ProAc Response D Two loudspeaker Measurements

Sidebar 2: Measurements

ProAc specifies the D Two's voltage sensitivity as 88.5dB/W/m; my estimate was significantly lower, at 85dB(B)/2.83V/m. However, while the speaker's nominal impedance is 8 ohms, the impedance of my sample (S/N 000628) drops below 8 ohms only in the lower midrange, and remains above 10 ohms throughout the upper midrange to the mid-treble (fig.1). The electrical phase angle is also relatively small over much of the audioband, meaning that, in combination with the generally high impedance magnitude, this ProAc will therefore be a relatively easy load for an amplifier to drive. The somewhat low-powered Leben integrated amp was a good match for it.

Fig.1 ProAc Response D Two, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There is a slight discontinuity in the impedance traces at 300Hz; when I investigated the enclosure panels' vibrational behavior with an accelerometer, I did indeed find a fairly strong resonant mode at 300Hz, as well as another, lower-level mode at 400Hz, both of them present on all surfaces (fig.2). I would have expected these modes to add a slight degree of midrange congestion at high playback levels, though JM wasn't bothered by any coloration problems that could have been laid at the feet of these modes.

Fig.2 ProAc Response D Two, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle between 40 and 50Hz between the two impedance peaks suggests that this is where the port tuning frequency lies. The woofer's nearfield response does indeed have its minimum-motion notch at 42Hz (fig.3, blue trace), which is the frequency of the lowest note of the four-string bass guitar and double bass. The peak in the port's output (fig.3, red trace) is a little broader than usual, but rolls off above 80Hz. Higher in frequency, the woofer crosses over to the tweeter (fig.3, green trace) just above 2kHz, with a slight peak apparent in its output at 1200Hz. Though another woofer peak is visible at 4.5kHz, this is suppressed by the crossover filter. The tweeter's output is generally even within its passband, but this driver does appear to be balanced about 3dB too high in level for an optimal match to the woofer.

Fig.3 ProAc Response D Two, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer (blue) and port (red), plotted below 350Hz and 300Hz, respectively.

Fig.4 shows how the individual drive-unit responses combine on the tweeter axis in the farfield, averaged across a 30° horizontal window. The midrange is impressively smooth, though a slight rising trend is apparent in the treble. The apparent boost in the upper bass will be partly due to the nearfield measurement technique, but the D Two does appear to have a touch of the British-monitor bump in this region. Why didn't John Marks hear this? I suspect that the tweeter being a little hot balances this, so that the listener's ear latches on to the extremes at the treble and bass ends of the spectrum as being correct, and therefore perceives the broad trough in between as making the speaker sound a little laid-back. This is the classic "smile" balance that has been so popular over the years. (It's called "smile" because that's how it appears on a graph.) However, JM did comment on the ProAcs' excellent presentation of detail when driven by the Leben amplifier—I suspect that the Ohm's Law interaction between the tubed amp's relatively high source impedance and the manner in which the speakers' impedance rises between the upper midrange and the mid-treble tend to cancel the "smile," at least to some extent.

Fig.4 ProAc Response D Two, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.

JM auditioned the ProAcs without their grilles. This was just as well, as the grilles reduced the mid-treble energy by up to 2dB (fig.5), which will indeed emphasize the speaker's laid-back character.

Fig.5 ProAc Response D Two, effect of grille on tweeter-axis response.

The Response D Two's horizontal dispersion will also have an effect on the perceived balance in all but very large rooms. Fig.6 reveals that the woofer gets a little more directional at the top of its passband, giving rise to a slight flare off axis at the bottom of the tweeter's passband. This will also work against the "smile" character. As usual with a design using a 1" tweeter, the D Two's radiation pattern narrows in the top two octaves, which will tend to balance the tweeter's hot on-axis output in the same region. Vertically (fig.7), a deep suckout develops in the ProAc's output for listening axes above the top of the cabinet. The stands used should be tall enough to place the listener's ears on or just below the tweeter axis, to produce the optimal balance.

Fig.6 ProAc Response D Two, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis on midrange side, reference response, differences in response 5–90° off axis on tweeter side.

Fig.7 ProAc Response D Two, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

Finally, there are no surprises in the D Two's step response (fig.8), which reveals that both drive-units are connected in positive acoustic polarity, with the tweeter's output leading that of the woofer but smoothly integrated with it. A small reflection can be seen about 1 millisecond after the woofer's output, which correlated with a small mode at 1235Hz in the cumulative spectral-decay plot (fig.9). However, other than the decay of this mode, the D Two's sound is superbly clean.

Fig.8 ProAc Response D Two, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 ProAc Response D Two, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

I am not surprised that John Marks was impressed by the sound of the ProAc Response D Two—this is a well-engineered loudspeaker from a designer with a long track record of producing good-sounding speakers.—John Atkinson

US distributor: Modern Audio
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