Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Emotiva Pro Airmotiv 4s's frequency response in the farfield, and an Earthworks QTC-40, with its small, ¼"-diameter capsule, for the nearfield responses. As the Pro Airmotiv 4s is a powered speaker, I can't publish a conventional sensitivity measurement. However, presenting 330mV of pink noise to the speaker's input and setting its level control to "50" gave an SPL on the tweeter axis of 84dB(B) at 1 meter.
The Airmotiv's small enclosure was impressively rigid, with just a few low-level vibrational modes apparent. Fig.1 is a waterfall plot calculated from the output of an accelerometer fastened to the center of one of the sidewalls. The highest-level mode lay at 340Hz, but its level was still very low in absolute terms. A slightly stronger mode, at 254Hz, could be found on the top panel, but the radiating area of the top panel is very small; this mode should have no audible consequences.
Fig.2 shows the Airmotiv 4s's response above 300Hz on its tweeter axis, averaged across a 30° horizontal window, and with the nearfield responses of the woofer (blue trace) plotted below 300Hz, and that of the port (red) plotted below 1kHz. The treble and bass switches were each set to "0" for these measurements; the complex sum of the nearfield outputs is shown as the black trace below 300Hz. The peak in the upper bass is almost entirely an artifact of the nearfield measurement technique, and the overall high-pass rolloff is much faster than a conventional reflex design's 24dB/octave, suggesting that the Emotiva's bass alignment is sixth-order, to prevent its small woofer from being driven to its excursion limit by infrasonic signals. The port has a sharply defined peak in its output just below 1kHz and peaks between 50 and 80Hz, while the woofer's minimum-motion notch occurs at 60.5Hz. This is the frequency where all the output comes from the port and suggests modest low-frequency extension.
In the time domain, the Emotiva's step response (fig.6) indicates that the tweeter is connected in inverted acoustic polarity, the woofer in positive polarity. The cumulative spectral-decay plot (fig.7) is impressively clean, other than a slight degree of delayed energy in the low treble.
Fig.1 Emotiva Pro Airmotiv 4s, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 1V; measurement bandwidth, 2kHz).
Fig.2 Emotiva Pro Airmotiv 4s, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue), port (red), and their complex sum (black), respectively plotted below 300Hz, 1kHz, 300Hz.
The Emotiva's farfield response is smooth and even, if with a slight rising trend from the midrange through the high treble. Fig.3 shows the effect on this response of switching the speaker's treble control to "+2" and "–2": the range covered by the tweeter can be seen to hinge up and down by 2dB at 10kHz. Switching the bass control from "0" to +2" raised the woofer's output by 1dB at 100Hz, and the port's by 2dB at 60Hz. Switching the control to "+4" raised these levels by another 1 and 2dB, respectively.
Fig.3 Emotiva Pro Airmotiv 4s, effect of treble switch on HF-axis response (5dB/vertical div.).
The Airmotiv 4s's lateral dispersion, referenced to its tweeter-axis response, is shown in fig.4. Though there is a slight flare off axis at the bottom of the region covered by the tweeter, the radiation pattern is otherwise wide and even below 10kHz, suggesting accurate, stable stereo imaging. In the vertical plane (fig.5), a deep suckout develops in the crossover region more than 5° above the tweeter axis, suggesting that high stands will work best with this speaker. The Airmotiv 4s maintains its treble balance up to 15° below the tweeter axis, suggesting that this speaker will work well as a studio monitor when mounted on the mixing console's meter bridge.
Fig.4 Emotiva Pro Airmotiv 4s, 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.5 Emotiva Pro Airmotiv 4s, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.
Fig.6 Emotiva Pro Airmotiv 4s, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Emotiva Pro Airmotiv 4s, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Overall, Emotiva Audio's Pro Airmotiv 4s offers impressive measured performance, especially when you consider that it costs just $299/pair.—John Atkinson
