ProAc Response 4 loudspeaker Measurements

Sidebar 2: Measurements

As I explained in Jack English's March review (p.98), the logistical problems of shipping the 355-lb-each ProAc Response 4s to Santa Fe for measurement were formidable. As I was visiting the East Coast in March, it seemed more sensible to measure—and take a listen to—the speakers in situ in Jack English's listening room.

I measured the speaker's impedance magnitude and phase with the DRA Labs MLSSA system, although for conformity's sake, I plotted them out using the Audio Precision software (fig.1). The magnitude varies considerably, reaching a maximum value of 26.3 ohms at 480Hz, with minima of 4.6 ohms in the upper bass and 4.5 ohms in the mid-treble. The phase angle gets a little extreme in the crossover region between the woofers and midrange domes, though, as is almost always the case, the worst angles appear when the impedance magnitude is high. The Response 4 should be relatively easy to drive, though its tonal balance will be significantly modified by an amplifier possessing a high source impedance—a classic tube design, for example. The speaker's sensitivity on the HF axis (measured using B-weighted noise) was somewhat lower than specified, at 85.5dB/W/m vs 89dB/W/m.

Fig.1 ProAc Response 4, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The bass end of the impedance curve had me puzzled. The rear-facing, 3"-diameter port appears to be tuned to a very low 16Hz, to judge by the saddle between the two low-frequency peaks. Yet a third impedance peak is evident at 70Hz. The woofer alignment is actually more complicated than a simple port tuning: The two woofers are loaded into different-sized chambers, with these opening into a third internal chamber connected with the outside world via the port. The effect of this arrangement can be seen in fig.2, which shows the individual nearfield responses of the two woofers and the port (footnote 1). Despite their different internal loadings, the two woofers have broadly similar bandpass responses centered on 100Hz. Each woofer has a minimum-motion point at 16Hz, the port tuning frequency seen in fig.1. (At this frequency, the resonance imposes such a high pressure on the back of the drive-unit cone that it can't move; all the system's output at this frequency therefore comes from the port.) But each woofer also has such a point at 65Hz, and the port has a second maximum at the same frequency before rolling off sharply with an approximate 32dB/octave slope.

Fig.2 ProAc Response 4, nearfield woofer and port responses.

To work out how these individual responses add up at the listening position is not a trivial task, given the fact that the two woofers are widely spaced and the port is on the cabinet rear. Summing the three outputs—magnitude and phase—in the ratio of the roots of their areas gives the result shown to the left of fig.3. The port compensates for the lower minimum in the woofer outputs, but it doesn't appear to fill-in the notch at 66Hz. This curve would suggest a lack of low bass, but Jack did find the ProAc to have extended low frequencies.

Fig.3 ProAc Response 4, anechoic response on HF axis at 1m, corrected for microphone response, with complex sum of nearfield woofer and port responses below 500Hz.

On the right-hand side of fig.3 is shown a composite of the nearfield response of one of the midrange domes, and the quasi-anechoic response on the tweeter axis at a distance of 1m. The Response 4 has a superbly flat midrange and low-treble above the 350Hz crossover point, with the dome units rolling out with a steep 24dB/octave slope below that frequency. The tweeter has a bit of an on-axis boost centered on 10kHz, which will add both a degree of air to the speaker's sound and a slight sibilance emphasis. Though it wasn't possible to carry out a set of off-axis measurements in JE's room, I would expect this on-axis peakiness to flatten out a little to the side of the speaker. It's recommended that you not toe the speaker in to the listening seat, therefore, though firing them straight ahead will probably render the sound too mellow, given the rolloff above 16kHz.

To investigate how everything added up, I measured the response of each speaker at Jack English's listening seat, 115" from each speaker. I used a 90kHz sampling rate and a long, 360ms time window, and computed the average power, which is shown in fig.4. The response trend through the midrange and treble is commendably flat. (Ignore the myriad peaks and dips—they're room effects and will not have any significant audible effects.) In the lower midrange, however, a large suckout is centered on 300Hz. This is mainly due, I suspect, to destructive interference at the microphone position between the direct sound from the speaker and the reflection from the floor, which follows 3.5ms later. I did suspect from fig.3 that the integration between the midrange units and the woofers was not quite optimal, the latter rolling out before the former are fully up to level. However, JE noted no audible problems in this frequency region, nor was I bothered by any leanness in my own auditioning.

Fig.4 ProAc Response 4, power-averaged, 1/10-octave smoothed response of Left and Right speakers at JE's listening seat.

Jack did note in his review that he had a problem optimally tuning the Response 4's bass. When I visited, he had ended up with the speakers placed very close to the sidewalls, to eliminate an excess of energy at 125Hz. However, as can be seen in fig.4, this excites a big room mode at 61Hz, and results in a lack of energy between 25Hz and 40Hz. This was still not optimum, as I found it led to lumpy, poorly defined bass performance on recordings that had significant low-bass energy. That the speaker does have low-bass extension, however, is shown in this graph by the return to the 0dB level below 25Hz. Nevertheless, I suspect that tube amplifiers, such as JE's Conrad-Johnsons, will not get the best from the Response 4.

In the time domain, the big ProAc is not particularly coherent. Fig.5 shows the impulse response at a distance of 1m. The floor reflection can be seen at the 6.5ms mark; also, a reflection is noticeable about 1.3ms after the main impulse, but I'm not sure whether this was from the edge of the speaker baffle or from the microphone holder. The step response calculated from the impulse response is shown in fig.6: The individual arrivals from the tweeter, midrange units, and woofers can be clearly discerned; all seem to be connected with the same positive-going polarity.

Fig.5 ProAc Response 4, impulse response on HF axis at 1m (5ms time window, 30kHz bandwidth).

Fig.6 ProAc Response 4, step response on HF axis at 1m (5ms time window, 30kHz bandwidth).

The waterfall, or cumulative spectral-decay, plot calculated from the impulse response (fig.7) reveals a very clean initial decay throughout the midrange and treble. A residual resonant mode can be discerned at 4.6kHz—the cursor position—but this is well down in level. It's not surprising that this speaker sounds so clean.

Fig.7 ProAc Response 4, cumulative spectral-decay plot (0.15ms risetime).

Putting aside the problems of getting the Response 4's bass end to sound both even and extended in Jack's room (which is problematic in this region), I was mightily impressed by this speaker. It featured effortless dynamics, a clean, neutral midrange, and highs that never called attention to themselves. And as for the soundstaging: Jack played me Midori's Carnegie Hall recital on Sony Classical; the images of the violin and piano were set well back behind the speaker positions, unambiguously hanging in space, with a complete absence of image bloat or wander. In my humble opinion, the ProAc Response 4 is a true Class A performer (although, at $18,000/pair, it ought to be).—John Atkinson

Footnote 1: All the frequency-response measurements were made using JE's current reference amplifier, the tubed Conrad-Johnson Premier Eight.—John Atkinson
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