Earthworks Sigma 6.2 loudspeaker Measurements

Sidebar 3: Measurements

The Sigma 6.2 was of just-below-average voltage sensitivity, at an estimated 86dB(B)/2.83V/m. However, its impedance plot of magnitude and phase against frequency (fig.1), which stays between 7 and 8 ohms across much of the audioband with very little electrical phase shift, indicates that the speaker is an easy load for the partnering amplifier to drive. The impedance minimum at 43Hz indicates the tuning of the rectangular port, which in turn implies modest bass extension.

Fig.1 Earthworks Sigma 6.2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

A small depression at 500Hz in the impedance magnitude trace appears to be associated with a strong cabinet panel resonance. Fig.2 is a cumulative spectral-decay plot calculated from the output of a plastic-tape accelerometer fastened to the center of a Sigma 6.2's back panel above the terminals; a strong ridge of delayed energy can be seen at 488Hz. The mode was strongest on the back panel and was much lower on the side and top panels. It could be clearly detected with a stethoscope while I played the "Chromatic Scale" track from Stereophile's Test CD 3 (the track is also included on our forthcoming Editor's Choice CD), as could a mode at 336Hz. It's possible that the presence of these modes correlated with the slight degree of midrange congestion I noted. It should also be noted that this behavior might be different with the black-finished MDF Sigma 6.2.

Fig.2 Earthworks Sigma 6.2, 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.)

Fig.3 shows the Earthworks' farfield response averaged across a ±15 degrees horizontal window centered on the tweeter axis. The ring-radiator tweeter gives extension well above the audioband, but the mid-treble is a little elevated in absolute terms. All things being equal, this might well correlate with the speaker's rather bright balance. It will also contribute to the astonishing amount of recorded detail laid bare by the Sigma 6.2. In the bass, the woofer's minimum-motion notch (red trace) coincides, as expected, with the impedance minimum in fig.1, but the port's output (blue) peaks a little higher in frequency. The port's output is free from upper-frequency resonances, which is good, considering it faces the listener. The summed low-frequency response (black) peaks up in the midbass, but this will be due almost entirely to the nearfield measurement technique; the Sigma 6.2's bass alignment actually appears to be a little overdamped, which ties in with the speaker's very-well-defined low frequencies but slightly lightweight balance.

Fig.3 Earthworks Sigma 6.2, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the nearfield responses of the woofer (red trace) and port (blue), and their complex sum (black), plotted below 300Hz, 1kHz, and 300Hz, respectively.

The slight asymmetry of the tweeter's placement on the mounting plate will spread the effects of diffraction from the edges. The manner in which the Sigma 6.2's response changes on the inner edge (closer to the tweeter) is shown to the front of figs.4 and 5. The salient points of these graphs are the limited dispersion of the tweeter above 10kHz—the speaker will tend to sound mellow in large rooms or if listened to from too far away in smaller, well-damped rooms—and the off-axis flare at the bottom of the tweeter's passband, between 3kHz and 5kHz. Interestingly, the presence-region boost that will result from this in-room coincides with a depression in the same region in the direct sound (fig.3), which will tend to compensate. There is a notch in the farfield response at the same frequency as the cabinet resonance noted in fig.2, which might suggest that the panel is at least partially canceling the woofer's motion at this frequency.

Fig.4 Earthworks Sigma 6.2, lateral response family at 50" on tweeter axis, from back to front: response 90 degrees-5 degrees off-axis, response on HF axis, response 5 degrees-90 degrees off-axis on driver side of baffle.

Fig.5 Earthworks Sigma 6.2, 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 on driver side of baffle.

The speaker's dispersion in the vertical plane (fig.6) confirms what I experienced in my auditioning: that the Sigma 6.2 must be auditioned with the listener's ears level with or just below the tweeter. An enormous suckout develops just above the tweeter axis. However, this, along with the on-axis depression in the presence region, will also work against the tendency for the lateral dispersion to make the speaker sound bright in-room. As a result, the Earthworks' spatially averaged response in my listening room (fig.7) is one of the flattest-balanced I have experienced, meeting extraordinary ±0.7dB limits between 400Hz and 10kHz! No wonder I felt the speaker's midrange to be among the best-balanced I have heard in my room.

Fig.6 Earthworks Sigma 6.2, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45 degrees-5 degrees above axis, reference response, differences in response 5 degrees-45 degrees below axis.

Fig.7 Earthworks Sigma 6.2, spatially averaged, 1/3-octave response in JA's listening room.

However, this graph does show the somewhat shelved-down top octave (which results from the tweeter's limited dispersion in this region) and the lightweight bass balance. The Sigma 6.2 begs for the reinforcement its low frequencies would get from being used close to the room boundaries, but this would interfere with its astonishing imaging precision.

In the time domain, the Sigma 6.2's impulse response on the tweeter axis (fig.8) is free from high-frequency ringing, while its step response (fig.9) shows an excellent, time-coherent right-triangle shape, disturbed only by a ripple with a period of approximately 2ms and some small reflections of the tweeter's output, these perhaps from the "lip" of the woofer baffle. The farfield cumulative spectral-decay plot (fig.10) is very clean overall, but with some residual hash in the mid-treble and a slight cancellation notch at the frequency of the lower-frequency ripple noted in fig.9.

Fig.8 Earthworks Sigma 6.2, impulse response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 Earthworks Sigma 6.2, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.10 Earthworks Sigma 6.2, cumulative spectral-decay plot at 50" (0.15ms risetime).

It is very rare, in my experience, to find a loudspeaker that excels in both the frequency and time domains. Taken overall, its measurements suggest that the Earthworks Sigma 6.2 joins that small community.—John Atkinson

37 Wilton Road
Milford, NH 03086
(603) 654-6427
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