RBH WM-24, WM-30, FM-45 Measurements and Analysis

    

Figure 1. Impedance & Phase. FM-45 at left, WM - 24 & WM-30 at right.

At left in the FM-45 graph we see a local minimum at just under 40 Hz. It’s also a global minimum in that at just under 40 Hz we also find the minimum system impedance at just over 6 ohms. To the right of that point, at just over 100 Hz we find another local minimum of about 8.7 ohms. This saddle point, located as it is between the two local z peaks (found at 80 Hz and 150 Hz) arises as a consequence of the electrical interaction between the woofer/ported cabinet system and the midrange/sealed cabinet system.  The saddle point found at just over 7kHz is likely due to the MF/HF drivers setting up an electrical resonance with the highpass section of the crossover network. Also noteworthy is the system’s electrical phase response: Maxing out at 19.7 degrees (64 Hz), reaching a minimum of -45 degrees (20kHz). In the critical 300 Hz – 3kHz range the phase deviates a maximum of 15 degrees from 0.

At right are the impedance curves of the WM-24 & WM-30. At left in the graph we see the typical peak-trough-peak impedance response of an LF system employing passive radiators. In the case of the WM-24 & WM-30 the 4” active driver pairs are augmented by a pair of passive radiators. At right we see a z peak-trough-peak, located at roughly 500 Hz lower, though similar in appearance than that seen in the FM-45 impedance plot. As was the case with the FM-45, it is likely due to the MF/HF drivers setting up an electrical resonance with the highpass section of the crossover network.

 Figure 2. Amplitude response plots. FM-45 at left, WM-24 (Red) & WM-30 (Blue) at right.
1m, on-axis, 2.828Vac, dB re 20 µPa. 1/12^th Octave smoothing used for enhanced visual clarity.

In Figure 2 we see at left the amplitude response plot of the FM-45 and at right the amplitude response plots of the WM-24 & WM-30.

Using the measured 89 dB sensitivity as a reference, the FM-45 produces a response plot that remains largely within +/- 3dB of that figure from about 45 Hz to 20kHz, stepping slightly ( a fraction of a dB) out of bounds only in the 5kHz to 9kHz range. Apparent in the plot is the slight upward tilt of the system’s amplitude response noted earlier in the subjective assessment portion of this review. Depending on where you’re seated in relation to the FM-45s this can present as a slight emphasis on detail. The effect diminishes, of course as the listening position is shifted off-axis.

The FM-45 LF response you’ll likely encounter in practice will certainly be influenced by placement and your listening space’s acoustics. Factoring in room gain and so forth, useful extension to somewhere in the neighborhood of 40 Hz seems a reasonable outcome. LF extension to at least the 40 Hz – 45 Hz range is often judged as essential should a loudspeaker system aspire to successfully presenting the listener with a musically satisfying experience. And in this case, the FM-45 meets those criteria.

Referencing the measured 90 dB sensitivity figure, the WM-24 & WM-30 both produce amplitude response plots that remain largely within the +/-3dB range from about 70 Hz to 20 kHz, stepping out of bounds by a fraction of a dB only in the 10 kHz to 12 kHz region. Sounding slightly reserved in the upper mid-bass/lower MF region and likewise in a portion of the HF region, the WM-24s and WM-30 otherwise presented similarly in many ways, sonically speaking, to the FM-45.

Noteworthy in the measurements is the high degree of similarity between the amplitude response of the WM-24 & WM-30. There also exists, especially in the 500 Hz to 20 kHz range the already mentioned family resemblance in both products to the FM-45.

Figure 3.  WM-24 & WM-30 amplitude response match comparison

In Figure 3 we see a comparison between the amplitude response plots of the WM-24 & WM-30.

Across the majority of 20 Hz to 20 kHz range they match to within 1 dB.  This partially explains the rock-solid stability of the images and the exceptionally clear pans across the soundstage presented by the WM-24s and WM-30 during the listening sessions. Excellent!

Figure 4. Spectral component comparison of two WM-24s

In listening to the various RBH products it was clear how well matched, in terms of timbre, they are to each other. Curious to take a closer look at this, a spectral analysis of the two WM-24s was done.

Above in Figure 4, the red & blue plots are the spectral components or harmonic profile of the two WM-24's acoustical version of a chord, sampled from Gershwin's "Rhapsody in Blue" as played on a Steinway Concert Grand piano.

When broken down into the individual spectral components the strong similarity between the two is immediately apparent. (The bits that look out of whack in the 150 - 180 Hz range in the red plot are owing to environmental noise). Here we see another reason for the impressive rock-solid stability of the images and the exceptionally clear pans mentioned earlier. Clearly, a high degree of quality control has gone into the selection and assembly of the components comprising the WM-24s. Overall, it’s fair to say RBH has in fact attained a substantial degree of timbre matching with these products. Excellent!