Audio Physic Kronos loudspeaker Measurements

Sidebar 3: Measurements

The Kronos offered a little higher sensitivity than the norm, at an estimated 89dB(B)/2.83V/m, though this is 2dB below the specified figure. Note that this is the sensitivity of the midrange-treble head unit, as the woofer section is powered. Similarly, the speaker's impedance graph (fig.1) shows the behavior of the head unit only. The magnitude stays close to 5 ohms throughout the unit's passband. However, the combination of 4.9 ohms and a -46 degree electrical phase angle at 250Hz will cause problems for amplifiers that can't deliver much in the way of current.

Fig.1 Audio Physic Kronos, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.1 is free from the wrinkles and glitches that would imply the existence of cabinet resonances. Even so, I did find two resonant modes to be present in the lower midrange, one on the curved front of the bass enclosure (fig.2), the other on the curved front of the head unit (fig.3). These are of reasonably high Q, so might not be fully excited with typical music. However, if they are excited, their frequencies are sufficently low that they might add a slight thickening to the sound of midrange-dominant instruments.

Fig.2 Audio Physic Kronos, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the woofer cabinet's front panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 Audio Physic Kronos, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the head unit's front panel below the coaxial driver. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The Kronos woofer module offers a considerable degree of adjustment, in order to optimize its in-room integration with the head unit. Rather than show the actual subwoofer responses with all the possible settings of its controls, I took a nearfield measurement of its response with the crossover switched out of circuit and with the Room Gain control set to "0." I then subtracted that reference response from a series of nearfield responses taken with Room Gain varying from "5" to "30" with the Crossover control switched off, then from another series of responses taken with Room Gain set to "0" and Crossover varying from "1" to "5." Neither control affects the behavior of the midrange-treble unit, incidentally.

The results of this work are shown in fig.4. Setting Room Gain to "30"—top, red trace at the left of the graph—boosts the low bass by 9dB at 20Hz and 16dB at 10Hz, while setting it to "5" (black trace) provides just 1dB of bass boost. The smaller the room in which the Kronos is used, the lower Room Gain should be set to achieve flat 20Hz response. To the right of this graph are shown the crossover curves, with the slowest low-pass rolloff being "5" (black trace) the steepest "1" (red)—the opposite of what I was expecting. Taken with the effect of the subwoofer's polarity-invert switch and the volume control, the Kronos owner should be able to get an even transition in the lower midrange between the woofer module and the head unit.

Fig.4 Audio Physic Kronos, effect of woofer Room Gain control (left) set to (from bottom to top) "5," "10," "15," "20," "25," "30"; and of the woofer Crossover control set to (from bottom to top) "1," "2," "3," "4," "5."

The unequalized nearfield response of the woofers—with Crossover off and Room Gain set to "0"—is the red trace shown to the left of fig.5. The alignment is overdamped, with a gentle rolloff below 100Hz. This is sensibly chosen, in my view, because it will allow the speaker to work well in small rooms without sounding ponderously heavy, as do so many bulky speakers. But in larger rooms, the Room Gain setting can be increased to flatten and extend the low frequencies, while maintaining the superb feeling of bass control that MF noted in his auditioning. The green trace in fig.5 was taken with Room Gain set to "30"—the Kronos' bass still shelves down slightly, but is otherwise superbly extended—and with Crossover set to "1" to give the least amount of overlap with the head unit. Switching in the Subsonic filter dropped the 20Hz level by 3dB, which doesn't seem like much, but probably adds just enough woofer headroom to cope with the explosions that are mandatory in movie soundtracks.

Fig.5 Audio Physic Kronos, 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 midrange unit responses, taking into account acoustic phase and distances from the nominal farfield point, plotted below 300Hz (black); the complex sum of the nearfield midrange unit responses, compensating for the distances from the nominal farfield point, plotted below 800Hz (blue); the nearfield response of the woofers with Room Gain set to "0" and Crossover off, plotted below 1kHz (red); and the nearfield response of the woofers with Room Gain set to "30" and Crossover to "1," plotted below 500Hz (green).

At the other end of the spectrum, fig.5 shows that the Kronos' head unit is superbly flat and extended above 1kHz, with just a slight lack of energy apparent at 6kHz. However, measuring the speaker's response in the two octaves between 1kHz and the crossover to the woofers proved problematic, due to the use of three drive-units to cover the midrange. Measured on the tweeter axis at 50", the farfield response featured a severe suckout between 300Hz and 600Hz (fig.5, black trace).

Looking at the individual responses of the front and side midrange units (not shown) revealed that the front midrange unit handles the region above 600Hz, the side midrange units that below 600Hz. However, the front unit's LF rolloff is on the slow side, meaning that its output is only about 6dB below that of the side units in the octave before they hand over to the woofers. Were the units equidistant from the listening position, this overlap would present no problems. But the front unit is 10" closer to the listener, meaning that its output will cancel that of the side units in the region where they overlap. To investigate that this was the case, I calculated the nominal farfield response of the midrange units by summing their outputs without the effect of the time delay. This response is shown as the blue trace in fig.5: the suckout has gone, and the Kronos' midrange response extends smoothly down to the crossover to the woofers.

The effect of the on-axis midrange suckout can also be seen in the Kronos head unit's dispersion plot (fig.6) (footnote 1). Just the differences between the off- and on-axis responses are shown in this graph; as the suckout fills in to the speaker's sides, this results in "wings" appearing in the graph.

Fig.6 Audio Physic Kronos head unit, 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.

Perhaps not too much should be made of this interference between the midrange units. These are all quasi-anechoic measurements, and in the listening room, the effect of the head unit's measured midrange suckout will be drastically modified both by the room's reverberant field and by the effect of the woofer module, depending on its level, polarity, and the crossover frequency chosen. Nevertheless, in my own auditioning of the Kronos in Mikey's room I was bothered by a slight lack of body to midrange-dominant sounds, which made the woofers sound a little disconnected even when Mikey flipped their polarity with the remote.

Note from the dispersion plot that the response around 1kHz falls off to the sides compared with the couple of octaves above that, where the tweeter radiates in a hemispherical manner. Given how flat the Kronos' on-axis response is in the treble, I suspect that it was this contrast between the upper midrange and low treble radiation patterns that led Mikey to feel the speaker's presence region featured "a slight but broad elevation." Fig.6 also shows that the ring-radiator tweeter's output falls quite rapidly off-axis above 10kHz. This nicely correlates with his feeling that the Kronos' top octave was on the mellow side.

In the time domain, the head unit's step response (fig.7) reveals that the tweeter and midrange units are all connected in the same, positive acoustic polarity. To my surprise, with the woofer polarity switch set to Non-Inverted, the woofers' acoustic output was inverted; ie, in opposite polarity to that of the head unit's drivers. Perhaps this was why MF preferred the sound with the woofers set to Inverted. The tweeter-midrange module's cumulative spectral-decay plot (fig.8) is superbly clean, except in the region where the midrange units overlap.

Fig.7 Audio Physic Kronos head unit, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Audio Physic Kronos head unit, cumulative spectral-decay plot at 50" (0.15ms risetime).

Some impressive speaker engineering has gone into Audio Physic's Kronos, particularly in the way in which its woofer section has been designed to work with the room acoustics to achieve superb bass quantity without sacrificing bass quality. However, I was puzzled by the measured interaction between the midrange units.—John Atkinson



Footnote 1: The sheer bulk of the woofer cabinet made it impractical to measure the head unit's farfield behavior with the woofers in place. Instead, I mounted the head unit on a high stand. The measured behavior will not, therefore, include the baffling effect of the woofer cabinet. I don't think it affects the lateral dispersion; however, I have not shown the vertical dispersion, which will be affected.—John Atkinson
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