Phase Technology PC 8.5 Speaker System Review

Phase Technology speakers come from a company that has manufactured speakers sold under many famous brand names for the past thirty-five years. The new three-way, floor-standing PC8.5 speaker system contains a number of unique or unusual design and construction features that yield performance beyond that of many competitively priced speakers.

Approximately halfway up the speaker panel is a 10-inch woofer that employs the RPF (rigid polymer foam) construction exclusive to Phase Technology products. The RPF process forms a cone-shaped radiator, which is driven by a voice coil at its apex. According to Phase Technology, an RPF driver is essentially free of the usual cone break-up effects at high frequencies. The solid, light, and rigid cone radiates from its flat surface, which is conventionally supported by plastic surround. The front of the white RPF cone is covered with a dark-gray foam plastic material.

In the PC8.5, the woofer operates in a sealed enclosure. At 350 Hz there is a crossover to a 5-1/4-inch RPF piston midrange driver, which is similar in design to the woofer and mounted above it on the centerline of the speaker panel.

The entire design of the PC8.5 reflects special care to minimize diffraction effects. These can occur in virtually every speaker system wherever a discontinuity exists in an acoustic path. Typical examples are the edges of a speaker mounting hole, a cabinet edge, a grille frame, etc. The result of diffraction is usually an irregularity in the speaker’s frequency and phase response, usually in the upper-middle and high-frequency ranges.

Diffraction can be minimized either by altering the shape or dimensions of the responsible surface discontinuity or by isolating the driver’s radiation from the discontinuity. In the PC8.5, both methods are used. The metal outer rings of the woofer and midrange driver baskets are screwed to the speaker panel and covered with rings of absorbent rubber, about 1/4 inch thick, whose outer surfaces lie in the plane of the panel. Although this is done for both drivers, it seems likely to be most useful for the midrange.

But it is the PC8.5’s tweeter, in whose operating range (from 2,000 Hz upward) diffraction effects are likely to be most troublesome, that has received the most unusual corrective treatment. The tweeter itself is a fairly conventional l-inch, soft-paper-dome radiator. (Phase Technology and its parent company, United Speaker Systems, hold the patent on this widely used tweeter design.) The tweeter dome is surrounded by a “Unicell Acoustic Treatment,” a specially molded piece of dense plastic foam that extends into the mounting hole to a depth of about an inch and in front of the panel by about 3/8 inch. The foam section is 6-1/4 inches square and occupies a total volume of 40 cubic inches. When the speaker's removable grille is installed, its wooden frame hugs the top and sides of the foam panel, extending a smooth surface out to the edges of the grille frame, which are rounded to make a smooth transition to the edges of the cabinet.

The function of the Unicell treatment, according to Phase Technology, is to “slow and gradually stop the sound waves before they can be diffracted by any surface or edge.” If acoustic energy from the tweeter does not reach the grille or cabinet edge, there can be no diffraction-caused response irregularity resulting from that edge.

The PC8.5’s crossover network is a complex system, initially designed with theoretically ideal values and then optimized by trial and error for the cabinet and driver characteristics. The cabinet, externally conventional in appearance, is unusually sturdy. It is constructed of 1-inch particle board, extensively braced internally and stuffed with absorbent padding. The 2-inch-thick speaker panel is exceptionally rigid. The woofer and midrange/tweeter portions of the system are brought out to separate gold-plated binding posts, which can be joined for conventional installations or separated for bi-wired or bi-amplified operation.

The PC8.5 system’s specifications include a frequency response of 35 to 20,000 Hz ±2 dB, a sensitivity of 89 dB sound-pressure level (SPL) at 1 meter with a 2.83-volt input, and at 4-ohm nominal impedance. It is recommended for use with amplifiers rated to deliver from 25 to 120 watts per channel. Dimensions are 39-1/2 x 13 x 11-3/4 inches, and weight is about 90 pounds. The cabinet is available finished in dark oak, walnut, or black. Price: $1,200 a pair. Optional spiked feet are included.

Lab Tests

The averaged room response of the two speakers was unusually wide and uniform, even at low frequencies where room-boundary interaction is unavoidable. The uncorrected response curve was flat within ± 5 dB from 20 to 20,000 Hz.

The close-miked woofer response varied only ±1.5 dB from 45 to 250 Hz, falling at 12 dB per octave above and below that range. When this was spliced to the averaged room curve, it produced a composite response that varied only ±3.5 dB from 33 to 20,000 Hz. Other than a slight dip at 500 Hz probably caused by a room-boundary interaction, the response was extremely smooth, with a nearly linear downward slope of less than 7 dB from 65 to 20,000 Hz.

The manufacturer advises that the grilles should be left on the speakers since they form an integral part of the diffraction-control system. We were able to confirm this to our complete satisfaction. Removing the grille from a speaker produced a jog of about ±3 dB in its frequency response in the vicinity of 3,000 Hz. Replacing the grille effectively eliminated this effect and restored the system’s inherent smooth response. Our FFT analysis confirmed that this was purely a diffraction effect and not the result of a resonance of some sort, since it was present only during the test signal and was not sustained after it had ended.

The tweeter’s response was possibly the smoothest and flattest we have measured from a speaker (very similar results were obtained with both swept-sine-wave and FFT measurements). Its variation of ±0.4 dB from 2,800 to 11,000 Hz is more like that of an amplifier than of a speaker. From 11,000 to 20,000 Hz the response sloped off smoothly. The system’s group delay (a measure of phase linearity) varied only about 0.1 millisecond overall from 4,000 to 26,000 Hz.

Another measurement that confirmed the superior performance of the PC8.5 was the “waterfall” plot, a 3-D display of thirty-two successive FFT response measurements at intervals of less than 0.1 millisecond. This plot reveals vividly the various resonances and sustained “ringing” frequencies (and their durations) in a speaker’s output. Most speakers do not produce very gratifying results in this test, but the PC8.5 showed only a very low-level ringing at about 10,000 Hz, at a level of some -40 dB relative to the primary output, that lasted about 2 milliseconds.

The system’s impedance was a minimum of 3.55 ohms at 200 Hz, and it varied between that value and 3.8 ohms over the 100- to 200-Hz range. The maximum impedance, at the bass resonance frequency of 39 Hz, was 17.5 ohms. There were smaller peaks of 8 ohms at 385 Hz and 4.9 ohms at 2,000 Hz. Although at some frequencies the impedance was slightly lower than the rated 4 ohms, its phase angle remained within a range of ±45 degrees, indicating that the PC8.5 should not present a difficult load to any amplifier designed to drive 4 ohms.

Sensitivity was 89.5 dB SPL at 1 meter with a 2.83-volt input of pink noise. The woofer’s distortion at a drive level of 3 volts (equivalent to a 90-dB SPL in our sensitivity tests) was between 0.5 and 1 percent from 40 to 400 Hz, rising to 4 percent at 30 Hz and 8 percent at 20 Hz. The midrange driver’s distortion was also very low, from 0.2 to 0.8 percent between 200 and 2,000 Hz.

The horizontal dispersion of the PC8.5 was only moderate over a 45-degree angle. There was about a 6-dB difference between the on-axis and off-axis response from 2,000 to 10,000 Hz and an increasing spread at higher frequencies. At 30 degrees off-axis, however, the response was very similar to the axial readings, typically within 2 or 3 dB up to at least 12,000 Hz.

Finally, we subjected the system to a single-cycle tone-burst test to establish how much short-term power it could handle at frequencies of 100, 1,000, and 10,000 Hz without obvious distortion or damage. At each frequency our Carver Mono-Block amplifier clipped before the speaker showed any signs of distress. The respective power outputs were 1,375 watts, 1,635 watts, and 1,255 watts into impedances of 3.7 to 4.6 ohms.

Comments

In our preliminary listening, before any measurements were made, it was apparent that the PC8.5 had a very smooth, easy sound quality, without roughness, shrillness, or any of the other problems that can prevent a speaker from sounding “right.” Its frequency coverage was wide and well balanced, with no tendency toward brightness or dullness. In fact, it sounded truly “flat,” and the confirmation of its flat response in our measurements came as no great surprise.

The lower frequencies were reproduced solidly but without the “boom” in the upper bass that in so many loudspeakers impairs the natural reproduction of male speech. One of the most surprising and impressive features of the speaker’s sound was the way that the lowest bass frequencies (when present in the program material) could be felt as well as heard, even at low volume. This effect, not unlike that of a good subwoofer, is quite rare in speakers of this size and price.

In other words, the PC8.5’s frequency response (no matter how defined) was wide, uniform, and free from audible aberrations that could impair the natural quality of its sound. Our measurements demonstrated convincingly that it had the fundamental qualities of a full-range, smooth, well-dispersed response and low distortion.

Not only did the system sound superb with respect to these measurable qualities, but it created a spatial image that almost overshadowed its basic performance as a transducer. Listening with closed eyes to a good orchestral recording from a CD, we found the illusion of attending a live performance remarkably convincing. The sound image was usually only slightly wider than the speaker spacing, but its height and depth made it the next best thing to a genuine live listening experience.

The PC8.5’s designers believe that its imaging quality is related to its exceptional response flatness and freedom from diffraction. Though I have no solid proof, I am inclined to agree. In many ways, the Phase Technology PC8.5 performs at a level that is rarely, if ever, found in conventional dynamic speaker systems, and certainly not at or near its price. It is a top value in today’s market, and I suspect that it could give strong competition to some speakers selling for several times its price.

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