Era Acoustics Design 4 loudspeaker & SUB10 subwoofer Measurements

Sidebar 3: Measurements

It came as no surprise to find that the tiny Era Design 4 was not very sensitive, 2.83V drive raising just 83.5dB(B) at 1m. However, its impedance remained above 8 ohms at almost all frequencies, with a minimum value of 6.2 ohms at 204Hz (fig.1). This speaker will not ask for much in the way of current from the partnering amplifier. In fact, it might work well with high-powered tube designs.

Fig.1 Era Acoustics Design 4, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance traces are free from the wrinkles and discontinuities that would imply the existence of panel resonances. Nevertheless, investigating the cabinet's vibrational behavior with a simple plastic-tape accelerometer, I found some fairly high-level resonant modes at 395Hz, 490Hz, and 680Hz. Fig.2 shows a cumulative spectral-decay plot calculated from the output of the accelerometer when it was fastened to the center of the cabinet sidewall. These resonances should be sufficiently high in frequency that their audibility will be minimal, though I do wonder if they might be the source of the slight lack of lower-midrange definition I noted.

Fig.2 Era Acoustics Design 4, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel level with the lower woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle centered on 51Hz in the impedance-magnitude traces suggests that this is the tuning frequency of the rear-facing port—significantly lower in frequency than I would have expected from a speaker of this size. The low port tuning, according to an e-mail I received from Signal Path's Jim Spainhour, was done so that the speaker could be mounted on a wall "and not have one-note bass pounding away." The plot of the woofer's nearfield output in fig.3 indicates that the minimum-motion frequency is indeed 51Hz, though the plot of the port's output reveals that its peak is displaced a little below that frequency. The sum of the individual responses (top trace below 200Hz in fig.3), which takes into account both acoustic phase and the greater distance of the port from a nominal farfield measuring point, features the usual upper-bass rise, most of which will be an artifact of the nearfield measuring point.

Higher in frequency in fig.3, the woofer's output rises through its passband before crossing over to the tweeter around 2kHz. The tweeter's output is flat overall, but definitely shelved up by a few dB compared with the midrange level. If balanced by a somewhat exaggerated upper bass—the old "LS3/5a trick"—this treble balance will actually sound pleasing, and it does aid the speaker's presentation of recorded detail. But it will also be unforgiving with "hot" or poor recordings, as I found in my auditioning.

Fig.3 Era Acoustics Design 4, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the port and woofer, and their complex sum, plotted below 600Hz, 300Hz, and 300Hz, respectively.

Despite the use of a small-diameter woofer, there is still a trace of lateral off-axis flare at the bottom of the tweeter's passband (fig.4), and, as expected, the 1" tweeter gets quite directional above 10kHz. But the contour lines in this graph are otherwise even and smooth, correlating with the excellent stereo imaging I noted. In the vertical plane, a strong suckout centered on 2kHz develops at extreme off-axis angles above and below the tweeter axis (fig.5), but as long as the listener sits within a few inches of that axis, he will perceive a fairly neutral treble region.

Fig.4 Era Acoustics Design 4, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off-axis, reference response, differences in response 5–90° off-axis.

Fig.5 Era Acoustics Design 4, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

The Era SUB10 powered subwoofer's comprehensive range of adjustments allows its output to optimally integrate with that of the satellite speakers. Fig.6 shows its response measured in the nearfield without the low-pass filter engaged (black trace), and with the filter set to its maximum (red), middle (blue), and minimum (green) positions. I measured the Era system's spatially averaged response in my listening room, both with (fig.7, red trace) and without (blue) the subwoofer, using SMUG Software's FuzzMeasure. On its own, the Era Design 4 offers full output down to 85Hz before rolling off quite sharply, even with the usual "room gain" at low frequencies. The notch between 300 and 500Hz is, I expect, due to the normal floor-bounce cancellation, while the in-room treble is both flat and well balanced with the upper bass. With the stereo subwoofers, I ran the Design 4s full-range and set the SUB10s' filter, polarity, and level controls by ear to give the best integration with the satellites. The red trace in fig.7 suggests I did well, though I obviously have a liking for exaggerated low bass! The levels of the subwoofers should have been brought down by at least 6dB for a truly neutral in-room low-end response.

Fig.6 Era Acoustics SUB10, nearfield response without low-pass filter (black) and with filter set to Max (red), 12:00 (blue), and Min (green).

Fig.7 Era Acoustics Design 4s, spatially averaged, 1/6-octave response in JA's listening room with (red) and without (blue) two SUB10 subwoofers.

In the time domain, the Design 4's step response (fig.8) indicates that both of its drive-units are connected in positive acoustic polarity, and the excellent integration between the HF and LF steps correlates with the good integration between them in the frequency response's crossover region. The cumulative spectral-decay plot (fig.9) is superbly clean.

Fig.8 Era Acoustics Design 4, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 Era Acoustics Design 4, cumulative spectral-decay plot at 50" (0.15ms risetime).

This is excellent measured performance, even if you don't take into account the Era Design 4's price of $600/pair.—John Atkinson

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Signal Path International
1824 30th Avenue NE, Suite 1
Bellevue, WA 98005
(704) 391-9337
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