Boston Acoustics M350 loudspeaker Measurements
Sidebar 3: Measurements
I performed the farfield measurements on the Boston Acoustics M350 using DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. For the nearfield and spatially averaged room responses, I used an Earthworks QTC-40 microphone.
My estimate of the M350's voltage sensitivity on its tweeter axis matched the specified 89dB(B)/2.83V/m, which is a little higher than the norm. Specified as a 4 ohm load, the M350's impedance magnitude (fig.1, solid trace) remained between 4 and 6 ohms for much of the audioband, dropping to a minimum value of 2.85 ohms at 138Hz. While there is a taxing combination of 5.5 ohms magnitude and 47° electrical phase angle at 36Hz, music rarely has significant energy at this frequency; the M350 won't present any drive problems to 4 ohmrated amplifiers and receivers.
The traces in fig.1 are free from the small discontinuities that would indicate the presence of enclosure resonances of some kind. Nevertheless, when I investigated the vibrational behavior of the panels with a piezoelectric plastic-tape accelerometer, I found a very strong resonance on the side panels level with the upper woofer (fig.2). However, while this resonance was detectable elsewhere on the sidewalls and on the speaker's rear panel, it was much lower in level. The affected radiating areas must therefore be small, but this behavior might well be the reason I noted some lower-midrange congestion in my auditioning.
Turning to the frequency domain, the four woofers behaved identically; their combined output, measured in the nearfield, is shown as the blue trace in fig.3. It peaks between 80 and 240Hz, crossing over to the midrange unit (green trace) somewhat lower in frequency than the specified 400Hz. The port is tuned to a fairly high 45Hz, indicated by the fact that that is where the woofers' response has a sharply defined notch, due to the pressure on the backs of the cones from the port resonance holding them stationary. The port's nearfield output (red trace) peaks between 30 and 70Hz and rolls off sharply above that range. While a resonance at 350Hz is visible in the port's midrange output, this is well down in level and should have no audible consequences, particularly as the port faces away from the listener.
Below 300Hz, the black trace in fig.3 shows the sum of the nearfield responses, taking into account acoustic phase and the different distances from a nominal farfield microphone position. The rise in the upper bass is mostly an artifact of the nearfield measurement technique. Above 300Hz in this graph, the black trace shows the M350's farfield response averaged across a 30° horizontal window centered on the tweeter axis. It is impressively smooth and even, though there is a hint of a suckout at the crossover frequency between the midrange driver and tweeter. There is also a slight rising trend visible throughout the treble. Whether this will result in an excess of high-frequency energy in the speaker's balance will depend on its dispersion in the same region.
Fig.4 shows the differences in the M350's response at 5° intervals to the speaker's side, which means that the response on the tweeter axis appears to be a straight line. While the contour lines are smooth and even, there is a slight flare at the base of the tweeter's passband, above which the output drops relatively sharply off axis. This will work against the excess of energy in the same region on the tweeter-axis response. In the vertical plane (fig.5), the M350's response is maintained over quite a wide angle, suggesting that it is immune to reasonable changes in ear height, though this is not what I found in my auditioning. The tweeter is just 34" from the floor, which would mean typical listeners would need to slouch a little to be on this axis.
The red trace in fig.6 shows how this quasi-anechoic behavior summed in my listening room. For comparison, the blue trace is the behavior of the Vandersteen Treo loudspeaker, which I reviewed in March. Both traces were generated by averaging 20 1/6-octavesmoothed spectra, taken for the left and right speakers individually using SMUGSoftware's FuzzMeasure 3.0 program and a 96kHz sample rate, in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. This eliminates the room acoustic's effects, and integrates the direct sound of the speakers with the in-room energy to give a curve that I have found correlates reasonably well with a speaker's perceived tonal balance. The first thing to note is that while I found the M350s to sound rather dark, they actually put out into the room more energy in the top two audio octaves than do the Vandersteens. In fact, the Bostons' in-room response meets very tight, Ò1dB limits above 500Hz, and the match between the two speakers in the upper midrange and treble was superb. I suspect that the suckout visible between 200 and 500Hz is due to interference between the direct sound and its reflections from the sidewalls. This suckout is much less wide than with the Treos, and an excess of energy in the octave below it corresponds to the overwarm balance I noted. (The ports were fully open for this measurement.) The M350s' low-frequency rollout is relieved by the 32Hz diagonal mode in my room, but their output is 10dB down at 20Hz.
Turning to the time domain, the step response on the tweeter axis (fig.7) suggests that the tweeter and midrange are connected in inverted acoustic polarity, the four woofers in positive polarity, this confirmed by examining each drive-unit individually (not shown). Of more significance is that the decay of each unit's step blends smoothly with the start of the unit next lower in frequency. This correlates with the excellent frequency-domain integration of their outputs seen in fig.3. The farfield, cumulative spectral-decay plot on the tweeter axis (fig.8) is superbly clean overall, though with a very slight amount of delayed energy visible at the cursor position (3240Hz). (As always, ignore the black ridge just below 16kHz in this graph, which is due to interference from the computer's video circuitry.)
Other than that strong cabinet-wall resonance in the lower midrange, the Boston Acoustics M350 measures superbly well, even without taking into account its relatively affordable price.John Atkinson