Dunlavy Audio Labs SC-IV/A loudspeaker Measurements

Sidebar 3: Measurements

As specified, the Dunlavy SV-IV/A is a sensitive speaker, my B-weighted measurement of 91dB/2.83V/m being within measurement error of the specified 92dB figure. (Note that with its low impedance, the speaker actually draws closer to 2W from the amplifier than 1W to achieve this sound-pressure level.) The variation of the impedance magnitude and electrical phase angle are plotted in fig.1. The value hovers around the 4 ohm line for much of the audio band and reaching a minimum value of 2.9 ohms at 63Hz. The phase angle is low, however, minimizing the strain on the partnering amplifier. From this graph, the woofer system resonance appear to lie at 37Hz, which is a little high for the speaker to reach its -3dB point at 20Hz as specified.

Fig.1 Dunlavy SC-IV/A, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Other than a glitch at 6kHz—which I assume is due to one of the resonant electrical circuits incorporated in the crossover—the plots in fig.1 are free from cabinet-induced problems. Nevertheless, using a simple plastic-tape accelerometer to investigate the vibrational behavior of the large cabinet panels revealed the existence of some strong resonant modes between 300Hz and 500Hz.

Fig.2, for example, is a waterfall plot calculated from the accelerometer's output when it was fastened to the center of the cabinet sidewall—it shows a strong resonance at 390Hz. Predicting the effect of this on music is difficult as its audibility will depend on the effective radiating area of the panel and the masking provided by the drive-units. All things being equal, the subjective effect of such resonances reduces with increasing sensitivity—the speaker doesn't have to be driven as hard to achieve the same sound levels, meaning that the cabinet resonances are less strongly excited—and the SC-IV/A is indeed a sensitive design.

Fig.2 Dunlavy SC-IV/A, cumulative spectral-decay plot of accelerometer output fastened to center of cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The biwiring terminals allowed me to investigate the crossover between the twin woofers and midrange units. Set at 70Hz, with the expected first-order slopes, it allows the woofers basically to act as subwoofers. They have a slight peak in their output, centered on 750Hz, but this is well suppressed by the crossover. The bass extension was less than specified, as I expected from the impedance plot, the SC-IV/A's -6dB point lying at 30Hz. (To check this measurement, I used both my regular B&K microphone and Joe D'Appolito's excellent Mitey Mike II.)

This LF behavior can be seen in fig.3, which shows the complex sum (amplitude and phase) of the woofer and midrange nearfield responses (on the left), spliced at 300Hz to the response at a 50" distance on the tweeter axis, averaged over a 30-degree horizontal window. This is still excellent LF extension, and in a room, the typical gain at low frequencies will bring the big Dunlavy within spitting distance of true 20Hz performance. It is also possible that the use of the widely spaced woofers will give lower bass extension in-room that either one on its own.

Fig.3 Dunlavy SC-IV/A, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with complex sum of the nearfield woofer and midrange responses plotted below 300Hz.

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