ProAc Response 1S loudspeaker Measurements

Sidebar 2: Measurements

The Response 1S's B-weighted sensitivity was to spec at 85dB/W/m. Its impedance plot (fig.1) suggests that it will be relatively easy to drive, with a minimum magnitude of 5 ohms between 200 and 300Hz.


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

Moving to the frequency domain, fig.2 shows the individual amplitude responses of the woofer, tweeter, and port. As suggested by the impedance plot, the port's bandpass peaks just above 70Hz, which coincides, as it should, with the minimum-motion point of the woofer. (At this frequency, the back pressure on the woofer cone is so great that it remains still, and all the speaker's output comes from the port.) The 1.5"-diameter port does have a big peak in its output at 820Hz, presumably due to a resonance of some kind. However, as the port is mounted on the speaker's rear panel, facing away from the listener, it's possible that its subjective effect will be minimal. Further up in frequency, the woofer has a noticeable step in its response just above 1kHz—before it crosses over to the tweeter around 4kHz. Its output drops rapidly above that frequency. The tweeter's response is pretty flat within its passband.


Fig.2 ProAc Response 1S, individual quasi-anechoic responses of woofer and tweeter on tweeter axis at 45" and corrected for microphone response, with nearfield woofer and port responses below 300Hz and 1kHz, respectively.

Fig.3 shows how these individual responses sum on the tweeter axis at a microphone distance of 45". The balance is commendably flat, apart from the step and notch in the woofer's output between 1 and 2kHz. From my experience, aberrations like this tend to add a nasal character to a speaker's presentation. However, I note that WP didn't remark on any such coloration. In the bass, the speaker rolls off steeply below 80Hz, due to the woofer output being out of phase with the port below the latter's tuning frequency. The $*–6dB point is a high 66Hz; as WP writes above, that's the nature of a 5" woofer in a small, ported enclosure.


Fig.3 ProAc Response 1S, anechoic response on tweeter axis at 45" averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses below 300Hz.

Laterally, as might be expected from the small cabinet, the Response 1S offers reasonably good dispersion (fig.4), though the notch in the low treble deepens off-axis. The change in response is also a little more untidy on the side of the baffle away from the offset tweeter. The woofer's dispersion can be seen to narrow somewhat in its top octave—more so than might be expected from its radiating diameter. Vertically (fig.5), the speaker maintains its on-axis balance as long as the listener sits with his or her ears between the tweeter and the bottom of the woofer. Above that height, the treble gets more pronounced, with a large suckout eventually developing in the crossover region.


Fig.4 ProAc Response 1S, horizontal response family at 45", normalized to response on tweeter axis, from back to front: differences in response 90°–5° off-axis on other side of baffle; reference response; differences in response 5°–90° off-axis on tweeter side of baffle.


Fig.5 ProAc Response 1S, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 15°–5° above tweeter axis; reference response; differences in response 5°–15° below tweeter axis.

In the time domain, the ProAc's step response (fig.6) reveals that, although the tweeter and woofer are connected with the same positive acoustic polarity, the speaker is not time-coherent, in that the woofer-output lags the tweeter by about 250µs. The irregularities in the woofer's decay presumably correspond with the step in its output above 1kHz; I suspect it's a surround problem rather than a cone-breakup mode.


Fig.6 ProAc Response 1S, step response on tweeter axis at 45" (5ms time window, 30kHz bandwidth).

The corresponding cumulative spectral-decay, or waterfall, plot (fig.7) does reveal some delayed energy in this region, as well as some low-level hash at the bottom of the tweeter's passband. Overall, however, the initial decay is very clean—always a good sign. For comparison, fig.8 shows the waterfall plot of an earlier sample of the Response 1S (serial number P02293), which used the earlier woofer. Though the initial decay is still clean, this driver was hashier at the top of its passband, with a pronounced step at 5kHz.


Fig.7 ProAc Response 1S, serial number P02808, cumulative spectral-decay plot at 45" (0.15ms risetime).


Fig.8 ProAc Response 1S, serial number P02293, cumulative spectral-decay plot at 45" (0.15ms risetime).

Finally, using a simple accelerometer to examine the cabinet's resonant behavior revealed one strong mode to exist, just above 400Hz (fig.9). This is high enough in frequency, however, that its subjective effect should be small.


Fig.9 ProAc Response 1S, cumulative spectral-decay plot of accelerometer output fastened to center of enclosure side panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth 2kHz.)

All in all, a good set of measurements, apart from the woofer anomaly in the high midrange. Given the enthusiasm of WP's comments, and my positive impressions during my own auditioning, it just goes to prove that any one measurement cannot be used to predict the quality of a loudspeaker. Rather, what's important is the balance between all the measured aspects of performance; ProAc designer Stewart Tyler appears to be a master at getting that balance right.—John Atkinson

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funambulistic's picture

I first heard these little gems in '98 or so. My impression was that they were more of a musical instrument than transducer and everything I threw at them sounded lovely. I am sure they were not the most "accurate" speakers around (otherwise, most of the music I demoed would have sounded terrible). Unfortunately, at the time, the price was too dear... Ruark was another speaker company that had that "instrument" sound. RIP Wes!