Epos Epic 2 loudspeaker Measurements
Sidebar 3: Measurements
The Epos Epic 2's B-weighted sensitivity on its tweeter axis was 90dB/2.83V/m, which both agrees with the specified 90dB and is 3dB higher than the norm. The electrical impedance drops below 6 ohms only between 120 and 500Hz, with a minimum magnitude of 4.2 ohms at 190Hz (fig.1). While there are the usual extreme electrical phase angles in the bass, the worst-case combination of 6 ohms magnitude and 35° phase angle will not be very taxing for an amplifier rated at 4 ohms.
There is a small discontinuity in the impedance traces around 260Hz. Investigating the enclosure's vibrational behavior with a plastic-tape accelerometer revealed a strong resonance at 258Hz on the side panels (fig.2), which I could hear through a stethoscope as I played the half-stepspaced toneburst track from Editor's Choice (CD, Stereophile STPH015-2). However, it was much less audible than I was expecting, presumably due to its very high Q (Quality factor). As Bob Reina commented that the Epic 2 offered a "warm, inviting, detailed, uncolored presentation of all midrange textures," I must assume that this resonance measures worse than it sounds.
I measured the Epic 2's frequency response in the farfield with DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. For the nearfield measurements I used an Earthworks QTC-40 microphone, whose ¼" capsule provides only a minimal obstacle to the flow of air through the port. I performed all the acoustic measurements with the speakers' grille removed, the front baffle providing a smoothly contoured environment for both drive-units. To the right of fig.3 are shown the farfield responses of the tweeter (blue trace) and woofer (red); to the left are shown the nearfield responses of the woofer (red) and port (green). The port's output is well behaved, with any midrange misbehavior well suppressed. The woofer's output is relatively flat, though with a slight midrange prominence. There is also a suspicious-looking glitch at the frequency of the cabinet side-panel resonance. The woofer's minimum-motion notch occurs at 48Hz, confirming that this is the tuning frequency of the large port on the rear panel, as suggested by the saddle in the impedance-magnitude trace centered on the same frequency.
At the upper end of its passband, the woofer rolls off with a fast, fourth-order, 24dB/octave slopea far cry from the first-order, 6dB/octave slopes featured by the first Epos speaker, the ES14, of 1987, while the tweeter rolls in with a third-order, 18dB/octave slope. The acoustic crossover is set at 3kHz, and the soft-dome tweeteranother departure from Epos traditioncan be seen to be balanced 45dB too high in level above 7kHz.
Fig.4 shows how these individual outputs sum on the tweeter axis in the farfield. The slight boost in the upper bass will almost entirely be an artifact of the nearfield measurement technique; the speaker will be flat down through the upper bass, and down 6dB at the port tuning frequency of 48Hz. The upper midrange is again a bit peaky, though this can be seen to better balance the exaggerated top-octave response on the other side of a trough at the base of the tweeter's passband. The plot of the Epic 2's lateral dispersion (fig.5) shows that this presence-region trough is balanced by a slight excess of off-axis energy in the same region. The tweeter's excess of energy on axis above 7kHz will also be offset by a sharper-than-usual rolloff to the speaker's sides in its top two octaves. In a normally furnished room of typical size, the Epic 2's treble will sound more neutrally balanced than is suggested by figs. 3 and 4.
In the vertical plane (fig.6), a sharply defined suckout appears in the crossover region above the tweeter axis. The Epic 2 needs to be used with a stand that places its tweeter at or above the listener's ears. This graph also shows that the presence-region trough does fill in 510° below the tweeter axis, which is confirmed by the speaker's step response on the tweeter axis (fig.7). Both drive-units are connected with positive acoustic polarity, but the decay of the tweeter's stepthe sharp up/down spike just before the 4 millisecond markdoesn't blend quite smoothly with the woofer's step. Moving the microphone a little below the tweeter axis would have the effect of shifting the woofer's step forward in time so that its rise better blends with the decay of the tweeter's step. Some slow undulations are visible in the decay of the woofer's step, but the Epic 2's cumulative spectral-decay plot is very clean, particularly in the treble (fig.8).
This is good measured performance for a speaker costing $799/pair, but it does indicate that, to get the optimal performance from a pair of Epic 2s, some care needs to be taken in setting them up.John Atkinson