JMlab Utopia loudspeaker Measurements

Sidebar 2: Measurements

The Utopia goes very loud with only a few amplifier watts. I calculated its B-weighted sensitivity as 92.5dB/2.83V/m, which is very high—around 6dB higher than the industry average. It does demand some current from the amplifier, however, the impedance (fig.1) remaining between 3 ohms and 6 ohms over much of the audio band. And although the electrical phase angle (dashed trace) remains low in the midrange and treble, in the bass the speaker gets highly capacitive while the magnitude is still low. At 60Hz, for example, the Utopia combines 4 ohms with a -50 degrees phase angle, which is going to stress wimpy power amplifiers.

Fig.1 JMlab Utopia, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The tuning of the port is indicated by the saddle in the solid magnitude trace at a very low 22Hz. Slight wrinkles in these traces imply the presence of resonances of various kinds. Those above 20kHz are going to be due to the tweeter's ultrasonic behavior and are likely to be subjectively harmless, but those in the midrange indicate possibly audible cabinet problems. Despite its solid construction, from its impedance plot the Utopia does appear to have some resonances present between 100Hz and 200Hz. Fig.2, for example, shows a cumulative spectral-decay plot calculated from the output of a simple piezoelectric-tape accelerometer fastened to the side of the woofer enclosure. Several resonant modes can be seen, the highest of which, at 188Hz, coincides with the severest wrinkle in the impedance plot and could be detected on all of the speaker's cabinet surfaces. It was particularly strong, for example, on the top of the upper-midrange enclosure.

Fig.2 JMlab Utopia, cumulative spectral-decay plot of accelerometer output fastened to side of woofer cabinet. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Though J-10 didn't find anything untoward in the speaker's lower midrange, I was surprised to find this mode at all, given the solidity of the Utopia's construction. However, given the speaker's very high sensitivity, it is probable that the direct sound of the drive-units will mask any radiation from the cabinet walls.

A low-level mode can be seen around 100Hz in figs.1 and 2. This makes its presence known in the nearfield acoustic measurement of the port output (the lower trace in fig.3), and slightly affects the nearfield output of the woofer (top trace in fig.3). The port output is free from higher-frequency resonant modes, and this graph confirms that the rectangular port is tuned to 22Hz or so, implying excellent bass extension. The woofer appears to cross over to the twin midranges at 300Hz.

Fig.3 JMlab Utopia, nearfield responses of woofer, port, and midrange units, adjusted for differences in radiating size.

The graph in fig.4 is a composite, the complex sum of the nearfield woofer, port, and midrange responses being spliced at 300Hz to the farfield response on the Utopia's tweeter axis, averaged across a 30 degrees horizontal window. Due to the proximity effect, this measurement technique always results in a slight boost in the bass. Even so, the Utopia appears to be flat down to 20Hz. There is a slight notch at the port problem frequency of 105Hz, but, again, JS noted no problems in this region in his auditioning. The midrange and treble are overall very flat, though the trace is a little rougher-looking in the higher frequencies—I suspect the wide baffle gives rise to small reflections of the tweeter's output.

Fig.4 JMlab Utopia 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 midrange, woofer, and port responses plotted below 300Hz.

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