Infinity Primus 360 loudspeaker Measurements

Sidebar 3: Measurements

The 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.)

Fig.1 Infinity Primus 360, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

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."

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).

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 the rich side compared with, for example, the Canton Vento 809 DC (reviewed by Wes Phillips elsewhere in this issue). The lows are 6dB down from their peak level at the port tuning of 42Hz, the lowest note of the four-string double bass and electric bass.

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.

The ultrasonic peak at the metal-dome tweeter's resonant frequency of 21kHz is suppressed on-axis but develops to the speaker's sides (fig.4). More important, note how evenly spaced the contour lines are in this graph, and how consistently the Infinity maintains its off-axis response across the audioband up to 9kHz or so. This means that the reverberant field generated by the speaker in a room will be as uncolored as the on-axis response. Again, this is impressive speaker engineering, even if you don't take the Primus 360's price into account. In the vertical plane (fig.5), the Infinity maintains its flat response over quite a wide listening window. Only if the listener stands will he perceive a suckout at the upper crossover frequency.

Fig.4 Infinity Primus 360, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5º off-axis, reference response, differences in response 5–90º off-axis.

Fig.5 Infinity Primus 360, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 20–5º above axis, reference response, differences in response 5–15º below axis.

The Primus 360's step response (fig.6) indicates that the tweeter and midrange unit are connected in positive acoustic polarity and the woofers in negative polarity, which, in conjunction with the phase shift associated with the crossover filters, ensures good frequency-domain integration between the units. Note how the overshoot of each unit's step smoothly hands over to that of the next lower in frequency—again, something that correlates with good integration in the frequency domain.

Fig.6 Infinity Primus 360, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Finally, the Primus 360's waterfall or cumulative spectral-decay plot (fig.7) is among the best I have measured, with a very clean initial decay and almost no ridges of delayed energy apparent. This Infinity may be bargain-priced, but it demonstrates extraordinarily competent loudspeaker engineering. I was a little bothered by its lively cabinet but overall the Primus 360 raises the bar for designs that compete at more than twice its price.—John Atkinson

Fig.7 Infinity Primus 360, cumulative spectral-decay plot at 50" (0.15ms risetime).

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Robin Landseadel's picture

I recall reading this review when it first appeared, noting the unusually fine test results as regards frequency response and off-axis response.

There's a thrift store in town, frequently has recent audio gear, with a lot of Home Theater audio gear as of late. Last month they had a pair of Infinity Primus 360's for $39.99, a pair of the matching, shorter, 250's at $29.99 and a C25 center channel for $8.99. All very clean, with the only damage evident being a missing plastic post for one grill cover, which I removed anyway. Very few scuffs/nicks. On top of all that they also had an early Oppo Universal DVD for $25. Last year, found an Onkyo TX RS606 for $50, and a 3 small subs, including a Polk PSW 50 that seems to do most of the heavy lifting, around $30 a pop. I've got 5.3, 360 sound for a total of less than $300, wire included.

This all goes into the garage, reconfigured into a studio for painting. 20' x 14' x 8'. I've got the speakers raised off the floor so the tweeters are slightly above ear level, with the C25 turned on its side for better imaging. All speakers are crossed over to the subs at 80 hz with the Onkyo's onboard EQ raising mids and reducing treble slightly.

This is one of the best systems I've heard so far. Of course, there are audible flaws, but what's striking is how natural and grain-free vocals are and how dynamic everything is. Piano sound is unusually plausible. Raising the speakers off the floor takes out a bit of the boom, crossing over to the subs seems to get rid of the rest, having three subs loafing at low levels gives me organ pedals & makes the room shudder when playing Sarah McLachlan's "I Love You" from "Surfacing". However, there is a residual audible resonance around 4k that doesn't tamp down with the eq change, the tweeter being the one thing I would change in the speaker design if I could.

I've owned a few classic speakers, have heard many others, spent many years using Stax earspeakers with a matching tube energizer as a audio reference for recording. The Primus speakers, set up as they are now, are as musically informative as the Stax Earspeakers, with better imaging and dynamic scale.

I was looking today at the specs for the Wilson Audio Sabrina speaker—product of the year, this year—and compared the measurements for the Primus 360. While I have no doubt that the Sabrina does some things better than the Primus speakers [and at $17,000 they should], it appears that the Primus 360 does some things better than the Sabrina. In my case, at $39.99.

In any case, as far as I'm concerned, the Primus 360 is a classic.