Revel Performa M20 loudspeaker Measurements

Sidebar 3: Measurements

All the acoustic measurements of the Performa M20 were made using DRA Labs' MLSSA system and calibrated B&K and Mitey Mike II microphones. My estimate of the Revels' voltage sensitivity was a little lower than specified, at 84.5dB(B)/2.83V/m. Given the sophistication of Revel's measurement facilities and the agreement I had obtained between my figure and their figure for the Performa F30, I was surprised by this disparity. I rechecked my figures; no change.

The speaker's impedance was assessed using an Audio Precision System One; the results are shown in fig.1, with the tweeter-level control set to its maximum and minimum positions. The lowest impedance is with the control set to "+1," in which case the minimum magnitude is 4 ohms between 2 and 3kHz. The electrical phase angle varies quite considerably, with a worst-case combination of 5.9 ohms and 42 degrees capacitive angle occurring at 100Hz. Good 4-ohm-rated amplifiers will work best with this speaker.

Fig.1 Revel Performa M20, electrical impedance (solid) and phase (dashed) with tweeter control set to ±1dB (2 ohms/vertical div.).

The impedance traces are free from the small glitches that would indicate cabinet-wall resonances. A cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the center of the enclosure sidewall (fig.2) did indicate a mode just above 380Hz, with another at 200Hz. Both were low in level, however, particularly the latter, and I heard no problem with lower-midrange congestion that might correlate with this behavior.

Fig.2 Revel Performa M20, cumulative spectral-decay plot of accelerometer output fastened to side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The saddle at 37Hz between the twin bass peaks in the fig.1 impedance-magnitude trace indicates the tuning of the rear-facing reflex port. This is confirmed by the sharp notch at the same frequency in the woofer's nearfield response, shown to the left of fig.3. The port's output is the bandpass response broadly peaking between 25Hz and 75Hz—a peak at 700Hz can be seen in the midrange, but the subjective effect of this will be suppressed by the fact that the port faces away from the listener. The topmost trace, to the left of fig.3, is the complex sum of the woofer and port responses. It peaks slightly, due to the 2pi nature of the nearfield measurement, which results in a -10dB point of 28Hz compared with Revel's specified 33Hz.

To the right of fig.3 is shown the M20's quasi-anechoic response on the tweeter axis at 50", averaged across a 30 degrees horizontal window. It is astonishingly flat overall, with only a slight downward shelf above 10kHz (the opposite of the floorstanding F30). This measurement was taken with the tweeter control at its central, "0" position. Fig.4 shows the effect of this control set to its maximum and minimum positions, with the fig.3 on-axis response subtracted. It basically shelves the entire region covered by the tweeter up or down by 1dB.

Fig.3 Revel Performa M20, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with nearfield woofer and port responses and their complex sum plotted below 300Hz, 1kHz, and 300Hz, respectively.

Fig.4 Revel Performa M20, effect on response of HF control set to "±1dB," normalized to response on tweeter axis at 50" (2dB/vertical div.).

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