Sidebar 3: Measurements
I used DRA Labs' MLSSA system, a calibrated DPA 4006 microphone, and an Earthworks microphone preamplifier to measure the Totem Element Fire V2's quasi-anechoic frequency- and time-domain behavior in the farfield. I used an Earthworks QTC-40 microphone, which has a small, ¼" diameter capsule, for the nearfield responses and Dayton Audio's DATS V2 system to measure the impedance magnitude and phase.
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. Footnote 2: A tweeter has a shallower acoustic center than a woofer. To bring the drive units into time-alignment with a speaker that uses first-order crossover filters, the baffle therefore needs to be stepped-back (Vandersteen, Dunlavy) or sloped-back (Thiel, Spica).
Fig.1 Totem Acoustic Element Fire V2, electrical impedance (solid) and phase (dashed) (5 ohms/vertical div.; note expanded vertical scale).
While the Element Fire V2's nominal impedance is specified as 8 ohms, the impedance magnitude (fig.1, solid trace) is higher than 10 ohms over almost the entire audioband. The minimum impedances were 8.05 ohms at 32Hz and 9.2 ohms at 165Hz. As the electrical phase angle (fig.1, dotted trace) is high in several frequency regions, as a result, the effective resistance, or EPDR (footnote 1), does drop below 5 ohms from 10Hz to 139Hz, with minimum EPDR values of 4.1 ohms at 25Hz and 39Hz, and 5.2 ohms at 143Hz. The Element Fire V2 is therefore a relatively easy load for the partnering amplifier. Totem specifies the Element Fire V2's voltage sensitivity as 88dB, though with no voltage mentioned; my estimate was lower, at 84dB(B)/2.83V/1m. However, the Fire V2's impedance averages 15 ohms and 2.83V is equivalent to 0.53W into that load. Adjusting my estimate for a 15 ohm impedance gives a sensitivity of 86.75dB/W/1m, which is closer to the specified figure.
Fig.2 Totem Acoustic Element Fire V2, cumulative spectral-decay plot calculated from output of accelerometer fastened to the center of a sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The enclosure emitted fairly loud "bonks" when I tapped its panels with my knuckles. Using a plastic-tape accelerometer, I found a strong resonant mode at 234Hz on the top, side, and rear panels, as well as a slightly lower-level mode at 625Hz (fig.2). (I doubt that it is a coincidence that there is a discontinuity in the impedance traces at this frequency.) Although these modes have a high Q (Quality Factor), which will work against audibility, the relatively high levels might result in midrange coloration.
Fig.3 Totem Acoustic Element Fire V2, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the woofer (blue) and port (red), respectively plotted below 350Hz and 1kHz.
The saddle centered on 35Hz in the impedance magnitude trace implies that this is the tuning frequency of the port on the Totem's rear panel. The woofer's nearfield response (blue trace below 350Hz in fig.3) had the expected reflex tuning notch at this frequency, but there is also a discontinuity in its output at 234Hz, the frequency of one of the panel resonances in fig.2. The port's nearfield response (fig.3, red trace) peaks at the tuning frequency, but there are some resonant peaks present in its midrange output. Again, these occur at the frequencies of the panel resonances in fig.2. The woofer's farfield response (blue trace above 350Hz) overlaps that of the tweeter (green trace) between 1.3kHz and 4kHz before rolling off relatively quickly above that region. There is a slight rise above 7kHz in the tweeter's output.
Fig.4 Totem Acoustic Element Fire V2, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.
The complex sum of the woofer's and port's nearfield responses (fig.4, black trace below 300Hz) only has a small upper-bass peak due to the nearfield measurement technique, which assumes the baffle extends to infinity in both planes. This suggests that the Element Fire V2's low-frequency alignment is slightly overdamped, which will be optimal for placing the speaker close to the wall behind it. However, the low frequencies are well-extended for a fairly small standmount, the output not reaching –6dB until the port tuning frequency.
The Totem's manual says stands should be used that place the listener's ears level with the Element Fire V2's tweeter. I therefore examined the loudspeaker's farfield response, averaged across a 30° horizontal window centered on the tweeter axis. The result is shown as the black trace above 300Hz in fig.4. The response is relatively even through the midrange and in the top two audio octaves, but there is a major suckout in the region where the woofer and tweeter outputs overlap. This measurement was taken without the grille; repeating it with the grille in place gave an almost identical result with both samples.
Fig.5 Totem Acoustic Element Fire V2, 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.6 Totem Acoustic Element Fire V2, lateral response family at 50", from back to front: responses 90–5° off axis, tweeter-axis response, responses 5–90° off axis.
Fig.7 Totem Acoustic Element Fire V2, 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 Totem's horizontal radiation pattern, normalized to the response on the tweeter axis, which therefore appears as a straight line, is shown in fig.5. Fig.6 also shows the Fire's horizontal radiation pattern but this time showing the actual responses. Both graphs indicate that the suckout in the low treble fills in more than 30° to the speaker's sides, which is presumably why the manual states that the initial setup should be without any toe-in, ie, with the speakers firing straight ahead. Fig.7 shows the Element Fire V2's dispersion in the vertical plane, again normalized to the response on the tweeter axis. The suckout in the crossover region does appear to fill in above and below the tweeter axis.
Fig.8 Totem Acoustic Element Fire V2, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.9 Totem Acoustic Element Fire V2, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Turning to the time domain, the Totem's step response on the tweeter axis (fig.8) indicates that both drive units are connected in positive acoustic polarity. However, the tweeter's output arrives first at the microphone, which will be due to the drivers being mounted on a flat front baffle (footnote 2). This time delay is the cause of the suckout on the tweeter axis seen in fig.4. There are some small ripples in the decay of the woofer's step which correlate with a small ridge of delayed energy at 4.4kHz in the Element Fire V2's cumulative spectral-decay, or waterfall, plot (fig.9). However, the decay is clean in the tweeter's passband below the ridge at 24kHz, this due to the titanium dome's fundamental resonance.
The Totem Element Fire V2's measured performance indicates that this speaker is easy to drive, has a clean midrange and high treble, and has relatively extended low frequencies for what is a fairly small standmount. However, I am bothered by its lively enclosure, and the designer's decision not to use a low-pass filter on the woofer will make setup tricky if the Fire V2 is not to sound recessed in the presence region.—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. Footnote 2: A tweeter has a shallower acoustic center than a woofer. To bring the drive units into time-alignment with a speaker that uses first-order crossover filters, the baffle therefore needs to be stepped-back (Vandersteen, Dunlavy) or sloped-back (Thiel, Spica).
