The plot below was taken on a monopole RD-75. This is an inroom response at 3m75 from the driver. (This is the same driver as used in the FAQ graphs.) Measurement interval is 21.3 milliseconds with a 1024 FFT. (Hence the lower resolution in the low end of the spectrum.) Vertical resolution is 6dB/division, hence 3dB between horizontal lines.
Although it looks like the cavity resonance is not damped, it is in the near field. The peaks are system response related in this particular environment. (Like the 200 Hz room dip, recurring in most 4 and 5 m measurements.) Observe the harmonic relation ship between the peaks starting from approximately 1500 Hz. The 1500 Hz peak (see cursor) is tied to a ribbon resonance and can be observed in an impedance plot. (See also the waterfall display and distortion plots of the RD 75's in the "Planar Magnetic Ribbon Testing") Although the RD75 is thought to be a mostly purely resistive load, resonances can be observed in an impedance plot. (See bottom of the page.)
The first cursor is placed at a ribbon resonance peak @ 1453 Hz, correlating with previously observed resonances. A prototype Clearview CXR-22 is in the loop, hence the "flat" (+/- 2dB) response out to 20 Khz.
To verify the locations of the resonances, I measured the impedance of the RD 75 monopole set-up. Results below The first resonance occurs at 123 Hz. It is hardly visible in the plot, I had to use the cursors LAUD and take note of the locations. The second resonance shows up at 540 Hz. Correlates with peaks as seen in the distortion plot. The region left and right of this point are resonant prone. The third resonance is visible at 1420 Hz. Correlates with the peak in the monopole frequency/magnitude plot on this page. Further on we see the impedance slightly rising because of the inductance of the strip-voicecoil. The white line below the impedance plot is the impedance phase. Virtually flat.
Observe that the cavity resonance does not show up on the impedance plot as it is not a ribbon resonance but rather an acoustic reinforcement.
