Sidebar 3: Measurements
Before beginning any measurements, I powered up the WiSA transmitter/hub, installed the SA Cockpit app on my iPad mini, connected the app to the hub via my Wi-Fi network, paired the hub with one of the speakers, and connected my MacBook Pro's USB port to the hub. Apple's AudioMIDI utility revealed that the USB port accepts 16- and 24-bit integer data sampled at all rates from 44.1kHz to 192kHz. The USB Prober app identified the WiSA hub as "Stereo Hub HT" from "Hansong (Nanjing)" and indicated that the USB port operated in the isochronous adaptive mode rather than the preferred isochronous asynchronous mode.
I used the SA Cockpit app to disable any equalization settings then connected the analog output of my DRA Labs MLSSA system to the XLR jack on the speaker's rear panel. (The speaker was set to Front Left.) This input's impedance was close to 15k ohms in the bass and midrange, dropping inconsequentially to 7k ohms at the top of the audioband. I used a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the System Audio Legend 40.2 Silverback DS's farfield behavior and dispersion. I used an Earthworks QTC-40 mike for the nearfield responses.
System Audio doesn't specify the 40.2 Silverback's sensitivity. Sending the speaker a pseudorandom noise signal with a 20kHz bandwidth and an amplitude of 100mV peak–peak gave an SPL of 95dB(C)/slow ballistics at 1m, measured with the Studio Six SPL Meter app on my iPhone. The sensitivity was set to "0dB" with the rear-panel switch for this measurement. Setting it to "–6dB" reduced the SPL by exactly 6dB; setting it to "+6dB" increased the SPL by 6dB.
Overall, the System Audio Legend 40.2 Silverback DS offers excellent measured performance, even without the equalization options possible with the WiSA transmitter/hub.—John Atkinson
Footnote 1: This means that the loudspeaker is firing into hemispherical space rather than a full sphere. A speaker that has a truly flat response in the usual "4pi" space will therefore appear to have a boosted upper-bass output with a nearfield measurement, the center frequency of that boost depending on the physical dimensions of the speaker and the woofer alignment. See stereophile.com/content/measuring-loudspeakers-part-three-page-6. Footnote 2: The speaker JA measured seems to have had RAM TWEAK DS1 enabled. See Kal's discussion in the main review text.—Jim Austin
Before beginning any measurements, I powered up the WiSA transmitter/hub, installed the SA Cockpit app on my iPad mini, connected the app to the hub via my Wi-Fi network, paired the hub with one of the speakers, and connected my MacBook Pro's USB port to the hub. Apple's AudioMIDI utility revealed that the USB port accepts 16- and 24-bit integer data sampled at all rates from 44.1kHz to 192kHz. The USB Prober app identified the WiSA hub as "Stereo Hub HT" from "Hansong (Nanjing)" and indicated that the USB port operated in the isochronous adaptive mode rather than the preferred isochronous asynchronous mode.
I used the SA Cockpit app to disable any equalization settings then connected the analog output of my DRA Labs MLSSA system to the XLR jack on the speaker's rear panel. (The speaker was set to Front Left.) This input's impedance was close to 15k ohms in the bass and midrange, dropping inconsequentially to 7k ohms at the top of the audioband. I used a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the System Audio Legend 40.2 Silverback DS's farfield behavior and dispersion. I used an Earthworks QTC-40 mike for the nearfield responses.
System Audio doesn't specify the 40.2 Silverback's sensitivity. Sending the speaker a pseudorandom noise signal with a 20kHz bandwidth and an amplitude of 100mV peak–peak gave an SPL of 95dB(C)/slow ballistics at 1m, measured with the Studio Six SPL Meter app on my iPhone. The sensitivity was set to "0dB" with the rear-panel switch for this measurement. Setting it to "–6dB" reduced the SPL by exactly 6dB; setting it to "+6dB" increased the SPL by 6dB.
Fig.1 System Audio 40.2 SB, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall level with the lower woofer. (Measurement bandwidth, 2kHz).
The System Audio's enclosure seemed inert when I rapped it with my knuckles. When I investigated the panels' vibrational behavior with a plastic-tape accelerometer, I found no significant resonant modes on the loudspeaker's top and back. The side panels were slightly livelier, with a low-level mode dominant at 859Hz with the accelerometer level with the lower woofer (fig.1) and another mode at 594Hz with it level with the midrange unit. As these modes are low in level, have a high Q (Quality Factor), and are relatively high in frequency, they are unlikely to have audible consequences.
Fig.2 System Audio 40.2 SB, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the woofer and midrange nearfield responses plotted below 310Hz.
The two woofers behave identically below 200Hz, though the upper woofer extends a little higher in frequency than the lower. They crossed over to the midrange unit around 250Hz. (The midrange unit's output appeared to extend below this frequency, but this might be due to crosstalk from the closely adjacent woofers.) The black trace below 310Hz in fig.2 shows the summed nearfield response of the woofers and midrange unit. The 3dB rise in the mid- and upper-bass is due to the nearfield measurement technique, which assumes that the drive units are mounted in a true infinite baffle (footnote 1). The System Audio's low frequencies are impressively extended for a relatively small loudspeaker; the output is down by 6dB at 27Hz, though the high-pass rolloff is much faster than a sealed enclosure's 12dB/octave slope.
The black trace above 310Hz in fig.2 shows the 40.2 Silverback DS's quasi-anechoic farfield response at a 50" microphone distance, averaged across a 30° horizontal window centered on the tweeter axis, taken without the grille. The response is generally even, though there are some small peaks and dips in the upper midrange and a gently rising trend in the top two audio octaves, reaching +5.1dB at 15.9kHz (footnote 2). Repeating this measurement with the grille reduced the level between 3kHz and 12kHz by 2dB.
Fig.3 System Audio 40.2 SB, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.4 System Audio 40.2 SB, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 20–5° above axis, reference response, differences in response 5–15° below axis.
Fig.3 shows the 40.2 Silverback DS's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line. The radiation pattern is impressively even and well controlled, which is known to correlate with accurate and stable stereo imaging. The dispersion narrows above 10kHz, which suggests that the excess of top-octave energy on-axis could be ameliorated by not toeing in the speakers to the listening position. The System Audio speaker's radiation pattern in the vertical plane, again normalized to the response on the tweeter axis, which is 35" from the floor with the speaker supported on its feet, is shown in fig.4. A sharply defined suckout at 2.66kHz develops more than 10° above the tweeter axis, indicating that this is the crossover frequency between the midrange unit and the tweeter.
Fig.5 System Audio 40.2 SB, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
In the time domain, the 40.2's step response on the tweeter axis is shown in fig.5. The speaker's crossover, implemented with DSP, adjusts the timing of the four drive units so that they arrive simultaneously at the microphone. The step response is therefore a perfect time-coincident right triangle, though the excess of top-octave energy seen in fig.2 correlates with a significant overshoot at the start of the step. Note that there is a latency of 27.5ms, the step not arriving at the microphone unit until 31.2ms rather than the usual 3.7ms.
Fig.6 System Audio 40.2 SB, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The 40.2's cumulative spectral-decay, or waterfall, plot (fig.6) is very clean overall, though some low-level delayed energy is present at the top of the midrange unit's passband. (As always with my cumulative spectral-decay plots, ignore the ridge of delayed energy close to 16kHz, which is due to interference from the MLSSA host PC's video circuitry.)
Footnote 1: This means that the loudspeaker is firing into hemispherical space rather than a full sphere. A speaker that has a truly flat response in the usual "4pi" space will therefore appear to have a boosted upper-bass output with a nearfield measurement, the center frequency of that boost depending on the physical dimensions of the speaker and the woofer alignment. See stereophile.com/content/measuring-loudspeakers-part-three-page-6. Footnote 2: The speaker JA measured seems to have had RAM TWEAK DS1 enabled. See Kal's discussion in the main review text.—Jim Austin
