Epos M5 loudspeaker Measurements

Sidebar 3: Measurements

The nicely finished Epos M5 offered just-above-average voltage sensitivity, at an estimated 88dB(B)/2.83V/m. Its impedance magnitude remained above 4 ohms over most of the audioband, dropping to minima of 3.6 ohms at 250Hz and 6.5kHz (fig.1). The electrical phase angle is generally moderate, though there is a somewhat demanding combination of 5 ohms impedance and –45º phase at 4kHz. Even so, a 4-ohm–rated amplifier will have no problem driving this speaker.

Fig.1 Epos M5, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There is no sign of the usual metal-dome tweeter resonance in this graph. The M5's impedance traces are also free from the small wrinkles and discontinuities that would indicate the presence of major cabinet resonances. Even so, a simple accelerometer fastened to the cabinet surfaces revealed a mode at 188Hz (fig.2). This may well be too low in level to be subjectively bothersome. The other modes are higher in frequency and will therefore be even less audible.

Fig.2 Epos M5, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

From left to right, fig.3 shows the individual outputs of the port, woofer and tweeter. The usual reflex notch in the woofer's response occurs at 52Hz, the frequency expected from the impedance plot, which has a saddle in the magnitude trace at the same frequency. As this represents the port's tuning frequency, only moderate bass extension can be expected from the M5. The port's output peaks in this region, then rolls off cleanly, with very little midband hash. Looking higher in frequency, the crossover between the woofer and tweeter is set at a high 3.7kHz, both units rolling out cleanly outside their respective passbands.

Fig.3 Epos M5, acoustic crossover response on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 350Hz and 900Hz, respectively.

The on-axis response in fig.3 indicates that the M5's tweeter appears to be set a couple of dB too "hot." However, when you look at the overall response averaged across a 30º horizontal window centered on the tweeter axis (fig.4), you can see that the tweeter's restricted top-octave dispersion results in a good balance with the woofer. Even so, there does appear to be a slight lack of energy in the crossover region, perhaps due to the tweeter not coming in early enough to fully take over from the woofer. All things being equal, this might make the M5 sound a little polite, but only if this lack is preserved off-axis. As expected from the earlier graphs, the M5 offers only moderate low-frequency extension, the bass lying 6dB down at the port tuning frequency. The slight bump in the upper bass is entirely due to the nearfield measurement environment used; the M5's woofer alignment is, if anything, slightly overdamped, which will emphasize the slight peak in its output in the octave between 800Hz and 1.6kHz, adding a sense of detail to the speaker's presentation.

Fig.4 Epos M5, anechoic response on tweeter axis at 50", averaged across 30º horizontal window and corrected for microphone response, with the complex sum of the nearfield responses, taking into account acoustic phase and distance from the nominal farfield point, plotted below 300Hz.

The M5's horizontal radiation pattern is shown in fig.5. In moderate-sized rooms, the slight off-axis flare between 3kHz and 5kHz will tend to compensate for the lack of on-axis energy in the same region. As a result, the speaker will sound evenly balanced in the treble. In the vertical plane (fig.6), the presence-region depression fills in below the tweeter axis but increases above that axis. Both indicate that the M5 needs to be used on fairly high stands that place the tweeter at or just above the listener's ear level.

Fig.5 Epos M5, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5º off-axis, reference response, differences in response 5–90º off-axis.

Fig.6 Epos M5, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5º above axis, reference response, differences in response 5–45º below axis.

The M5's step response (fig.7) indicates that both drive-units are connected in positive acoustic polarity, with the tweeter's output slightly leading that of the woofer. One hands smoothly over to the next, however, indicating good frequency-domain integration of the outputs (fig.8). The lack of presence-region energy seen in fig.4 is not due to interference between the two drive-units' outputs, therefore, but to a misalignment of the filter turnover frequencies, presumably to compensate for the flare in the tweeter's dispersion at the bottom of its passband. This is confirmed by the M5's waterfall or cumulative spectral-decay plot, measured in the farfield (fig.9), which does not show post-impulse energy released in this region. However, there is a slight ridge of energy evident just below 6kHz, as well as something untoward going on around 1kHz. (This is associated with the slight undulations in the tail of the step response in fig.7, before the first true boundary reflection at the 7.5ms mark.) I suspect that neither will result in audible problems.

Fig.7 Epos M5, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Epos M5, step responses on tweeter axis at 50" of tweeter (red) and woofer (blue) (5ms time window, 30kHz bandwidth).

Fig.9 Epos M5, cumulative spectral-decay plot at 50" (0.15ms risetime).

This is impressive measured behavior for a relatively inexpensive minimonitor.— John Atkinson

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