Aerial Acoustics Model 7T loudspeaker Measurements
Sidebar 3: Measurements
My estimate of the Aerial Acoustics Model 7T's B-weighted sensitivity on its tweeter axis was 86.6dB/2.83V/msignificantly lower than the specified 89dB. The electrical impedance remains between 4 and 8 ohms over much of the audioband (fig.1), but drops to 3.5 ohms in the upper bass and low treble, and to 3 ohms in the low bass. Although the electrical phase angle remains low at almost all frequencies, this speaker will work best with a good amplifier rated at 4 ohms.
The traces in fig.1 are free from the wrinkles and discontinuities that would suggest the presence of cabinet resonances. However, when I investigated the vibrational behavior of the enclosure panels with a simple plastic-tape accelerometer, a single mode at 530Hz was present on the sidewalls adjacent to the midrange unit (fig.2), and a second mode at 370Hz at other places, including the top panel. These resonances were all low in level, of high Quality Factor (Q), and of sufficiently high frequency that they should not introduce any colorations.
The saddle centered between 20 and 30Hz in the impedance-magnitude trace suggests that the rear-facing port is tuned in this region; the corresponding minimum-motion point in the woofers' output (fig.3, blue trace), which is when the back pressure from the port resonance holds the woofer cones still, occurs at 27Hz. The port's output (red trace) peaks between 20 and 40Hz, and the upper-frequency rolloff is clean, with no pipe resonances visible. The woofers behave identically, and their combined output crosses over to the midrange unit (green trace) a little higher than the specified 400Hz. The crossover slopes appear to be symmetrical third-order, and the woofer's low-pass rolloff is very clean. All of the drive-units appear to offer even, flat responses within their passbands, though a small peak is evident between 2 and 3kHz. The ring-radiator tweeter's response extends to 30kHz, the upper limit of this graph.
Fig.4 shows how these individual responses sum in the farfield on the 7T's tweeter axis, without the grille. With the grille in place (not shown) there is less energy in the mid-treble, and a little more above 10kHz. The bump in the upper bass is entirely due to the nearfield measurement technique, which assumes that the drive-units are mounted in a baffle infinitely wide and tall. However, it does look as if the 7T's woofer alignment is a little overdamped, trading bass weight against clarity. The Aerial's low frequencies are still extended, however, with the usual 24dB/octave rolloff below the port tuning frequency. Other than the small peak between 2 and 3kHz, the Model 7T's response is superbly flat. This graph was produced with serial no.074541; when I repeated the measurement on no.074542, the results (not shown) were identical, which demonstrates excellent pair matching but also confirms that the small peak in the low treble is characteristic of the design. I note that Kal Rubinson didn't comment on any brightness or coloration that might have resulted from this behavior, but I suspect that it does slightly emphasize recorded detail.
The Aerial 7T's lateral dispersion (fig.5) is textbook in the midrange and low treble, with the off-axis output smooth, even, and free from "hot spots"all things that correlate with excellent, stable stereo imaging. This graph does indicate that the tweeter's output to the sides falls off more rapidly than the norm in the top audio octave and above; the 7T might sound too mellow in very large or overdamped rooms. In the vertical plane (fig.6), the use of high-order crossover filters means that the speaker's balance doesn't change over a usefully wide window centered on the tweeter axis, which is a high 41" above the floor.
The Aerial's step response on the tweeter axis (fig.7) suggests that all four drive-units are connected with positive acoustic polarity, and that the decay of each unit's step blends smoothly with the rise of the driver next lower in frequency. This correlates with the excellent integration of their outputs seen in the speaker's frequency response (fig.4). The cumulative spectral-decay plot (fig.8) is extremely clean, other than a slight amount of delayed energy at the frequency of the small peak in the mid-treble.
The measured performance of the Aerial Acoustics Model 7T confirms both the speaker's excellent sound quality and the expertise of its designers.John Atkinson