Stand Loudspeaker Reviews

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Acora SRB's frequency response in the farfield, and an Earthworks QTC-40 mike for the nearfield and in-room responses. My initial measurements were performed on one of the original samples auditioned by Julie Mullins, serial number TIB0010. After Acora found out that the first samples had been incorrectly assembled with the tweeters wired in the wrong polarity, which resulted in a massive suckout in the crossover region, they submitted new samples. I performed a complete set of measurements on serial number TIB0021. Except where noted, all the following comments apply to the new sample.

Acora specifies the SRB's sensitivity as 86.5dB/W/m; my estimate was within experimental error of this, at 85.9dB(B)/2.83V/m. Although the SRB's impedance is specified as 8 ohms, the solid trace in fig.1 shows that the impedance magnitude remains below 8 ohms for almost the entire midrange and treble. The minimum magnitude is 5.05 ohms between 930Hz and 1kHz. The electrical phase angle (dashed trace) is occasionally high, though the impedance magnitude is also high at the same frequencies, ameliorating the drive difficulty for the partnering amplifier. Nevertheless, the "equivalent peak dissipation resistance" (EPDR, footnote 1) is close to 4 ohms for much of the audioband and drops to 2.54 ohms between 120Hz and 130Hz. As is often the case, this loudspeaker will work best with amplifiers that are comfortable driving loads below 4 ohms.

There are two very slight discontinuities between 700Hz and 1kHz in the traces in fig.1 that imply that there are resonances of some kind present at these frequencies. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found a single resonant mode just below 900Hz on all the panels (fig.2). The use of a very dense enclosure material has pushed this resonance higher in frequency than is usually found in a conventional loudspeaker this size. Although the frequency of the resonance is close to that of the musical note B5, its Q (Quality Factor) is very high, which means it should not affect sound quality.

The saddle centered at 66Hz in the impedance magnitude trace suggests that this is the tuning frequency of the small port on the rear panel, which is confirmed by the fact that the minimum-motion notch in the woofer's output (fig.3, blue trace), which is when the port resonance holds the cone stationary, also lies at that frequency. The port's output (fig.3, red trace) peaks between 40Hz and 100Hz, though the clean upper-frequency rolloff is disturbed by two high-Q resonant modes between 600Hz and 800Hz. As these twin peaks are low in level and the port faces away from the listener, I doubt they will have any effect on sound quality.

The black trace below 300Hz in fig.3 shows the complex sum (amplitude and acoustic phase, the latter taking into account the fact that the port is on the back panel) of the nearfield woofer and port responses. The boost in the response in the upper bass will in part be due to the nearfield measurement technique, which assumes that the radiators are mounted in a baffle that extends to infinity in both vertical and horizontal planes. The SRB's woofer alignment does appear to be somewhat underdamped, however.

What is the optimal listening/measuring axis? Acora's Scott Sefton told Editor Jim Austin and reviewer Julie Mullins that the preferred listening axis was anywhere from slightly below the tweeter axis to the midpoint between the woofer axis and the tweeter axis. As Sefton had clarified that these axes were perpendicular to the slant of the baffle, the midpoint between the tweeter and woofer translated to the axis level with the tweeter, so that is where I measured the SRB's farfield behavior.

The response, averaged across a 30° horizontal window centered on the tweeter axis, is shown as the black trace above 300Hz in fig.3. Though the response in the crossover region is flat, there is a broad peak in the upper midrange, which implies a lack of baffle-step compensation in the crossover. The tweeter's output between 5kHz and 20kHz is 3–5dB higher than it is in the presence region, almost matching the level of the midrange peak.

The manual recommends that the loudspeakers not be toed-in to the listening position, so perhaps the SRB's balance will improve to its sides. Fig.4 shows the Acora's horizontal radiation pattern, with the off-axis responses normalized to the response on the tweeter axis, which thus appears as a straight line in the center of the graph. The upper-midrange emphasis does smooth out a little. The speaker also gets quite directional in the top octaves, which will compensate for the excess of on-axis energy in this region. The Acora's vertical dispersion is shown in fig.5, with the off-axis responses shown up to 45° above and below the response on the tweeter axis. A suckout develops in the crossover region immediately above the tweeter axis, confirming Acora's recommendation about the optimal listening axis being below the tweeter.

To examine the SRB's spatially averaged response in my room, I set the loudspeakers up on 24" stands, positioned 90" apart and 11' 4" from the position of my head. This was where the Marten Oscar Duos that Michael Fremer reviewed in the November 2020 issue had worked well. The Acoras were not toed-in to the listening position.

The red trace in fig.6 shows the SRB's spatially averaged response in my room, the blue trace that of the Marten Oscar Duo, with the loudspeakers' levels matched in the midrange (footnote 2). The Acoras' low frequencies are shelved down below about 80Hz, which suggests that the Acora speaker needs to be placed closer to the wall behind it than I could arrange in my room. The SRB's output is significantly higher than that of the Oscar Duo in the upper midrange, but less so in the treble.

Interpreting the Acora SRB's in-room behavior is difficult, as its balance will depend very heavily on the nature of the music being played. If the level at 900Hz is taken as the neutral reference, as is likely with some recordings, the low treble will sound polite, the lower midrange will be shelved down, and the low frequencies will be lightweight. But if, with other recordings, the bass and midrange sound correctly balanced, the upper midrange will sound very forward and also very articulate.

In the time domain, the Acora SRB's step response on the tweeter axis (fig.7) indicates that both drive-units are connected in positive acoustic polarity. The tweeter's step arrives first at the microphone and the decay of its step smoothly blends with the positive-going start of the woofer's step, which confirms the good blend of their outputs in the frequency domain. There is a strong ripple in the decay of the woofer's step, which correlates with some delayed energy in the region of the upper-midrange peak in the Acora SRB's cumulative spectral-decay plot (fig.8). The decay in the region covered by the tweeter, however, is superbly clean. (As always with my CSD plots, ignore the small ridge just below 17kHz, which is due to interference from the computer monitor's line-scan frequency.)

I was intrigued by the Acora SRB's use of a massive enclosure, but I was puzzled by some aspects of its measured performance.—John Atkinson

Footnote 2: I generate this graph by averaging 20 1/6-octave–smoothed spectra, taken for the left and right speakers individually using a 96kHz sample rate, in a vertical rectangular grid 36" wide by 18" high and centered on the position of my head.