Infinity Prelude MTS loudspeaker Measurements
The first Infinity Prelude speaker Stereophile reviewed, the slightly larger P-FR was astonishingly sensitive, at an estimated 95.5dB(B)/2.83V/m. The MTS clocked in at 90dB(B)/2.83V/m sensitivity as specified, which is still significantly higher than average—the speaker will produce high levels with only a few amplifier watts.
The impedance, however, averages 4 ohms, as can be seen from fig.1, though the electrical phase angle is usefully low. The curve remaining below 6 ohms in the bass was taken with the speaker set to Full-Range operation. With the tower set to High-Pass, the impedance rises with dropping frequency in the bass due to the insertion of a series capacitor. This does result in a more demanding combination of 5 ohms magnitude and 45 degrees phase angle at 95Hz, but a good 4 ohm-rated amplifier or receiver should be able to take this in stride. As MF put it, this speaker is a "tube amp's best friend."
Fig.1 Infinity Prelude MTS, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Other than the glitch at a very high 28kHz due to the metal-dome tweeter resonance, the impedance curves are free from wrinkles that would otherwise indicate the presence of mechanical resonances. But fastening a simple accelerometer made from piezoelectric plastic tape to the center of one of the curved metal sidewalls gave the cumulative spectral-decay plot shown in fig.2. One strong mode can be seen at 480Hz. This might well be high enough in both frequency and Quality Factor (Q) not to degrade the speaker's sound—as a rule of thumb, the higher both these parameters, the less audible any effect of the panel resonance will be. This resonant mode is very high in level, though it should be noted that I took this measurement with the tower supported on upturned cones rather than on the base module, to allow what resonances are present to develop to their fullest extent. And the radiating area of the panel is very small, which will also minimize the mode's audibility. Which is presumably why MF didn't notice any midrange congestion or lack of clarity.
Fig.2 Infinity Prelude MTS, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the curved sidewall adjacent to the tweeter. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
The Prelude P-FR had a superbly flat response on-axis. The Prelude MTS's response, averaged across a 30 degrees horizontal window on the tweeter axis, is almost as flat (fig.3, right-hand trace). A big peak due to the tweeter resonance can be seen just below 30kHz, but this is way above the range of human hearing. To the left of this graph are three traces: The rightmost is the complex sum of the woofers and midrange unit response set to High-Pass, measured in the nearfield. Setting the switch to Full-Range decreases the rate of rolloff by 6dB/octave, giving a low-frequency response that's 2dB down at 100Hz rather than the 124Hz shown here. This would actually give a better crossover to the powered subwoofer, the response of which, again measured in the nearfield, is the leftmost trace in fig.3. For completeness' sake, the middle trace in the graph is the response of the subwoofer taken without its low-pass filter in circuit.
Fig.3 Infinity Prelude MTS, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange and woofer responses (set to High-Pass) and the nearfield responses of the subwoofer, with and without its low-pass filter.