Moth Audio Cicada loudspeaker Measurements

Sidebar 3: Measurements

Despite its not being a horn design, Moth Audio's Cicada had a very high sensitivity: an estimated 92.5dB(B)/2.83V/m. And with an impedance that remains above 11 ohms at all frequencies (fig.1), the Cicada is very efficient. As with the other two speakers in this survey, the Cicada's impedance traces are marred by the peaks typical of resonant behavior. The peak at 180Hz is associated with a major panel resonance (fig.2). Despite the speaker's high sensitivity, the fact that this mode is present on all cabinet surfaces works toward its audibility—I can't help thinking it was why AD thought female voices sounded "almost imperceptibly husky."

Fig.1 Moth Audio Cicada, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 Moth Audio Cicada, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel 12" from top. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

As with the other two speakers, the saddle in the impedance magnitude trace indicates that the Cicada's vent does behave to some extent as a conventional reflex port, tuned in this case to 50Hz—not coincidentally, the frequency of the drive-unit's minimum-motion point (fig.3, red trace). But peculiarly, the port's response (blue trace) peaks above and below this frequency, which results in an upper-bass peak in the speaker's summed LF response (black trace). What would otherwise be the port's smooth low-pass rolloff above 80Hz is spoiled by a major resonance at 180Hz and a smaller one at 320Hz. Both result in small suckouts in the woofer's nearfield response, which, because of the phase relationship between the two sources, results in almost total cancellation at 187Hz.

Fig.3 Moth Audio Cicada, anechoic response on driver axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with nearfield responses of the woofer (red) and port (blue) and their complex sum, taking into account acoustic phase and distance from the nominal farfield point (black), plotted below 300Hz, 800Hz, and 400Hz, respectively.

Higher in frequency, the response trend is surprisingly even, but broken by peaks in the low treble and at 10kHz. All things being equal, this might add some audible "steeliness" to the Cicada's sound. However, and as AD noted, the speaker's output drops off rapidly off-axis above 5kHz or so (fig.4), which will allow the subjective impact of this narrow peak to be ameliorated by the listener moving to the side of the drive-unit's whizzer cone. Also as AD found, the low-treble peak gives rise to "excess forwardness or 'bite'" when the Cicada is listened to on-axis. However, this graph shows that the low-treble peak is also confined to a narrow range of listening angles, and confirms that the Cicada's treble balance can be flattened out quite nicely by experimenting with less toe-in (though the tradeoff is that some treble instruments will then sound too soft). In the vertical plane (not shown), the dispersion is fairly even as long as the listener sits with his ears 33" from the floor, level with the drive-unit or just below. Sit too high and the mid-treble disappears.

Fig.4 Moth Audio Cicada, lateral response family at 50", normalized to response on driver 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.

In the time domain, the Moth's step response (fig.5) is basically time-coherent, but broken up by some high-frequency ringing and a leading spike, presumably emanating from the small whizzer cone. The Cicada's cumulative spectral-decay plot (fig.6) is disappointing, with major ridges of delayed energy at 2.2kHz and 10kHz, the frequencies of the peaks in the on-axis response.

Fig.5 Moth Audio Cicada, step response on driver axis at 50" (5ms time window, 30kHz bandwidth).

Fig.6 Moth Audio Cicada, cumulative spectral-decay plot at 50" (0.15ms risetime).

Like the other two speakers reviewed this month by Art Dudley, the Moth Audio Cicada manages to sound acceptable—even good—despite its measured performance, not because of it. But against that must be placed its high sensitivity and efficiency, which will allow it to work synergistically with the flea-powered amplifiers loved by so many.—John Atkinson

Moth Audio
1746 Ivar Avenue
Hollywood, CA 90028
(323) 467-4300
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