring-clip input terminals. The M.5 speaker comes in mirror-image pairs, designed to be installed with the tweeters toward the outside.

The M.5's nominal impedance is 6 ohms, and its sensitivity is rated as 88 dB sound-pressure level (SPL) at 1 meter with a 2.83-volt drive level. Response is given as 68 to 23,000 Hz at the - 3-dB points.

We placed the speakers on 39-inch pedestals, which put their tweeters close to ear level for a seated listener. They were about 6 feet apart, 4 feet from the side walls, and 2-1/2 feet from the wall behind them.

The averaged room-response curve from the two speakers was impressively uniform, within ±2.5 dB from 100 to 2,500 Hz and remaining strong down to 60 Hz. The averaged output dipped about 5 dB at higher frequencies but climbed back to its lower-midrange level in the range of 15,000 to 20,000 Hz.

The close-miked woofer response, combining the separately measured cone and port contributions, reached its maximum between 60 and 110 Hz and dropped off smoothly above that, to - 5 dB at 1,500 Hz; the measurement was no longer valid at higher frequencies. This curve spliced smoothly to the room-response curve above 1,000 Hz, resulting in a composite frequency-response variation of ±2.5 dB from 42 to 2,300 Hz, ±2 dB from 2,000 to 12,000 Hz, and ± 2.5 dB from 12,000 to 20,000 Hz.

Nonetheless, the effective lower limit of the speaker's response was actually in the vicinity of 65 to 70 Hz, as rated. Although some audible fundamentals were present at 50 Hz and lower, the distortion and fluttering as the cone approached its excursion limits rendered the system ineffective at those frequencies.

The on-axis frequency response at 1 meter, measured with swept one-third-octave pink noise, was generally similar to the room measurements, without their extended bass response. It varied ±2 dB from about 150 to 3.000 Hz, dipping another 2 dB between 4,000 and 9,000 Hz. The high-frequency dip was strongly affected by the position of our test microphone, apparently being caused by interference between the outputs of the tweeter and woofer.

A similar measurement smoothed and repeated at 45 degrees off-axis, indicated good high-frequency dispersion from the M.5's tweeter. Starting to diverge slightly above 1,000 Hz, the off-axis response (on the side near the tweeter) was down by a fairly constant 3 dB all the way to 10,000 Hz and down only 6 dB at 20,000 Hz. When we measured with the microphone offset in the opposite direction, the curve was identical to the first up to 4,000 Hz, with an off-axis loss of almost 5 dB at 10,000 Hz and almost 10 dB at 20.000 Hz. Either way, the horizontal dispersion of the M.5's tweeter was better than that of most speakers we have measured.

The system's quasi-anechoic MLS frequency response exhibited the usual slight irregularities, on the order of ±2 dB, between 300 and 5,000 Hz. Measurements at different angles and distances confirmed that a response peak or null at about 8,000 Hz was truly an interference effect, changing widely with slight changes in test conditions.

The tuning of the system's enclosure and port was close to ideal, as evidenced by its nearly identical bass impedance peaks of 12 and 13 ohms at 40 and 110 Hz. The impedance reached lows of 4 ohms at 60 to 70 Hz and 3.6 ohms at 300 Hz.

Sensitivity at 1 meter, with a 2.83-volt input of pink noise, was 90 dB on the axis of the woofer and 88 dB on the tweeter's axis. When we drove the speaker with single-cycle tone bursts at 1,000 and 10,000 Hz, our amplifier clipped in the range of 1,200 to 1,500 watts without damage to the speaker or audible distortion. At 100 Hz, the woofer cone reached its physical limits with alarming rasping sounds at an input of 74 watts into its 9.5-ohm impedance, though the distortion had already become unpleasantly audible (and visible on an oscilloscope) with as little as 27 watts input.

With 2.83 volts input, the woofer's distortion was between 0.5 and 2 percent over the range of 42 to 2,000 Hz. The effective crossover to the port output occurred at about 120 Hz, however, and its distortion at that frequency was about 5 percent, rising to about 15 percent at the speaker's rated lower limit of 68 Hz.

Although the shape of the M.5's response curve might suggest that it would sound muted or distant, the actual sound was quite the opposite - smooth, crisp, and well defined. Its bass extension was sufficient to prevent most music from sounding thin, and the absence of artificial emphasis in the upper bass was notable. Best of all, it shared the outstanding imaging qualities of its larger (and costlier) siblings in the H.I. line.

Few speakers of comparable size and weight can deliver as balanced and musical a sound as the M.5. It could benefit from the addition of a subwoofer, but if that level of performance is desired it would seem more sensible to get one of the larger AR H.I. speakers in the first place.

Perhaps the M.5's chief limitation (though for most people it should not pose any problems) is its power-handling ability at frequencies below 100 Hz. This speaker leaves no doubt in the listener's mind when the woofer cone (whose actual effective diameter is only about 4 inches) has reached its limits, and almost any amplifier or receiver has more than enough power to make that happen from time to time. For listening to music below lease-breaking levels, however, the Acoustic Research M.5 is a truly fine-sounding speaker, and a notable value at its price.