Apogee Centaurus Slant 6 loudspeaker Measurements

Sidebar 3: Measurements

John Atkinson measured the Slant 6 in Stereophile's test lab and presented the graphs to me after I'd completed the auditioning and written the rest of the review.

The 6's calculated B-weighted sensitivity was 83.2dB for a drive level of 2.83V at 1m—a rather low value. The impedance plot (fig.1) shows a minimum impedance of 3.78 ohms at 120Hz—with a 4 ohm dip at 2kHz—and an impedance of less than 5 ohms above 1kHz. The port tuning can be seen as the impedance dip at 37Hz, implying reasonably good LF extension.

Fig.1 Apogee Slant 6, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The impedance curve suggests that the Slant 6 is a moderately demanding load on a power amplifier. When the impedance curve is considered along with the lowish measured sensitivity, the Slant 6 is clearly not a candidate for use with 30Wpc British integrated amplifiers. Instead, a fairly muscular amplifier with the ability to increase its power as the load impedance drops is recommended. A small tubed amplifier may be a good match tonally, but won't deliver the dynamics and bass extension of which the Slant 6 is capable. Incidentally, the 150Wpc (rated power into 8 ohms) McCormack DNA-1 Deluxe showed no signs of strain, even during loud passages and high playback levels. The lower-powered, $1295 DNA-0.5, reviewed by TJN in February, may be an ideal match for the Slant 6 in price, tonal balance, and power output.

Fig.2 shows the responses of the port (green trace), woofer (blue), and ribbon (red) individually measured on the ribbon axis. The maximum port output at 37Hz is coincident with the woofer's minimum-motion point (the dip in the woofer trace). Although specified at 1kHz, the crossover transition actually appears at 1154Hz (the point where the woofer and ribbon traces are equal in level). Though the woofer rolloff shows no significant breakup modes, even well out of its passband, there are a suspicious peak and dip in its output in the upper midrange. Finally, the ribbon's response is smooth and flat, but with a slight depression in the top octave.

Fig.2 Apogee Slant 6, acoustic crossover on ribbon axis at 53", 37" from the floor, corrected for microphone response, with nearfield woofer (blue) and port (green) responses plotted below 300Hz and 800Hz, respectively.

The Slant 6's output averaged over a 30° horizontal window, combined with the complex sum of the nearfield woofer and port responses (fig.3), shows a smooth overall response. The low-frequency –6dB point occurs at 30Hz—a respectable figure for a single 6.5" woofer, although this suggests the woofer is being driven fairly hard at the lowest frequencies at high spls. A slight rise in the bass spanning three octaves (40Hz–320Hz) may be partly responsible for the plummy quality noted during the auditioning. We can also see an excess of energy in the band between 1kHz and 3kHz (exactly the band in which the ear is most sensitive, both to quiet sounds and to small amplitude variations), which may account for my perception that the 6 has slightly exaggerated detail, a tendency toward brightness, and an emphasis on instrumental harmonics rather than fundamentals. Note that I didn't find the Slant 6 bright per se, but that I heard an emphasis in a certain band that increased the perceived amount of musical detail and emphasized instrumental harmonics over fundamentals.

Fig.3 Apogee Slant 6, anechoic response on ribbon axis at 53" averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses below 300Hz.

The suckout between 600Hz and 900Hz is a crossover effect caused by the driver outputs not adding in-phase directly in front of the ribbon. (The microphone was positioned 37" from the floor, almost exactly halfway up the ribbon, for all these measurements.)

Fig.4 shows how the Slant 6's response changes over the horizontal axis, with the on-axis response subtracted from all the traces so that only the differences in response to the loudspeaker's sides are shown. Moving off-axis on the ribbon side of the baffle, the midrange suckout fills in; on the woofer side, the suckout worsens. By placing the ribbons on the inside edges of the speakers (the way the 6es are meant to be set up), this suckout will not be a factor in the speakers' direct sound. However, the colored off-axis response will be reflected to the listener by the room's side walls, which possibly contributes to my feelings of a slightly lean balance, overall. As usual, some side-wall absorption will smooth the Slant 6's tonal balance. Note that the 6 has only moderate dispersion above 5kHz, which will add to the depressed top-octave on-axis balance.

Fig.4 Apogee Slant 6, horizontal response family at 53", normalized to response on ribbon axis, from back to front: differences in response 90°–5° off-axis on woofer side of baffle; reference response; differences in response 5°–90° off-axis on ribbon side of baffle.

Measuring the response as a function of vertical axis produced the graph of fig.5. Again, only the differences in response are shown (the straight line reference is at a measurement height of 37"). We can see a dramatic rolloff in the ribbon's response as we get outside a very narrow (±10°) vertical window. You don't need MLSSA to know this about the Slant 6—just stand up when listening, and you can hear the instant drop in upper midrange and treble energy. This ribbon's very narrow vertical dispersion—due to its vertical dimension being larger than the wavelengths of the sounds it produces—results in less reflected energy from the floor and ceiling, which is a good thing. Fig.5 confirms the necessity of getting just the right relationship between tilt angle and listener height to achieve a smooth tonal balance.

Fig.5 Apogee Slant 6, vertical response family at 53", normalized to response on ribbon axis 37" from the floor, from back to front: differences in response 45°–5° above ribbon axis; reference response; differences in response 5°–45° below ribbon axis.

The Slant 6's impulse response (not shown) was clean, with none of the ultrasonic ringing often seen in loudspeakers with metal-dome tweeters. The step response (fig.6) is more revealing of the 6's time behavior: the ribbon and woofer are wired with the same acoustic polarity (the ribbon's output is the first spike, the woofer's output the later hump), with the ribbon leading the woofer in time. Although wired with the same polarity, the two drivers share the same flat baffle, which means that the two drivers' acoustic centers aren't perfectly aligned, resulting in the ribbon energy leading the woofer energy. As the suggested setup has the listener off-axis on the ribbon sides of the baffle, this will result in an even larger time lead for the ribbon axis.

Fig.6 Apogee Slant 6, step response on ribbon axis at 53" (5ms time window, 30kHz bandwidth).

Finally, the cumulative spectral-decay, or waterfall, plot of fig.7 shows the Slant 6's ribbon to be very well-behaved. The Slant 6 stores very little energy, instead decaying quickly and cleanly, as seen by the white space and lack of garbage in the lower traces. This is to be expected from a low-mass ribbon driver, although not all ribbons decay this cleanly. Apart from some energy-storage problems at the top of the woofer's passband, the waterfall plot correlates with my impression that the 6 had superb transient performance.

Fig.7 Apogee Slant 6, cumulative spectral-decay plot at 53" (0.15ms risetime).

Nothing in the Slant 6's measured performance surprised me. The drastic change in the amount of ribbon energy at the listening position as a function of the vertical axis confirms that the 6's tonal balance is very dependent on the amount of tilt-back and listening height. Be prepared to adjust the front spikes, listen, and adjust again in an iterative process to get the optimal tonal balance.—Robert Harley

Apogee Acoustics
PO BOX 1316, Caloundr, Sunshine Coast
Queensland 4551, Australia
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