RBH 641-SE loudspeaker Measurements

Sidebar 3: Measurements

My estimate of the RBH 641-SE's voltage sensitivity was marginally higher than specified, at 85.7dB(B)/2.83V/m, but this is still about average. However, the speaker's impedance plot (fig.1) indicates that it is an easy amplifier load. The saddle in the magnitude trace at 44Hz indicates the tuning of the rear-facing port, and implies in turn only moderate bass extension.

Fig.1 RBH 641-SE, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

A slight wrinkle between 400Hz and 500Hz in the impedance traces implies the presence of some kind of resonance. Fig.2, a waterfall plot calculated from the output of an accelerometer fastened to the center of the back panel, reveals a strong resonant mode at 430Hz. Though it was detectable on all the cabinet surfaces, this mode might be too high in frequency to have an audible effect. However, I did wonder if it was related to the "warm" character I occasionally noted in the midrange, which affected the tonal characters of clarinet and violin.

Fig.2 RBH 641-SE, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's back panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The 641-SE's nearfield responses are shown to the left of fig.3. The woofer's output shows the expected notch at the port tuning frequency, but there is a second notch visible at 30Hz, which is odd. The port's output peaks almost an octave higher than the nominal tuning frequency, but rolls off smoothly above 100Hz. The crossover between the woofers and the midrange unit in this graph appears to be a little higher in frequency than the specified 150Hz, and the slopes look asymmetrical. Higher in frequency, the farfield responses of the midrange unit and tweeter are well-integrated on the tweeter axis, and the overall response is pretty flat.

Fig.3 RBH 641-SE, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield midrange, woofer, and port responses plotted below 600Hz, 1.7kHz, and 500Hz, respectively.

This flatness of the 641's upper-frequency response can also been seen in fig.4, which splices the speaker's farfield output, averaged across a 30 degrees horizontal window centered on the tweeter axis, to the complex sum of the nearfield responses, taking acoustic phase and the different path lengths of each driver to a nominal farfield listening point. But note that two complex sums are shown. The top, smooth trace is with the woofers actually wired out of phase with the upper-frequency drivers. With the woofers wired in phase, there is a large crossover suckout evident.

Fig.4 RBH 641-SE, 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 responses plotted below 500Hz. Bottom curve is with the woofers wired in phase with the upper-frequency drivers; top curve is with them wired out of phase.

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