Apogee Duetta II loudspeaker Measurements
Just how hard is the Duetta to drive? Contrary to rumor, it proved quite tolerable—no less than 5 ohms, almost purely resistive, on the "1" setting, and it would warrant an across-the-board specification of 6.4 ohms, which is an easy load in speaker terms. The amplifier problems arose, not from an excessively low impedance as in the case of the 1-ohm Scintilla, but from its chronic insensitivity (in normal speaker terms) of about 80dB/W. The impedance graph (fig.1) reveals the crossover located at around 500Hz.
Fig.1 Apogee Duetta, impedance magnitude.
Measuring the sound output in 1/3-octave bands, the mid-tweeter response, shown on fig.2, is pretty healthy down to 400Hz. The vertical marker is placed at 12.5kHz, and graphically demonstrates the declining final octave, even when measured directly on-axis, with the mic on the "hottest" angle. Down 3dB at 12kHz, it was -10dB by the 20kHz 1/3-octave band. This graph also shows the output of the bass driver and its excessive level when interfaced with the tweeter, typically 6-8dB too high. (Note that this curve will encompass some significant nearfield effects, particularly in the bass.)
Fig.2 Apogee Duetta, acoustic crossover.
Fig.3 is a composite of the axial response, plus the output at 10 degrees above axis, and 30 degrees off the horizontal axis (toward the center stage). This shows that the output off-axis is pretty well maintained in the desired manner. At 30 degrees to the horizontal, the 12kHz region picked up a little but with no significant improvement in extension. On driver integration alone, the Duetta can be rated fine.
Fig.3 Apogee Duetta, response on listening axis, at 10 degrees above axis, and 30 degrees horizontally off-axis
Finally, we come to the computer-averaged response assessed in the listening space (fig.4), a summation of some 64 responses. In my 80m$s3 room, the 30Hz and 25Hz bands were clearly excessive in level, with the 30Hz rising 12dB above the computed median. Note that the bass rise was part of a rising curve and not an isolated "boom." From the response, it is clear that the general energy trend is rather rich, with the midrange broadly weighted to a maximum in the 400Hz to 1kHz sector (the marker is at 1kHz). Above 1kHz, the output declined some 5dB, though in a fairly controlled manner. The average output held up to 10kHz with a desirably smooth rolloff thereafter. The dotted curve shows the effect of the 1.5 ohm bass resistor—a mild improvement in balance.
Fig.4 Apogee Duetta, spatially averaged response in MC's listening room.
Larger rooms would definitely improve the Duetta's bass-mid balance, though the low bass would remain overpowerful down to 30Hz. For the benefit of those readers not familiar with these room-averaged responses, I can assure you that it is possible to obtain a room curve using this method which runs within ±3dB 25Hz to 200Hz, ±2dB 200Hz to 8kHz, with a gentle rolloff thereafter. (This latter effect is due to the finite directivity of almost all commercial tweeters at higher frequencies.) Blind listening tests have shown a consistently good correlation between perceived response and the room measurement.—Martin Colloms