Mordaunt-Short Performance 6 loudspeaker Measurements

Sidebar 3: Measurements

I estimated the Performance 6's voltage sensitivity at 88dB(B)/2.83V/m, which is slightly but inconsequentially below the specified 89dB. The speaker's impedance (fig.1) mostly ranges between 4 and 8 ohms, with a minimum value of 3.6 ohms at 33.5Hz. Probably the point hardest to drive lies at 27Hz, where a fairly low 5.2 ohms is combined with a –39.4° capacitive phase angle; fortunately, with the exception of classical organ recordings and some techno, music has very little energy in this region.

Fig.1 Mordaunt-Short Performance 6, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance is free from the wrinkles and glitches that would indicate the presence of cabinet resonances. Investigating the cabinet's vibrational behavior with an accelerometer revealed very little in the way of resonances. About all I could find was a mode on the curved side panel at 227Hz (fig.2), though this was at a very low level.

Fig.2 Mordaunt-Short Performance 6, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet side panel level with the lower woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

There is a slight bend in the impedance traces between 100Hz and 200Hz, and I do wonder if this is associated with the unusual dip seen in the outputs of the two woofers in this same region (fig.3, blue and green traces). These traces also show that the two woofers differ slightly in their tuning frequencies, the lower woofer (green) extending very slightly lower than the upper unit (blue). Summing their outputs gives a notch at 34Hz, the frequency of the saddle centered on that frequency in the impedance magnitude trace (fig.1). The port's output peaks between 27Hz and 80Hz, with a slight offset toward the higher frequency, and is refreshingly free from resonant modes in the midrange.

Fig.3 Mordaunt-Short Performance 6, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the midrange unit, summed woofers, and port (black), with the response of the upper woofer (blue) and lower woofer (green).

The twin woofers cross over to the midrange unit just above 300Hz—the individual outputs of the drive-units and port are plotted in the ratios of their radiating diameters in this graph. The midrange unit itself has a couple of cone resonances visible in its output one and two octaves above the 3.2kHz crossover to the tweeter, but these are well suppressed. The tweeter dome resonance can be seen to lie at a high 26.5kHz.

Fig.4 shows the Performance 6's response averaged across a 30° horizontal window on the tweeter axis at 50", spliced to the complex sum of the individual nearfield low-frequency responses. While impressively flat overall, there is a slight shelving down of the mid-treble and a mild shelving up of the upper bass. The latter is almost entirely due to the nearfield measurement technique, which assumes a 2pi acoustic environment, which means that the LF extension doesn't perhaps go quite as deep as implied by the 34.5Hz port tuning frequency—I note that PM found that the speaker's bass, "though agile...lacks a little ultimate weight."

Fig.4 Mordaunt-Short Performance 6, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange, woofer, and port responses, taking into account acoustic phase and distance from the nominal farfield point, plotted below 300Hz.

The Mordaunt-Short's top octave appears to lack energy on axis, but this suckout does fill in a little to the speaker's sides (fig.5). This graph also reveals a beautifully controlled horizontal radiation pattern, with contour lines evenly spaced out to more than 60° off axis and no trace of the usual flare at the bottom of the tweeter's passband. This will contribute both to an uncolored room sound and stable stereo imaging. In the vertical plane (fig.6), a deep suckout in the upper crossover region develops more than 5° below the tweeter axis, which suggests that a high listening chair will be better than a low one.

Fig.5 Mordaunt-Short Performance 6, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.6 Mordaunt-Short Performance 6, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 10–5° above axis, reference response, differences in response 5–20° below axis.

As suggested by the speaker's flat on-axis response, the Performance 6's farfield step response (fig.7) reveals that each drive-unit's step smoothly integrates with that of the next lower in frequency. The individual step responses (fig.8) confirm that the tweeter (red trace) and midrange (blue) are connected in inverted acoustic polarity, the woofers (green) in positive polarity. The farfield cumulative spectral-decay plot (fig.9) is beautifully clean, suggesting a lack of treble grain to the Mordaunt-Short's sound.

Fig.7 Mordaunt-Short Performance 6, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Mordaunt-Short Performance 6, step response on tweeter axis at 50" of the tweeter (red), midrange unit (blue), and woofers (green). (5ms time window, 30kHz bandwidth.)

Fig.9 Mordaunt-Short Performance 6, cumulative spectral-decay plot at 50" (0.15ms risetime).

This is a beautiful-looking speaker that is equally beautifully engineered. Great things are coming from the UK these days.—John Atkinson

COMPANY INFO
Mordaunt-Short
US distributor: Marantz America
440 Medinah Road
Roselle, IL 60172
(630) 741-0300
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