Joseph Audio RM33si Signature loudspeaker Measurements

Sidebar 3: Measurements

The smartly finished Joseph Audio RM33si Signature appears to need a surprisingly large amount of power to play loud. Using DRA Labs' MLSSA system and a calibrated B&K 4006 microphone, I estimated the RM33si's B-weighted sensitivity at just 80dB(B)/2.83V/m. This is both significantly below average, and 8dB below the specified figure. Unless Chip clipping his Musical Fidelity amplifier on the Telarc cannons had damaged something, I have no explanation for the discrepancy, other than to note that the thermometer topped 100 degrees F the day I measured the speaker. However, the heat certainly didn't appear to affect any of the other speakers I measured that day, which both measured and sounded much louder with the same drive signal.

However, the RM33si's impedance curves (fig.1) indicate that the speaker is fairly easy to drive, with an impedance magnitude that drops below 8 ohms for only a brief period in the middle of the midrange. The complicated nature of the curves indicates the tuning of the base-mounted reflex port to lie at a very low frequency, approaching 20Hz. The output of the port could well be affected by the proximity of the floor; for the measurements, I placed the speaker on 1" ceramic cones. However, in my own auditioning, the port output did seem a little shelved-down in absolute terms.

Fig.1 Joseph RM33si, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Note the absence of any wrinkles in the fig.1 traces, implying freedom from any mechanical cabinet resonances. This was confirmed when I examined the Joseph's vibrational behavior with a simple plastic-tape accelerometer. The only resonance I could find on the baffle and sidewalls was a very-low-level mode at 50Hz (fig.2). This is obviously an optimally braced enclosure.

Fig.2 Joseph RM33si, cumulative spectral-decay plot of accelerometer output fastened to center of front baffle below midrange unit. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

In the right-hand portion of fig.3 is shown the RM33si's quasi-anechoic response, averaged across a 30 degrees horizontal window on the tweeter axis, spliced to the nearfield output of the midrange unit. Other than a slight rising trend, the speaker's midrange and treble output is superbly even. The midrange unit rolls off with a steep slope below a small peak at 125Hz but with a slight shelf centered on 55Hz, the frequency of the low-level cabinet mode seen in fig.2. The side-mounted woofer peaks a little between 40Hz and 50Hz, before starting a very steep rolloff above the lower crossover frequency of 125Hz.

Fig.3 Joseph RM33si, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield midrange, woofer, and port responses and their complex sum plotted below 500Hz, 250Hz, 250Hz, and 300Hz, respectively.

The port does peak a little at the woofer's minimum-motion frequency of 23Hz, but then goes a bit higher in level in the same frequency region where the woofer output peaks. The complex sum of the midrange, woofer, and port outputs appeared to have a slight depression around the crossover frequency, but this is a region in which it is notoriously difficult to get an accurate idea of what is going on without access to a full-size anechoic chamber. However, the Joseph's farfield response on pink noise (not shown) did have a slight lack of energy in this region.

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