Joseph Audio RM7si loudspeaker Signature Measurements

Sidebar 2: Signature Measurements

The Joseph's sensitivity was an estimated 84.5dB(B)/2.83V/m, a little lower than specified. However, the impedance (fig.1) was generally high in magnitude, dipping below 8 ohms only in the midrange and reaching a minimum of 6.8 ohms at 184Hz. The phase angle is generally moderate, implying that tube amplifiers rated at 8 ohms would have no difficulty driving this speaker. The saddle at 45Hz in the magnitude trace indicates the port's tuning frequency.

Fig.1 Joseph RM7si Signature, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The wrinkle apparent between 300 and 400Hz in this graph implies the presence of some kind of mechanical resonance in this region. Fig.2 is a waterfall plot calculated from the output of a simple plastic-tape accelerometer fastened to the center of one of the cabinet sidewalls. A very strong resonant mode can be seen at 367Hz; this mode could be detected on all the enclosure surfaces, but was strongest on the sidewalls, and, all things being equal, should be expected to obscure midrange clarity.

Fig.2 Joseph RM7si Signature, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 shows the responses of the two drive-units and the port. The latter covers a neat octave bandpass between 35Hz and 70Hz, with the woofer appearing well-behaved over the entire decade between 100Hz and 1kHz. The crossover appears to be set at 2kHz, and the very-steep-rollout slopes typical of the "Infinite Slope" topology are easily apparent. Some resonant modes in the woofer's metal cone can be seen in the mid-treble, but the proprietary crossover suppresses these by almost 30dB.

Fig.3 Joseph RM7si Signature, acoustic crossover on tweeter-axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz and 1kHz, respectively.

Unlike the tweeter in the regular Rm7si (see Stereophile, February 1996, p.165), the output of the Signature's silk-dome unit is a little suppressed in its lowest octave. As a result, there is a lack of energy immediately above the crossover point in the tweeter-axis response (fig.4), which has been averaged across a 30 degrees horizontal window. Whether or not this has a major or minor effect on the speaker's perceived balance will depend on its dispersion in the same region.

Fig.4 Joseph RM7si Signature, anechoic response on-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.