Mission m71 loudspeaker Measurements part 2
The lower trace in the bass in fig.4 is the m71's woofer response with the port sealed, which Mission recommends when the speaker is used close to a room boundary. Again, remember that the upper-bass output will be magnified by up to 3dB because this is a nearfield measurement, but the drive-unit still starts to roll off below 110Hz or so. Because of the slower rollout characteristic of a sealed-box alignment, the -6dB point is not as high as you might have thought at 59Hz, and the reinforcement of the bass due to the close proximity of the room boundaries might well give better bass overall in this condition. I do note, however, that Bob Reina preferred the sound without the ports plugged. I recommend trying the speaker plugged and unplugged, out in the room and close to the wall, to see which combination gives the sound you prefer.
Fig.5 shows how the m71's output changes as the measuring microphone is moved off-axis. (The traces in this graph have the on-axis response subtracted from them, to emphasize the changes.) As expected, the tweeter's off-axis output falls quite rapidly to the speaker's sides, which will compensate for the driver's on-axis rise in the same region. Apart from that, the contour lines in the treble are both smooth and evenly spaced, which tends to correlate with stable, well-defined stereo imaging.
Fig.5 Mission m71, lateral response family at 50", from back to front: differences in response 90 degrees-5 degrees off-axis, reference response on woofer axis, differences in responses 5 degrees-90 degrees off-axis.
However, a serious notch at 1.1kHz develops off-axis. From the impedance and nearfield measurements mentioned earlier, I suspect that what we are seeing here is increasing interference between the woofer output and the strong port resonance as the microphone moves to the speaker's sides. The subjective effect of this resonance is difficult to predict, but I note that Bob Reina was bothered by the speaker's forward presentation of the sound of the marimba, an instrument very sensitive to resonant problems in this region.
Fig.6 shows a similar plot for the Mission m71's vertical dispersion, again normalized to the response on the woofer axis. (I chose this axis because the woofer is above the tweeter.) This graph shows that the speaker maintains its good treble integration between the drive-units over quite a narrow vertical window of +5 degrees and -10dB. This means that the speaker is best auditioned with the listener's ears between the top of the cabinet and the tweeter, indicating the use of higher rather than lower stands.
Fig.6 Mission m71, vertical response family at 50", from back to front: differences in response 45 degrees-5 degrees above woofer axis, reference response, differences in response 5 degrees-45 degrees below woofer axis.
In the time domain, the Mission's step response on the woofer axis (fig.7) reveals that the two drive-units are connected with the same positive-going acoustic polarity. With second-order crossover filters, it is usual to invert the polarity of one of the drive-units, to compensate for the phase shift associated with the filters. However, Mission has placed the woofer above the tweeter, thereby moving the tweeter farther away from the listener, which has the same effect. Note, however, that fig.7 shows that the tweeter's output on this axis has not been moved enough to make the m71 time-coherent. In fact, this doesn't happen until the microphone is 20 degrees above the woofer axis, by which time a large crossover notch has developed in the overall output.
Fig.7 Mission m71, step response on woofer axis at 50" (5ms time window, 30kHz bandwidth).
Although there were no boundaries close enough to the speaker to produce reflections of its sound in the step response, some undulations can be seen with a period close to 1ms. This is more indication of a resonance in this neighborhood. The waterfall plot (fig.8) does confirm that something is going on here, but the resolution in this graph is relatively poor in this frequency region. Other than a very slight mode at 5kHz, the treble region in fig.8 is quite clean. (Ignore the black ridge at 16kHz, which is due to interference from my computer monitor.)
Fig.8 Mission m71, cumulative spectral-decay plot at 50" (0.15ms risetime).
Considering its low price, the Mission m71 performs respectably overall. However, I was bothered by that problem in the 1kHz region.—John Atkinson