Joseph Audio RM7XL Special Edition loudspeaker Measurements

Sidebar 3: Measurements

My estimate of the Joseph Audio RM7XL Special Edition's voltage sensitivity on its tweeter axis was within experimental error of the specification, at 85.5dB(B)/2.83V/m—slightly below the average of 87dB for the 630 or so speakers I have measured over the past 17 years. Its impedance remains above 8 ohms at almost all frequencies, dropping to a minimum value of 7.15 ohms at 200Hz (fig.1). This, in combination with the generally low electrical phase angle, will make the RM7XL a very easy load for an amplifier to drive.

Fig.1 Joseph Audio RM7XL Special Edition, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

It is difficult to see at the scale this graph is printed in the magazine, but there is a slight discontinuity in the impedance traces between 300Hz and 400Hz that correlates with the fairly strong panel resonance at 367Hz that I found on the cabinet's sidewall (fig.2), and also detected in earlier versions of this speaker. This cumulative spectral-decay plot was calculated from the output of an accelerometer fastened to the center of the enclosure's sidewall; another mode can be seen at 130Hz. However, Bob Reina didn't remark on any lack of clarity in the lower midrange that might result from this cabinet behavior. It's probable that the modes look worse than they sound.

Fig.2 Joseph Audio RM7XL Special Edition, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered on 43Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the 1.75"-diameter port. The corresponding minimum-motion point in the woofer's output (fig.3, bottom trace below 70Hz) lies at 40Hz, while the port's output (top trace below 70Hz) peaks a little higher in frequency. The woofer is basically flat on-axis within its passband, before crossing over to the tweeter at 1.5kHz. This graph reveals the tweeter to have a slight rising trend on-axis, but its response smoothly extends to the 30kHz limit of this graph.

Fig.3 Joseph Audio RM7XL Special Edition, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the port and woofer plotted below 300Hz.

However, while Joseph Audio made its name with designs featuring Richard Modaferri's Infinite Slope crossover topology, the rolloff slopes in fig.3 are gentler than, for example, those of the RM7si Mk.2 Signature—see fig.3 in the measurements sidebar accompanying my August 2000 review of that speaker. It looks as if the new speaker's tweeter rolls in with an 18dB/octave slope, while the woofer rolls out with a 24dB/octave slope. As a result, the inevitable resonant modes in the woofer's metal cone are not as well suppressed as they were in the earlier speaker. Even so, the highest-level mode, at 7kHz, is still 15dB down with respect to the tweeter's level at that frequency, and its very high Q (quality factor) might also work against its audibility, at least with continuous tones. (This mode will be excited by musical transients.)

Fig.4 reveals how these individual responses sum on the RM7XL's tweeter axis in the farfield, averaged across a 30° horizontal window. The speaker's response is remarkably smooth and even, though there is little sign of the usual nearfield response "bump" in the upper bass, suggesting that the woofer's alignment is a little overdamped. The lows are quite well-extended for a bookshelf design. The tweeter's output in its top two octaves is a couple of dB too high in level, which ties in with BJR's finding the tweeter to be very revealing.

Fig.4 Joseph Audio RM7XL Special Edition, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the port and woofer nearfield responses plotted below 300Hz.

The Joseph's lateral radiation pattern (fig.5) shows generally even contour lines, though with a slightly uneven flare in the bottom octave of the tweeter's passband. The tweeter's output falls off to the sides above 10kHz, but not quite as quickly as that of some other 1" domes. In the vertical plane (fig.6), the RM7XL maintains its even balance over quite a wide range of listener heights, a suckout in the crossover region developing only at extreme positions above and below the tweeter axis.

Fig.5 Joseph Audio RM7XL Special Edition, 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 Joseph Audio RM7XL Special Edition, vertical-response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

In the time domain, the RM7XL's step response (fig.7) indicates that the drive-units are both connected with positive acoustic polarity, while the individual steps of the tweeter and woofer (fig.8) reveal that the slight glitch at the 4ms mark in fig.7 is associated with the tweeter's output (red trace). The fact that the woofer's positive-going step coincides with the positive-going return to the time axis of the tweeter's step correlates with the good frequency-domain integration of the two units' outputs seen in fig.4.

Fig.7 Joseph Audio RM7XL Special Edition, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Joseph Audio RM7XL Special Edition, step responses on tweeter axis at 50" of tweeter (red) and woofer (blue) (5ms time window, 30kHz bandwidth).

Finally, the RM7XL's cumulative spectral-decay plot (fig.9) is very clean, though the residual woofer resonance at 7.2kHz just makes its presence known.

Fig.9 Joseph Audio RM7XL Special Edition, cumulative spectral-decay plot at 50" (0.15ms risetime).

Overall, the 'XL continues the tradition of excellent measured performance established by earlier versions of Joseph Audio's RM7 loudspeaker.—John Atkinson

Company Info
Joseph Audio, Inc.
P.O. Box 1529
Melville, NY 11747
(800) 474-4434
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