INFINITY PRIMUS 360
produce surprising results. Sometimes the more expensive speaker sounds merely like a louder, bassier version of the little guy, but without a corresponding increase in overall quality. At other times, the more expensive speaker can emphasize the strong points and soft-pedal the weaknesses of the original, resulting in even greater value per dollar. At still other times, the higher-priced speaker and its little brother present completely different sonic personae. As I listened to the Primus 360 and recalled the last time I�d heard the Primus 150, I concluded that their relationship didn�t neatly fit any of those categories. If you com-
pare the comments above to those in my review of the Primus 150, you might conclude that I think the 360 sounded like a louder 150, with deeper bass (although the bass did have a somewhat different character). That would accurately describe how the 360 affected my brain, but it would not address how the larger Primus impacted my heart and soul. To explain that, I need to drag my colleague Art Dudley into this. At the Home Entertainment 2004 show in New York City last May, Art and I sat on the panel of a �Meet the Editors�Stereophile� session. Art was gushing about the performance of a vintage horn speaker he�d just heard at
the New York Hilton. He was urging everyone to listen to how the sound from this speaker gently emerged and rolled over the listener, as does live music but as do very few speakers, most of which, according to Art, sound as if music is being �squirted at you.� How Art described good horn speaker sound pretty much sums up how the Primus 360 affected me. With every recording, there was a sense of high-level dynamic linearity and ease that did not vary by recording or loudness level. At no time did I feel that the 360 was straining or working hard to produce music, which enhanced realism with most recordings. The conclu-
M E A S U R E M E N TS
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he Infinity Primus 360 was significantly more sensitive than average, at an estimated 91.5dB(B)/2.83V/m. Though this is slightly lower than the specified 93dB, the 360 will still play very loudly with only a few watts of input power. However, with an impedance magnitude that drops below 4 ohms in the lower midrange and high treble and an electrical phase angle that is extreme in the upper bass (fig.1), the speaker needs to be partnered with an amplifier or receiver that can drive low impedances with aplomb. (The combination of 5.2 ohms and �45º phase angle at 93Hz will tax amplifiers rated at 8 ohms.) There are some wrinkles in the impedance traces between 150Hz and 400Hz that imply the presence of cabinet resonances of some kind. Using a simple plastic-tape accelerometer to investigate
the cabinet�s vibrational behavior, I found strong resonant modes present at 180Hz, 240Hz, and 490Hz on all surfaces. The last was the strongest (fig.2), but this will be too high in level to have serious subjective consequences. The lower-frequency modes are in a region where they are more likely to have an effect; I wonder if they contributed to Bob Reina�s finding the speaker to have some �warmth or ripeness in the midbass.� The saddle at 42Hz in the impedance-magnitude trace suggest that this is the tuning frequency of the front-mounted port, and nearfield analysis indeed shows that the port�s acoustic output peaks between 30Hz and 60Hz (not shown). The minimum-motion frequencies of the two woofers were a little different, however, at 30Hz (lower) and 40Hz (upper). The port�s output suffered from a peak at 330Hz; though this was well down in level, the nearfield woofer outputs did show a discontinuity at this frequency, suggesting the presence of an internal air-space resonance. This is too high in frequency to add warmth, but it might obscure midrange clarity. The crossover between the woofers and the midrange is specified as occurring at 350Hz, but there actually appeared to be a broad overlap between the three drive-units. Even so, the Primus 360�s farfield response, averaged across a 30º horizontal window on the tweeter axis (fig.3), shows good drive-unit integration. It is also extraordinarily flat, especially when you take the Infinity�s low price into consideration! Most of the peak in the upper bass will be due to the nearfield measurement technique, but the Infinity�s lows will be on
Fig.1 Infinity Primus 360, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.2 Infinity Primus 360, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet�s side panel level with the port (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 Infinity Primus 360, anechoic response on tweeter axis at 50", averaged across 30º horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer, midrange unit, and port responses, taking into account acoustic phase and distance from the nominal farfield point, plotted below 300Hz.
