Meadowlark HotRod Shearwater loudspeaker Measurements

Sidebar 3: Measurements

The Meadowlark Shearwater is of below-average voltage sensitivity, at an estimated 84.5dB(B)/2.83V/m. However, its impedance plot (fig.1) suggests that it's an easy load for the partnering amplifier to drive, the magnitude dropping below 8 ohms only in the midbass and lower midrange. A spike in the magnitude trace around 150Hz, coupled with a glitch in the electrical phase trace at the same frequency, suggests the presence of a major cabinet resonance. There is also a much smaller glitch just below 300Hz, and a cumulative spectral-decay plot derived from the output of a plastic-tape accelerometer fastened to the center one of the speaker's side panels (not shown) does reveal the presence of a panel resonance at 280Hz. This is probably too low in level to have any subjective impact. But a similar plot taken when the accelerometer was fastened to the center of the rear panel (fig.2) does reveal some motion at the frequency of the spike in the impedance plot.

Fig.1 Meadowlark Shearwater, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 Meadowlark Shearwater, cumulative spectral-decay plot of accelerometer output fastened to center of back panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

That this is due to an acoustic resonance, probably associated with the line that loads the Meadowlark's woofer, is confirmed by the nearfield responses shown to the left of fig.3. A suckout in the woofer's output at 150Hz coincides with a peak in the port output at the same frequency. The port also has peaks apparent at 300Hz, 450Hz, and 600Hz, the lowest harmonics of a 150Hz fundamental mode. Perhaps these contribute to the sense of lower-midrange "bloom" that Chip Stern noted in his auditioning.

Fig.3 Meadowlark Shearwater, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer and port plotted below 350Hz and 650Hz, respectively.

The impedance plot (fig.1) suggests that the Shearwater's port is tuned to 31Hz, as is confirmed by the notch apparent in the woofer's output at the same frequency, which implies reasonable bass extension. Higher in frequency, the use of first-order filters leads to significant overlap between the tweeter and the woofer, with the result that the specified crossover frequency of 2.7kHz is only approximate.

Fig.4 shows the complex sum of the woofer and port responses spliced to the Shearwater's response on the tweeter axis, averaged across a 30 degrees lateral window. The overall low-frequency response is well-extended, with a -6dB point around 30Hz, but a severe notch is apparent at 150Hz. This suggests that the peak in the port output at this frequency is in anti-phase to the woofer's output, reinforcing the suckout in the latter's response. However, it is fair to note that Chip noted nothing untoward in the upper bass, which suggests that this acoustic problem looks worse than it sounds.

Fig.4 Meadowlark Shearwater, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.

Higher in frequency, the Shearwater's balance is broken by small peaks and dips, though as the latter are around the same width and depth as the former, the subjective effect will be neutral overall. The tweeter peaks a little before rolling off above 11kHz. Though this rolloff will be primarily due to the HF unit's restricted dispersion in its top octave, the presence of the slight peak might have contributed to Chip's not wanting to sit too close to the loudspeaker.

This limited top-octave dispersion can be seen in fig.5, which shows the Meadowlark's radiation pattern normalized to the speaker's response on the tweeter axis. More important, the woofer grows increasingly directional at the top of its passband, which leads to a discontinuity in the off-axis dispersion behavior as the woofer crosses over to the tweeter, which will have very wide dispersion at the bottom of its passband. In all but large rooms, this excess of off-axis presence-region energy should make the speaker tend to sound a bit aggressive.

Fig.5 Meadowlark Shearwater, lateral response family at 50", from back to front: differences in response 90 degrees-5 degrees off-axis, reference response on tweeter axis, differences in response 5 degrees-90 degrees off-axis.

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