Floor Loudspeaker Reviews

I measured one of the Piega Coax Gen2 811 loudspeakers auditioned by RvB. It had the serial number 8057. I used DRA Labs' MLSSA system, a calibrated DPA 4006 microphone, and an Earthworks microphone preamplifier to measure the quasi-anechoic frequency- and time-domain behavior in the farfield. I used an Earthworks QTC-40 microphone for the nearfield responses. Before I started the testing, I made sure the short wire jumpers that connect the two pairs of binding posts were securely fastened.

The Coax 811's voltage sensitivity is specified as a high 92dB/W/m. My B-weighted estimate, measured on the tweeter axis, was significantly lower, at 85.1dB(B)/2.83V/m. Piega specifies the Coax 811's nominal impedance as 4 ohms. The impedance magnitude (fig.1, solid trace), examined with Dayton Audio's DATS V2 system, was close to 4 ohms at low frequencies, with a minimum value of 3.6 ohms at 88Hz. However, it rose above 8 ohms from the upper midrange onward. The shape of the magnitude trace suggests that when this loudspeaker is used with a tube amplifier that has a high source impedance, the high-frequency response will be boosted compared with the level in the midrange and bass. The electrical phase angle (fig.1, dotted trace) is occasionally high, which means that the effective resistance, or EPDR (footnote 1), drops below 3 ohms in the bass and between 90Hz and 330Hz. The minimum EPDR values are 2.41 ohms at 26Hz, 2.67 ohms at 66Hz, and 1.99 ohms at 150Hz. The Coax 811 is a relatively demanding load for the partnering amplifier.

The aluminum enclosure seemed very inert when I rapped it with my knuckles. The only resonant mode I found using a plastic-tape accelerometer lay at 293Hz on the center of the curved sidewalls, but was extremely low in level (fig.2).

The saddle centered on 27Hz in the impedance magnitude trace suggests that this is the tuning frequency of the two passive radiators mounted at the base of the front baffle. (Both radiators behaved identically.) The blue trace below 300Hz in fig.3 shows the woofers' response measured in the nearfield (the two woofers behaved identically); it has the expected notch at the reflex tuning frequency. The radiators' response is shown as the red trace in fig.3. Their output peaks at the tuning frequency before rolling off cleanly at lower and higher frequencies. I haven't plotted their output above 400Hz because the measurement was affected by crosstalk from the adacent woofers.

The blue trace above 300Hz in fig.3 shows the farfield response of the woofers. It crosses over to the coaxial mid/HF driver at 710Hz, with a sharp rolloff above that frequency. The coaxial unit's response (green trace, taken without the grille), has some small suckouts in the treble, with then the top octave shelved down by up to 4dB.

The complex sum of the woofers and passive radiators' nearfield responses is shown as the black trace below 300Hz in fig.4. The broad peak between 30Hz and 200Hz will be mostly due to the nearfield measurement technique, which assumes that the baffle extends to infinity in both horizontal and vertical planes (footnote 2).

The Piega speaker's woofer tuning is actually close to being maximally flat, with extended low frequencies. However, the balance may well sound too rich in small rooms or if the speakers are placed too close to the wall behind them.

The black trace above 300Hz in fig.4 shows the Coax 811's farfield response, averaged across a 30° horizontal window centered on the tweeter axis and taken without the grille. It is even through the midrange, but there are again small suckouts in the treble and a shelved-down top octave. The red trace in this graph shows the Piega's farfield response, again averaged across a 30° horizontal window centered on the tweeter axis, but now taken with the grille. The grille increases the depth of the treble-region suckouts and reduces the energy between 12kHz and the small peak at 18kHz.

The speaker's horizontal radiation pattern, normalized to the response on the tweeter axis, which therefore appears as a straight line, is shown in fig.5. The midrange dispersion is well-controlled, but the off-axis output at angles greater than 20° lacks energy between 2kHz and 5kHz. It is likely that this is due to destructive interference between the rectangular midrange diaphragm and the coaxially mounted tweeter in its center. As the tweeter's top-octave output also drops to the sides, this suggests that the Coax 811s need to be toed in to the listening position if the balance is not to sound polite or dull. Fig.6 shows the Coax 811's dispersion in the vertical plane, again normalized to the response on the tweeter axis. The dispersion is uniform up to 10° below that axis, which is useful considering that the tweeter is 44" from the floor, 8" higher than what a survey Thomas J. Norton performed for Stereophile in the 1990s indicated was the typical listener ear height.

In the time domain, the Piega's step response (fig.7) indicates that all the drive units are connected in positive acoustic polarity, with the coaxial tweeter's output arriving first at the microphone. The decay of its step cleanly merges with the positive-going start of the midrange diaphragm's step, the decay of which cleanly blends with the start of the woofers' step. This indicates an optimal crossover topology. Other than some low-level hash in the mid-treble region, something I have noted with other planar drive units, the Coax 811's cumulative spectral-decay, or waterfall, plot (fig.8) is clean. (As always with these plots, ignore the ridge of delayed energy close to 16kHz, which is due to interference from the measurement computer's video circuitry.)

The Piega Coax Gen2 811 offers respectable measured performance, though its treble balance will depend very much on the source impedance of the partnering amplifier. But I must give shoutouts to that exceptionally inert enclosure and the extended, powerful low frequencies.—John Atkinson

Footnote 2: Scroll down the page here for an explanation of this effect.