Athena Technologies AS-F2 loudspeaker Measurements
The Athena AS-F2 turned out to be much more sensitive than usual, at an estimated 93.5dB(B)/2.83V/m. Not only will it therefore play loudly with relatively low-powered amplifiers, but its plot of impedance magnitude and phase (fig.1) reveals it to be an easy load, with a minimum magnitude of 4.5 ohms in the lower midrange. The electrical phase angle gets a bit high in the upper bass, but the magnitude remains at 6 ohms or above, which will ameliorate any potential drive difficulty. The severe glitches in the impedance plots between 100 and 200Hz and around 450Hz, however, imply the presence of cabinet resonances.
Fig.1 Athena AS-F2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.2 is a waterfall plot calculated from the output of a simple plastic-tape accelerometer fastened to one of the enclosure side panels. A number of high-level resonant modes can be seen, the highest in level lying at 535Hz, 290Hz, and 156Hz. These modes could be found on all the AS-F2's panels, and given the large radiating area of these surfaces, they will have an audible effect. Certainly, in my very limited auditioning of the Athena, I was aware of some congestion in the lower mids, which is what I believe BJR is referring to as "warmth."
Fig.2 Athena AS-F2, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel 12" from the top. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
The saddle at 34Hz in the impedance magnitude trace indicates the tuning of the large port. This is confirmed by the sum of the twin woofers' nearfield responses, shown to the left of fig.3, which has a sharp notch at the same frequency. The port's output is the lowest trace in this graph; while it covers the expected passband, there are some severe peaks evident at the same frequencies of the resonant modes in fig.2. These resonances are high enough in level that they give rise to glitches in the woofers' output. The speaker's output appears to rise in the bass region, but this is an artifact of the nearfield measurement technique. The speaker's reflex tuning is actually well-balanced between bass definition and extension.
Fig.3 Athena AS-F2, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield woofer and port responses and their complex sum plotted below 300Hz, 300Hz, and 850Hz, respectively.
The AS-F2's farfield response on the tweeter axis is shown to the right of fig.3. The midrange and low treble are flat, apart from a slight peak between 500Hz and 700Hz, which collrates with the forward balanced noted in the auditioning. There is also a slight rise in the top audio octave. Whether this contributes to audible coloration will depend on the speaker's dispersion in the same region. Fig.4 reveals that the tweeter becomes quite directional above 8kHz, which will tend to balance the on-axis excess of energy in average-sized rooms. The AS-F2's radiation pattern tends to narrow at the top of the woofers' passband, but not to the degree that I would have expected. In the vertical plane (fig.5), the speaker's response changes quite significantly if the listener moves much above or below the 38"-high tweeter axis.
Fig.4 Athena AS-F2, 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.5 Athena AS-F2, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-15 degrees below axis.
The Athena's step response (fig.6) is absolutely normal, with all the drive-units connected in the same positive acoustic polarity. The cumulative spectral-decay plot (fig.7) is surprisingly clean for a speaker at this price, with very little hash apparent. This is presumably why BJR was so impressed by the Athena's upper-frequency clarity.
Fig.6 Athena AS-F2, on-axis step response at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Athena AS-F2, cumulative spectral-decay plot at 50" (0.15ms risetime).
It looks as if the Athena design team has maximized the AS-F2's bass performance. Apart from the enclosure resonances I found, they don't seem to have compromised performance in other areas to produce a speaker that is very competitively priced.—John Atkinson