Boston Acoustics A40 Speaker System Review

The Boston Acoustics A40 is a small two-way bookshelf speaker (it could reasonably be described as a “minispeaker”) that delivers exceptional performance at a very low price. The crossover between its 6-1/2-inch acoustic-suspension woofer and 3/4-inch dome tweeter is at 3,500 Hz, with 6-dB-per-octave slopes. Rated impedance is 8 ohms, and there are no balance or level adjustments.

The A40 is housed in a walnut-grain, vinyl-veneer cabinet with a black cloth grille on a molded plastic frame that snaps in place. The speaker’s insulated spring-loaded connectors are recessed into the rear of the cabinet.

Laboratory Measurements

For our room-response measurements, the A40s were placed against a wall about 8 feet apart and 5 feet above the floor. The microphone, placed at the rear of the room, was on the axis of the left speaker and about 30 degrees off the axis of the right speaker. Even without further correction or averaging, the swept warble-tone response of the speaker was unusually smooth. The very small difference between the left and right speaker curves above about 8,000 Hz attested to the excellent dispersion of the tweeters. When we had averaged the left and right speaker outputs and corrected the result for the known high-frequency absorption in the room, we obtained a frequency response flat within ±2.5 dB from 200 to 20.000 Hz.

The bass response was measured separately with close microphone spacing. The woofer output at 110 Hz was about 3 dB higher than in a broad plateau of ±1.5 dB from 250 to 2,000 Hz, and it fell off at 12 dB per octave below 100 Hz. (This type of measurement is not fully valid above about 1.000 Hz since the dimensions of the speaker cone become comparable to the wavelength of the sound.)

When the woofer curve and the overall room curve were combined, the composite response curve was flat within ± 3 dB from 70 to 20,000 Hz and was essentially free of major holes or peaks at any frequency.

The quasi-anechoic response of the speaker was measured with our INDAC FFT analysis system. The speaker was on a stand about 21 inches from the floor and at least 5 feet from any room wall. The microphone was located on the speaker axis 1 meter from the grille. The response was ± 5 dB from 180 to 18,000 Hz, both limits being set by the analysis system rather than the speaker. By excluding a slight peak at about 15.000 Hz, the overall variation was reduced to ± 4 dB.

When we rotated the speaker 45 degrees to measure its off-axis response, we were surprised to find only a ± 2-dB variation from a few hundred hertz to 15,000 Hz, which was about the flattest speaker response we have yet measured with the INDAC system. Encouraged by this result, we turned the speaker a full 90 degrees off axis so that the microphone was effectively measuring the output off the side of the speaker. The FFT analysis showed only a ±6-dB variation up to 16,000 Hz—better than many speakers can do along any axis!

The bass distortion was measured with the microphone close to the speaker cone at inputs of 1 watt and 10 watts (based on an 8-ohm impedance). At 1 watt, the distortion was well under 1 per cent down to 60 Hz, rising gently to 4.5 per cent at 30 Hz. The higher power, needless to say, resulted in a much more rapid increase of distortion, although it was still in the 2 to 2.5 per cent range down to 85 Hz and reached 12.5 per cent at 50 Hz.

The A40 impedance curve had broad minima of about 5 ohms in the 20- to 30-Hz range and between 200 and 300 Hz. Its maximum was about 20 ohms at the bass resonance of 90 Hz, and there was a broad rise to about 18 ohms in the 2,500- to 3,500-Hz crossover region. The sensitivity of the A40 was on the high side for a sealed system, with a 2.83-volt (1-watt) input of random noise in an octave centered at 1,000 Hz generating a sound-pressure level of 89 dB measured at a 1 meter distance.

Comment

Boston Acoustics supplied typical performance data for the A40 based on their own tests and production standards. We essentially duplicated the claimed performance despite our completely different test conditions.

In recent years we have auditioned and tested a number of very good speaker systems, most of them so good that we would be hard pressed to rank them in any order of quality. However, most of these speakers carried list prices between $150 and $300. The speakers in the under-$100 category that are most often chosen for budget systems generally fall short of meeting minimum high-fidelity standards. The Boston Acoustics A40 is a welcome exception to that rule. Obviously it cannot develop a room-shaking bass output—the laws of physics and economics make that impossible—but even that lack is unlikely to be noticed unless one is specifically listening for it. By way of compensation, the overall balance and smoothness of the A40’s sound are those of a good speaker by any standard, and it can hold its own in some very exalted circles if one is willing to forgo the deep-bass performance. Very few forward-radiating systems we have seen can match its dispersion. Extended listening to the A40 has shown us that there is nothing about its sound to identify it as coming from a smallish, inexpensive speaker.

With the A40, there is no longer a valid reason to degrade the performance of low-cost components (many of which are surprisingly good) by connecting them to inferior speakers. The A40 comes surprisingly close to matching the essential sound quality and character of the best and most esteemed speakers for a small fraction of their cost, and it is hard to imagine a room unable to accommodate a pair of them.

Boston Acoustics points out that the low price of the A40 is largely due to the use of vinyl-clad wood and the simple crossover network (one capacitor and one inductor) made possible by the choice of driver characteristics. In respect to sound quality per dollar, the A40 is surely one of the most cost-effective speaker designs we have seen in recent times; we are impressed.

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