Floor Loudspeaker Reviews

Sidebar 2: Measurements

The MB Quart 280 is definitely a 4 ohm design, to judge from the graph of modulus of impedance vs frequency (fig.1), measured with spot frequency tones but plotted with the Audio Precision System One software. Apart from the sealed-box resonant peak around 75Hz (of moderate height), the impedance doesn't rise above 5 ohms until 450Hz. Above the electrical crossover to the tweeter, which appears to be set to 1450Hz, the 280 presents an easy load to the driving amplifier, averaging 8 ohms between 3kHz and 20kHz. Coupled with its low sensitivity (my measurement indicated around 86.5dB/W/m), however, the 280 will need a good, gutsy power amplifier to be driven to best advantage. This is not a speaker that will work best with high–output-impedance tube amps or Japanese receivers with limited current delivery.

The frequency response of the speakers was measured in the listening area using pink noise and an Audio Control SA-3050A 1/3-octave spectrum analyzer. Nine sets of six averaged measurements were taken independently for left and right loudspeakers in a window 72" wide and varying from 27" to 45" high. The response shown is the average of these measurements, weighted slightly toward the sound heard at the listening position (footnote 1). This spatial averaging is intended to minimize the effect of room standing-wave problems on the measurement and gives a response curve that has proved to correlate reasonably well with what is perceived; it also gives an idea of the off-axis behavior of the speaker under test.

The nearfield bass extension, assessed with a sinewave sweep to get an idea of the true bass extension relative to the level at 100Hz, measured –6dB at 63Hz, about that to be expected from the small cabinet, although as can be seen from the spatially averaged in-room response (fig.2, taken with the speakers on the 17" stands), the rate of roll-off below the cabinet resonance is gentle. A very slight rising trend through the midrange can be seen, though there is no strong evidence for any problems around the crossover frequency, apart from the barest hint of boost in the 1.6kHz third-octave band. The broad peak in the treble is more due to the increased dispersion of the tweeter in the two octaves above crossover, leading it to contribute more to the room reverberant field than the directional woofer does below crossover.

In fact, compared with the similar responses taken for the Acoustic Energy AE1, and the Celestion SL600 and Celestion SL700 in the September '88 issue, the tweeter is probably slightly too low in level compared with the woofer, the HF balance being more like the SL600's than the SL700's. The tweeter response is smooth, however, with a well-controlled roll-off. While sweeping the sinewave tone, I noticed that the cabinet of one speaker very slightly buzzed at 225Hz and that both speakers were quite live in the 200–230Hz region. Otherwise, the cabinet walls were relatively dead.—John Atkinson

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Footnote 1: The in-room measurements are the first that I have performed in my new listening room. The sounds in the two rooms, new and old, are astonishingly similar after acoustic treatment, but there is a narrow peak at 63Hz, 4.5dB high, in the new room compared with the old, which I have not yet been able to remove. In addition, all the averaged responses taken in the new room have a slight loss of energy in the 250Hz bands compared with the old, which I assume is due to destructive interference in this region between the direct sound and the primary floor reflection.