Joseph Audio RM7si loudspeaker Big Science

Sidebar 2: Big Science

Other than impedance, all acoustic measurements were made with the DRA Labs MLSSA system and a calibrated B&K 4006 microphone. To minimize reflections from the test setup, the measuring microphone is flush-mounted inside the end of a long tube. Reflections of the speaker's sound from the mike stand and its hardware will be sufficiently delayed not to affect the measurement.

Because the altitude at Santa Fe reduces the sensitivity of all loudspeakers, I calculate sensitivity by comparing the measured, B-weighted level at 50" for a given voltage input, using a noise signal, with that obtained for a sample BBC LS3/5A that I've measured both in Santa Fe and at sea level. The RM7si is not very sensitive, 2.83V raising just 83.5dB (B-weighted) at 1m. While this is below the specified figure, it appears that the designer has traded-off sensitivity to obtain good bass extension. (Cabinet size, sensitivity, and bass extension are all related variables.) In addition, the Joseph's impedance magnitude (fig.1) is relatively high, with minimum values around 7 ohms in the midrange and 7.5 ohms in the high treble. Other than in the upper bass, the Joseph's electrical phase angle is mild, which, together with the highish magnitude, makes the speaker an easy load for an amplifier to drive.

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

The port tuning is indicated by the "saddle" in the impedance trace at 48Hz. A slight wrinkle in the fig.1 traces between 300Hz and 400Hz is probably due to a cabinet resonance of some kind.

For reference, fig.2 shows the individual drive-unit responses of the first RM7 sample. Note the very steep crossover slopes, with the tweeter's rolloff overlaid with a notch, and the raggedness at the top of the woofer's passband. This peakiness between 700Hz and 1800Hz could also be seen on the first sample's overall response (not shown) and, I am sure, correlates with the slightly bright balance I observed. (As J. Gordon Holt pointed out two decades ago, brightness per se is not associated with a tilted-up top-octave response but with problems lower down in frequency.)

Fig.2 Joseph RM7si, original sample, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz.

Fig.3 shows the acoustic crossover for the second RM7si sample. While the crossover point and steep filter slopes are identical, you can see that the new woofer is better-behaved just below crossover. The tweeter is flat overall within its passband, with the small peaks offset by small dips, but set a little lower in absolute level than in fig.2. The port output is the bandpass centered on 48Hz, though the woofer's minimum-motion point is lower in frequency.

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

The RM7si's overall response, averaged across a 30 degrees horizontal window, is shown in fig.4. Impressively smooth and even, it is broken only by a small excess of energy in the midrange and a slightly shelved-down treble. The former occasionally made its presence known as a very slightly nasal character, while the latter contributed to the speaker's mellow tonal balance. The overall bass response is well-tuned, and extends down to a quite low 39Hz (-6dB), this the frequency of the note E-flat just over an octave below the bass staff.

Fig.4 Joseph RM7si, final sample, 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.

The speaker's dispersion pattern in the horizontal plane (fig.5—just the changes in response are shown) quickly narrows at high frequencies, as expected from the tweeter's physical layout. In all but very lively rooms, this will add to the mellow perceived tonal balance. The woofer's output also falls more rapidly to the RM7's sides in the octave below the crossover frequency than the tweeter's in the octave above crossover (shown by the cursor position). In lively rooms, this might make the little Joseph sound rather bright, in contrast to the lack of top-octave energy in the room. It shouldn't be a factor in rooms with their usual share of absorptive material.

Fig.5 Joseph RM7si, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis, reference response, differences in response 5 degrees-90 degrees off-axis.

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