Red Rose Music R3 loudspeaker Measurements

Sidebar 3: Measurements

There are no surprises in the Red Rose Music R3's plot of impedance magnitude and phase against frequency (fig.1). The saddle in the magnitude trace at 46Hz indicates the tuning of the rear-facing port, the peak in the upper midrange is due to the crossover filters, and the rising impedance at ultrasonic frequencies is a result of the ribbon's inductance. The impedance drops below 4 ohms only in the lower midrange and in the mid-treble, and although the electrical phase angle occasionally reaches high values, this never coincides with low magnitude. Consequently, the R3 will not present much of a difficult load to the partnering amplifier. The estimated sensitivity was on the low side, at around 85dB/2.83V/m, though this will be affected by the speaker's on-axis response (see below).

Fig.1 Red Rose R3, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The traces in fig.1 are free from the wrinkles that would otherwise indicate the presence of cabinet resonances, mechanical or acoustical. Exploring the cabinet's vibrational behavior with a plastic-tape accelerometer revealed quite a high-level, high-Q resonance on the top panel at 230Hz; this can also be seen in a waterfall plot calculated from the output of the accelerometer fastened to the center of a sidewall (fig.2). Despite some flexure in the low bass, the side panels are well-braced, with the only high-level mode apparent at 625Hz, which is sufficiently high in frequency not to be much of a subjective problem.

Fig.2 Red Rose R3, cumulative spectral-decay plot of accelerometer output fastened to side wall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 shows the individual responses of the drive-units and the port. The latter is the sharply defined bandpass centered on the woofer's minimum-motion frequency of 46Hz. Though some upper-frequency peaks can been in its output, these are well-suppressed, and their subjective effects will be minimized by the fact that the port faces away from the listener. The woofer's response is basically flat within its passband, though a small peak appears just below the acoustic crossover frequency, which appears to be between 3kHz and 4kHz. The ribbon seems to come in a little too high in frequency to give a seamless on-axis match with the woofer, and also has a severe suckout in its output at 3kHz, which may be due to a notch filter in the crossover or to acoustic interference.

Fig.3 Red Rose R3, acoustic crossover on HF axis at 50", corrected for microphone response, with nearfield woofer and port responses below 300Hz and 1kHz, respectively.

The ribbon is also a little too "hot" in absolute level, which I could hear as a slightly fizzy quality on pink noise in the very large, ceiling-less room I use for my speaker measurements (the family generally refers to it as the "back yard"). Without the ameliorating effects of a room's reverberant field, the exaggerated top-octave region was emphasized by what sounded like a lack of energy in the crossover region.

However, even with the speakers in a room, MF did comment on the R3's relative lack of presence-region energy. This can be readily seen in fig.4, which splices the complex sum of the nearfield woofer and port responses to the farfield tweeter-axis response averaged across a 30 degrees lateral window. Not only does the ribbon roll in a little late, the woofer's output is suppressed in the octave below the crossover. This is due to the unit's increasing directivity in this region, and is relieved only by the presence-region peak previously noted.

Fig.4 Red Rose R3, anechoic response on tweeter 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.

Red Rose Music
943 Madison Avenue
(at the Whitney Museum)
New York, NY 10021
(212) 628-5777
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