Bose Acoustimass 5 series II Speaker System Review

The Bose Acoustimass 5, or AM-5, was not the first speaker system to use two small satellites and a separate bass module to provide maximum flexibility in placement with minimum visual impact, but it carried “invisibility” to new extremes while delivering surprisingly good sound quality and selling for a very attractive price.

Bose has now announced an improved version of this system, called the Acoustimass 5 Series II. The satellites have been restyled and actually reduced in size. Each measures only 6-3/8 inches high and consists of two roughly cubical enclosures about 3-1/8 inches square and 4-3/4 inches deep. The cubes can be independently rotated through almost 360 degrees, permitting the spatial properties of the speaker’s sound to be modified by reflecting some of its output off nearby walls. (Controlling the ratio of direct to reflected sound in the listening area has long been a Bose feature.)

Each sealed, molded-plastic cube contains a 2-1/2-inch driver that operates above approximately 200 Hz. The enclosures are filled with sound-absorbing material. Unlike the satellites in the original AM-5, the Series II satellites cannot be taken apart; the two cubes are permanently attached. Several types of brackets and stands are available for mounting the satellites in a variety of positions. The drivers are magnetically shielded so they can be placed close to a TV monitor without distorting the color.

Much of the special quality of the Acoustimass system derives from its unique bass module. In the original system, the module contained two 6-1/2-inch drivers whose front and rear outputs were loaded by separate cavities of different volumes. These cavities, resonant at frequencies about an octave apart, were ported to the room through a pair of 2-1/2-inch holes. The bass enclosure was equivalent to a bandpass filter, about one and a half octaves wide, whose steep high-frequency cutoff slope effectively prevented harmonics of the bass frequencies from being propagated into the room.

Bose engineers had determined that when these harmonics were audible, they enabled listeners to localize the bass source in a three-piece system if it was placed at some distance from the satellites. The Acoustimass bass module's complete freedom from this effect made it possible to place it anywhere in the room without degrading the front stereo image.

In the original AM-5, however, there was a slight gap between the low-frequency limit of the satellites and the high-frequency limit of the bass module, producing a measurable “hole” in the system’s frequency response in the vicinity of 200 Hz. With many types of program material the hole was not noticeable, but at times it could be heard as a somewhat disembodied low-bass presence.

This problem has been eliminated in the Series II. The upper limit of the bass module’s frequency response was extended while retaining (and even increasing) its very steep cutoff slope. The result is said to be a smooth, undetectable transition from the bass module to the satellite drivers together with an actual extension of the low-bass response.

The new bass module contains two 5-1/4-inch drivers (slightly smaller in diameter than the 6-1/2-inch drivers of the original AM-5) in a dual-cavity acoustic structure similar to the original design. But rather than each cavity’s having its own port to the room, both cavities are coupled to a third one, which in turn connects to the listening area through a molded plastic duct whose diameter flares smoothly from about 2-1/2 inches to about 4 inches at its exit from the box. The bass module’s frequency range is roughly 50 to 250 Hz.

The new Acoustimass bass module is approximately the same size as that of the original AM-5: 19-1/4 inches long, 14 inches high, and 7-1/2 inches wide. It can be placed almost anywhere, on any of its surfaces, as long as the vent is unobstructed (the opening should be at least 2 inches from other surfaces).

One end of the bass module contains a group of spring-loaded connectors for the input signals from the amplifier

and the outputs to the satellites (the crossover system is built into the module). The system’s automatic protection against overload damage reduces the signal level until the overload has been removed.

The Acoustimass 5 Series II is rated for operation with amplifiers able to drive loads of 4 to 8 ohms (the system is nominally 6 ohms) and with power ratings between 10 and 200 watts per channel. The Acoustimass bass module is available in a scratch-resistant black or white' satin finish, and the satellites are finished in a black or white polymer. Cables (about 18 feet long) are supplied for connecting the bass module to the satellites. The complete system, in its shipping carton, weighs 30 pounds. Price: $799.

Lab Tests

We placed the Acoustimass 5 Series II satellites on 26-inch stands, about 30 inches from the wall behind them and 4 to 5 feet from the side walls, for both measurements and listening tests. The bass module was placed on the floor about midway between the satellites and also about 30 inches from a wall.

We made room-response measurements first with both parts of each satellite facing directly forward, then repeated them with the drivers angled 45 degrees outward from the forward axis. The bass module’s output was measured with the microphone at its port opening.

With the satellites facing forward, the composite frequency response was very uniform in their range, varying ±2.5 dB from 280 to 20,000 Hz. When the drivers were angled outward, a rather extreme condition of operation, the output level was slightly lower above 1,500 Hz. The response curve ran parallel to the forward-facing response curve but 2 to 5 dB below it all the way to 20,000 Hz. Clearly, one’s choice of angles for adjustment of reflected sound is not going to affect the system’s basic sound qualities.

There were two points of maximum output in the range of the bass module, at 120 and 55 Hz. Between 50 and 250 Hz the output varied only ± 3 dB. It fell off at about 24 dB per octave below 50 Hz and at a remarkable 50 dB per octave from 250 to 330 Hz. There was a narrow band of high output, between 450 and 600 Hz, where the level was comparable to that of the satellites in the same range.

The sensitivity of the Acoustimass 5 Series II was a good 90 dB sound-pressure level (SPL) at 1 meter when it was driven by 2.83 volts of pink noise. Though this figure is slightly higher than the 87-dB SPL measurement obtained by Bose (using pink noise limited to a 200- to 20,000-Hz band), the difference is probably explained by our measurement signal, which extended down to 20 Hz, and our placement of the Acoustimass bass module near the satellite being measured.

The bass module’s distortion at a 2.83-volt drive level was about 0.5 percent over much of its range (130 to 230 Hz), increasing to 2 percent at 95 Hz and 4 percent at 65 Hz. Satellite distortion at the same level was 0.6 percent from 500 Hz to 3,000 Hz and increased to 4 percent at the 230-Hz crossover frequency. The satellite’s low-frequency response dropped at a rate of 15 dB per octave below 300 Hz (excluding the effect of the system's crossover network).

The system’s impedance reached its minimum of 4.7 ohms at 300 Hz and remained under 9 ohms throughout the satellite’s operating range. The maximum impedance of 30 ohms was at 53 Hz, with smaller peaks at 28 and 130 Hz. The impedance phase angle was less than -25 degrees above 130 Hz. At lower frequencies it increased to -180 degrees (lagging) at 20 Hz, with variations corresponding to the response variations in that range.

The Bose Acoustimass 5 Series II, like its predecessor, was able to handle enormous power inputs without damage or serious distortion. At 100 Hz, there was no sign of speaker overload or “bottoming” at 790 watts into its 8-ohm impedance, the point where our amplifier clipped. Similarly, the amplifier clipped at 810 watts into 8.5 ohms at 10,000 Hz and at 1,100 watts into 6 ohms at 1,000 Hz.

Measurements with our IQS FFT analysis system showed that the frequency-response curves of the satellites measured on-axis and at 45 degrees off-axis diverged smoothly above 2,000 Hz, with a difference of 6 dB or less up to 8,000 Hz. The gap widened to about 12 dB above that frequency and still more above 12,000 Hz. This is perfectly natural behavior for a 2-1/2-inch cone driver, and in the Acoustimass 5 Series II system it presents no problem. If desired, the user could aim one driver in each pair at the listening location, to obtain a full-range response, and the other at the wall behind for ambience enhancement by reflection.

Comments

When we tested the original Bose AM-5, we were impressed by its ability to sound like a very large system, much larger than its tiny satellites and inconspicuous Acoustimass bass module would suggest. It proved able to absorb the output of some very large amplifiers without damage or audible distress and was clearly a step forward from any previous three-piece system we had heard.

The tendency to produce a disembodied bass (in the frequency domain, not spatially) was perhaps the AM-5’s greatest weakness. When we heard the Series II version at a press conference, it appeared that the problem had been solved. Now that we have had the opportunity to test a production system, we can verify that the Acoustimass 5 Series II is free of that effect.

The Series II has the essential qualities of a first-rate conventional speaker system. Its frequency response is smoother than many highly regarded systems we have tested, and it spans the 45- to 20,000-Hz range with excellent dispersion. Its stereo imaging is very good, with accurate positioning laterally and vertically.

Listening to a wide variety of compact discs, we were constantly impressed by how much this system sounded like a larger, much more expensive speaker. There is no way that a person hearing (but not seeing) the Acoustimass 5 Series II would suspect that its powerful, room-filling sound emerges from satellite speakers that could easily be held on the palm of one’s hand and a compact bass module that can be hidden under or behind almost any piece of furniture.

Readers of Stereo Review are aware that three-piece speaker systems are now available from almost every speaker manufacturer. Of the many we have tested, all of them very good, the Bose Acoustimass 5 Series II is one of the most expensive—and also one of the best. Hear it and be convinced for yourself.

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