Aerial Acoustics Model 7B loudspeaker Measurements

Sidebar 3: Measurements

The big Aerial's estimated sensitivity was a little lower than specified, at 84.4dB(B)/2.83V/m, probably due to a lack of energy in the low treble. Its plot of impedance magnitude and phase against frequency (fig.1) revealed the 7B to be a moderately demanding load, with a nominal value in the midrange and bass closer to 4 ohms than the specified 6 ohms. The speaker is basically an 8 ohm design in the treble, however. The saddle in the magnitude trace at 25Hz reveals the tuning frequency of the 3"-diameter port.

Fig.1 Aerial 7B, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Other than one at the tweeter's ultrasonic resonance frequency at 25kHz, the impedance traces are free from wrinkles due to the presence of resonances of various kinds. Examining the panels' vibrational behavior with a simple plastic-tape accelerometer revealed some modes, but these were pushed high in frequency by the bracing. Fig.2, for example, shows a waterfall plot calculated from the accelerometer's output when it was fastened to the cabinet sidewall 12" from the floor. A single mode can be seen at 563Hz, and could be found on all surfaces. However, the fact that it is both high in frequency and low in level means that its subjective effect will be minimal.

Fig.2 Aerial 7B, cumulative spectral-decay plot of accelerometer output fastened to side wall 12" from base. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 shows the acoustic crossover on the tweeter axis, spliced to the nearfield responses of the midrange unit, woofers, and port. The port's output is centered as expected on the 25Hz tuning frequency, while the woofers hand over to the midrange unit around 430Hz, with steep crossover slopes. The overall response, averaged across a 30 degrees window on the tweeter axis and spliced to the complex sum of the woofer and port responses, is shown in fig.4. The bass is down 6dB at a low 23Hz, and the speaker's balance is superbly even and flat on this axis. There is a slight negative plateau in the low and mid-trebles compared with the midrange, which could explain the slightly lower sensitivity, and might also contribute to RD's feeling that the speaker sounded a little laid-back. The tweeter's ultrasonic resonance peak is 15dB high, but this is unlikely to be excited using digital sources.

Fig.3 Aerial 7B, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield and port woofer responses plotted below 300Hz and 700Hz, respectively.

Fig.4 Aerial 7B, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.

Fig.5 shows the speaker's lateral dispersion on the tweeter axis. (Just the changes in response are shown, which is why the tweeter-axis curve is a straight line.) There is a slight flare in the bottom of the tweeter's passband, which will work against the on-axis lack of energy in the same region in all but very large rooms, but the dispersion is otherwise superbly well controlled. This undoubtedly contributes to the speaker's excellent imaging. In the vertical plane (fig.6), the response doesn't change significantly over quite a wide range of listener height, which is just as well given the tweeter's 39" distance from the floor, which is a little on the high side.

Fig.5 Aerial 7B, lateral 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.6 Aerial 7B, vertical response family at 50", from back to front: differences in response 15 degrees-5 degrees above HF axis, reference response, differences in response 5 degrees-10 degrees below HF axis.

The step response (fig.7) indicates that all the drive-units are connected with positive acoustic polarity, while the cumulative spectral-decay plot (fig.8) is superbly clean, implying freedom from treble grain and hardness.

Fig.7 Aerial 7B, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

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

All things considered, these measurements indicate a superbly well-engineered loudspeaker. It's no surprise that Bob Deutsch liked its sound so much.—John Atkinson

Aerial Acoustics
P.O. Box 81248
Wellesley, MA 02481
(781) 235-7715