Epos ES25 loudspeaker Measurements
I calculated the ES25's sensitivity to be a little lower than specified, at 86.5dB/2.83V/1m (B-weighted). Its impedance (fig.1) is also moderately demanding, dropping to 5 ohms throughout the midrange, with a relatively severe electrical phase angle in the upper bass. The tuning of the large port is revealed by the impedance "saddle" at 33Hz, while the wrinkle in the fig.1 traces between 600Hz and 700Hz indicates some kind of resonant behavior. Indeed, the cabinet did prove to be very lively in this region, but it appears to be high enough in frequency to not have too much deleterious effect on music.
Fig.1 Epos ES25, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Fig.2 shows the individual response of the three drive-units and the port. The tweeter has the expected ultrasonic and inaudible peak at 26kHz, but also has a peak in the middle of its passband. It crosses over to the midrange unit at a highish 5.5kHz. The midrange unit is smoothly balanced in its passband, but suffers a large overlap with the woofer. The woofer itself appears to be more sensitive than it need be and hands over to the port below 60Hz or so.
Fig.2 Epos ES25, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield midrange, woofer, and port responses plotted below 500Hz, 300Hz, and 2.5kHz, respectively.
The overall response on the tweeter axis, together with the complex sum of the midrange, woofer, and port nearfield responses (taking the physical distance between the acoustic sources into account), is shown in fig.3. Though there's a slight lack of energy at the top of the midrange unit's operating range, which might correlate with JE finding the speaker "polite," the ES25's treble response is quite flat overall. But in the midrange, the overlap between the two drive-units leads to a seriously shelved-down output between 100Hz and 800Hz. This no doubt correlates with JE noting a "recessed" midrange.
Fig.3 Epos ES25, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield midrange, woofer, and port responses plotted below 300Hz.
I noticed this "hollow"-sounding character in my own auditioning of the ES25. It was particularly bothersome on classical orchestral music, which lost much of its essential lower-midrange power as a result. And the lack of midrange energy meant that the bass region sounded distinctly disconnected. I wondered if this was a deliberate design choice because the speaker was intended to be used up against the wall behind it, but my specification sheet made it clear that the ES25 is intended to be used in free space, well away from room boundaries.
However, the fact that the inputs to all three drive-units are accessible on the rear panel meant that it was trivial to fix this problem by reversing the electrical polarity of just the woofer. The result can be seen in fig.4, which shows the Epos's spatially averaged, 1/3-octave response in my listening room, taken with the woofer connected with both electrical polarities. With the "correct" woofer connection, the entire midrange region is sucked out by up to 6dB. With the woofer electrically inverted, the midrange is now pretty flat in-room. The result on music was equally impressive; orchestral music sounded warm and powerful, and the ES25's bass was now well-integrated with the lower midrange (if still a little lumpy-sounding).
Fig.4 Epos ES25, spatially averaged, 1/3-octave response in JA's room with woofers in "correct" electrical polarity (bottom at 400Hz) and inverted polarity (top).
Note the excellent low-bass extension in fig.4: with a judicious amount of LF boundary reinforcement, the ES25 puts out strong in-room bass down to the 32Hz 1/3-octave band and is just 3dB down from the 1kHz reference level at 25Hz.
Even with the woofer connected the "wrong" way, I still found the ES25 to be polite-sounding, its balance lacking sparkle in the treble. Partly, this is due to the on-axis suckout in the mid-treble, which can also be seen in fig.4. But fig.5, which shows the ES25's horizontal dispersion pattern—note that only the changes in response are shown—reveals the low treble to roll off quite rapidly to the speaker's sides compared with the regions above and below. This, I feel, will exacerbate the polite-sounding on-axis balance. Vertically (fig.6), the mid-treble suckout deepens if the listener sits much above or below the 33"-high tweeter axis.
Fig.5 Epos ES25, horizontal response family at 57", normalized to response on tweeter axis, from back to front: differences in response 90°–5° off-axis on tweeter side of baffle; reference response; differences in response 5°–90° off-axis on woofer side of baffle.
Fig.6 Epos ES25, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 10°–5° above tweeter axis; reference response; differences in response 5°–15° below tweeter axis.
In the time domain, the step response (fig.7) indicates that all three drive-units are connected with positive acoustic polarity. However, the broad overlap between the woofer and midrange unit and the time delay due to the physical distance between the drivers gives rise to destructive interference with this connection. It can just be seen from fig.7 that the slow-risetime output from the woofer arrives at the measuring microphone just as the decay of the midrange unit's output becomes negative-going.
Fig.7 Epos ES25, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Finally, the cumulative spectral-decay or waterfall plot (fig.8) is generally clean but marred by three slight resonant modes, these coincident with the peaks in the ES25's on-axis response. As JE didn't remark on any treble emphasis and I didn't notice any sibilant problems in my own auditioning, I must assume that these modes are relatively benign. I have to say that, following my very favorable impressions of Epos's ES11 and ES14, I was disappointed by the '25. I suspect that there is a better speaker possible using the same basic ingredients.—John Atkinson
Fig.8 Epos ES25, cumulative spectral-decay plot at 57" (0.15ms risetime).