Aerial Acoustics Model 8 loudspeaker Measurements

Sidebar 3: Measurements

With an estimated B-weighted sensitivity of 85dB/2.83V/m, the Aerial 8 will need a reasonably high-powered amplifier to sing at lifelike levels. However, its impedance plot (fig.1) reveals that it only drops below 5 ohms in the upper bass and lower midrange and that the electrical phase angle, other than in the mid-bass, is relatively benign. Other than the small wrinkle in the traces at 25kHz, due to the tweeter's primary dome resonance, this graph is free from cabinet-induced problems. The "saddle" in the magnitude trace at 20Hz indicates the tuning of the large, 2.5"-diameter port.

Fig.1 Aerial Acoustics 8, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

This low frequency is confirmed by fig.2, the leftmost trace of which shows the port's response measured in the nearfield. In absolute terms, the port output is rather low compared with that of the woofer (the fig.2 trace that peaks between 45Hz and 225Hz), while the output of the midrange unit seems to overlap the woofer somewhat before steeply rolling off below 200Hz. Other than a slight energy excess in the low treble, the rest of the 8's upper-range, response appears to be very flat. There is, of course, a large ultrasonic peak at the tweeter resonance frequency, but this will be subjectively innocuous (particularly if you only play CDs, which have no energy above 22kHz capable of exciting this resonance). It is too easy for audiophiles to see a metal-dome tweeter and assume the presence of a "metallic" coloration. In my experience, this character is almost always associated with problems at the top of the woofer's passband—even if it doesn't use a metal diaphragm! As Michael Fremer noted, the Aerial 8's tweeter doesn't sound at all "metallic."

Fig.2 Aerial Acoustics 8, anechoic response of midrange/tweeter on tweeter axis at 50", corrected for microphone response, with the nearfield midrange, woofer, and port responses plotted below 300Hz.

Fig.3 shows the 8's overall response, made by splicing the complex sum of the nearfield woofer, midrange, and port outputs to the speaker's farfield response, averaged across a 30 degrees window on the tweeter axis. In the bass, the port doesn't give full measure in the low-bass, but the overall rollout is moderate, the speaker being around 6dB down from the 1kHz reference level at a low 20Hz. This is almost a full-range design. The midrange and treble are very flat on this axis, but broad energy peaks can be seen in both the upper midrange and the mid-bass. MF was persistently bothered by the latter, which will as he noted, be room-dependent. He didn't remark on the former, which, all things being equal, might endow the speaker with a slight nasal coloration.

Fig.3 Aerial Acoustics 8, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of woofer and port responses plotted below 300Hz.

Things are never equal in the world of speaker design, however, and the Aerial 8's horizontal dispersion plot (fig.4) indicates a slight energy excess to the speaker's sides in the presence region, which will mitigate against the audibility of the peak an octave lower. And note the superbly well-controlled dispersion in the treble in this graph, something that always correlates with equally superb imaging, in my experience. In the vertical plane (fig.5), the 8 is relatively unfussy about listening axis as long as you don't stand up, in which case a deep suckout appears in the upper crossover region.

Fig.4 Aerial Acoustics 8, horizontal response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis; reference response; differences in response 5 degrees-90 degrees off-axis.

Fig.5 Aerial Acoustics 8, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-5 degrees above-axis; reference response; differences in response 5 degrees-10 degrees below-axis.

In the time domain, the 8's step response (fig.6) indicates that all three drive-units are connected with the same positive acoustic polarity, but that the speaker is not time-coherent. Fig.7, the Aerial's cumulative spectral-decay plot on the tweeter axis is astonishingly clean from resonant hash of any kind, with the exception of the harmless tweeter mode mentioned earlier and a slight problem at 1kHz, this associated with the on-axis peak at the same frequency. Again, however, I must note that MF was not bothered by any audible problems in this region.

Fig.6 Aerial Acoustics 8, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 Aerial Acoustics 8, cumulative spectral-decay plot at 50" (0.15ms risetime).

Finally, the Aerial's big cabinet panels are well-braced and rigid. A waterfall plot (fig.8) calculated from the output of a simple PVDF-tape accelerometer fastened to the side wall above the woofer reveals only a couple of vibrational modes, and the most significant of these, at 500Hz, is high enough in level that it probably won't lead to any subjective problems.—John Atkinson

Fig.8 Aerial Acoustics 8, cumulative spectral-decay plot of accelerometer output fastened to cabinet sidewall above woofer. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)