Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Simaudio Moon Voice 22's behavior in the farfield, and an Earthworks QTC-40 mike for the nearfield responses. As the loudspeaker's manual recommends that the grille be left off for the "best results," the measured behavior was taken without the grille.
Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
Fig.1 Simaudio Moon Voice 22, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The Moon Voice 22's sensitivity is specified as 89dB/2.83V/m; my B-weighted estimate was inconsequentially lower, at 88dB(B)/2.83V/m. The Voice 22's nominal impedance is specified as 6 ohms. The speaker's impedance magnitude (fig.1, solid trace) remains above 6 ohms in the upper midrange and treble, but drops below 4 ohms in the bass and lower midrange, with a minimum value of 3.4 ohms at 184Hz. The electrical phase angle (dotted trace) is occasionally high, which means that the equivalent peak dissipation resistance, or EPDR (footnote 1), lies below 3 ohms for much of the bass and midrange. The minimum EPDR values are 1.9 ohms at 34Hz, 2.1 ohms at 54Hz, 1.63 ohms at 117Hz, and 2.24 ohms at 373Hz. The Voice 22 needs to be paired with amplifiers that don't have problems with low impedances.
Fig.2 Simaudio Moon Voice 22, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (measurement bandwidth, 2kHz).
The enclosures' side and top panels emitted a "plink" when I rapped them with my knuckles. When I investigated these panels' vibrational behavior with a plastic-tape accelerometer, I found a strong resonant mode at 555Hz (fig.2). I always measure a loudspeaker's vibrational behavior with it sitting on three upturned cones, which allows the resonant modes to be fully developed. However, as the Voice 22 has a compliant pad inserted into its base, I repeated this measurement with the speaker sitting directly on the floor. This reduced the level of the resonant mode by 5dB. In any case, as the mode is both relatively high in frequency and has a high Q (Quality Factor), its effect on music will be minor.
Fig.3 Simaudio Moon Voice 22, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield woofer (blue) and port (red) responses respectively plotted below 300Hz and 1kHz.
The Moon speaker's impedance-magnitude plot has a saddle in the bass centered on 42.7Hz, suggesting that this is the tuning frequency of the port on the rear panel. The woofer's nearfield response (fig.3, blue trace) has the expected reflex notch at this frequency, and the port's output (fig.3, red trace) peaks between 35Hz and 70Hz. The port's upper-frequency rolloff is clean overall, though some resonant peaks are present between 700Hz and 900Hz. These are low in level, however, and their audibility will be reduced by the fact that the port faces away from the listener. The complex sum of the woofer and port responses is shown as the black trace below 300Hz in fig.3. The apparent boost in the upper bass is mostly an artifact of the nearfield measurement technique. The Voice 22's reflex alignment appears to be maximally flat, in textbook fashion.
The Voice 22's farfield output, averaged across a 30° horizontal window centered on the tweeter axis (fig.3, black trace above 300Hz), has too much energy in the octave between 700Hz and 1.4kHz, though the speaker's output in the region covered by the tweeter is relatively even. In itself, that boosted upper midrange might add a touch of nasality to the speaker's tonal balance. However, the plot of the Moon speaker's horizontal dispersion, normalized to the response on the tweeter axis (fig.4), indicates that there is a shallow gulley off-axis in the same region. Other than in small rooms, this behavior will tend to flatten the perceived balance.
Fig.4 Simaudio Moon Voice 22, 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 Simaudio Moon Voice 22, 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 waveguide that surrounds the fairly large tweeter dome narrows the drive unit's top-octave dispersion in both the horizontal and vertical planes (fig.5). The latter graph suggests that the Moon Voice 22 will sound tonally balanced on or just below the tweeter axis. The cursor position in fig.5 indicates that the crossover between the two drive units occurs just below 1.7kHz, close to the specified 1.5kHz.
Fig.6 Simaudio Moon Voice 22, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
In the time domain, fig.6 shows the Voice 22's step response on the tweeter axis. The tweeter's output arrives first at the microphone, and it is connected in inverted acoustic polarity. However, the positive-going decay of the tweeter's step smoothly blends with the start of the woofer's step, which suggests optimal crossover implementation. The cumulative spectral-decay, or waterfall, plot (fig.7) is superbly clean.
Fig.7 Simaudio Moon Voice 22, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The Moon Voice 22's measured behavior suggests that the on-axis and off-axis behavior has been carefully managed to give a relatively neutral tonal balance, though the upper midrange may still sound a little forward.—John Atkinson
Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
