Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Trenner & Friedl Sun's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses. My estimate of the Sun's voltage sensitivity was 82dB/2.83V/m, which confirms the specified figure but is well below average. This is hardly surprising considering that the woofer cone has an effective diameter of just 3.5" (footnote 1). The Sun's specified impedance is 4 ohms; the graph of its impedance magnitude and electrical phase angle against frequency (fig.1) reveals that while the impedance drops slightly below 4 ohms between 200 and 400Hz, reaching a minimum value of 3.7 ohms at 270Hz, it remains above 6 ohms for the bass and treble regions. Even considering the combination of 5 ohms and a phase angle of –42° at 150Hz, this speaker won't give any 4 ohm–rated amplifier drive problems.
The black trace in fig.3 shows the Sun's farfield response on its tweeter axis, spliced at 300Hz to the complex sum of the nearfield outputs of the woofer and the four tiny ports at the corners of the rear panel. The response is superbly even between 300Hz and 10kHz, and the slight rise below 300Hz will be entirely a result of the nearfield measurement technique. The sharply defined notch just above 10kHz in this graph will be due to destructive interference between the tweeter's output and the reflections of that output from the edges of the woofer cone. This is a common problem with coaxial drive-units; of the speakers I've measured that use coaxial drivers, only the KEFs and TADs have been free from it.
As is to be expected with such a small enclosure, the Sun's lateral dispersion, normalized to the tweeter-axis response, is wide and even (fig.4), which corresponds with the excellent stereo imaging noted by KM. The apparent off-axis peak just above 10kHz in this graph is due to the interference notch in the on-axis output filling in to the speaker's sides, which will minimize the audibility of this suckout. The vertical dispersion is similar (fig.5), which is to be expected, given the symmetrical placement of the SEAS drive-unit on the front baffle.
Fig.6 indicates that the tweeter section of the coaxial drive-unit is connected in positive polarity, the woofer section in negative polarity. The tweeter's output arrives at the microphone slightly before that of the woofer, but as the decay of the tweeter's step blends smoothly with the start of the woofer's step, this will not be a problem: The Sun is time-coherent if not time-coincident. The waterfall plot on the tweeter axis (fig.7) features a superbly clean initial decay, with just a small degree of delayed energy apparent in the crossover region.
Footnote 1: The SEAS driver is specified as being 4.7" in diameter, not 3.5", but this is because it is usual to specify the diameter of the drive-unit's chassis, not that of the actual cone.
Fig.1 Trenner & Friedl Sun, electrical impedance (solid) and phase (dashed) (5 ohms/vertical div.).
The traces in fig.1 are free from the wrinkles that would indicate the presence of cabinet resonances. Nevertheless, I found a strong mode at 605Hz that was present on all cabinet panels, but strongest on the top (fig.2). A couple of lower-level modes are present close to 605Hz, but as Ken Micallef didn't comment on any midrange congestion that might have resulted from this behavior, it's possible that all these modes are too high in frequency and of too high a Quality factor (Q) to be maximally excited by music.
Fig.2 Trenner & Friedl Sun, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of top panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 Trenner & Friedl Sun, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue) and ports (red), plotted in the ratios of their radiating diameters, and complex sum of nearfield woofer and port responses plotted below 300Hz (black).
In fig.3, the nearfield output of the woofer (blue trace) has the expected minimum-motion notch at the port tuning frequency of 56Hz, and the summed output of the ports (red) peaks between 40 and 100Hz. Although KM comments that the Suns "cleanly produced bass notes of substance when present on the recording," the measured response doesn't really support that. I did at first wonder if KM's impression of "ample bass reproduction" was due to an underdamped woofer alignment, but the nearfield woofer and port responses in fig.3 and the shape of the impedance-magnitude trace in fig.1 suggest that the Sun has been tuned for clarity and definition rather than for apparent low-frequency extension. However, KM's placement of the speakers just over a foot away from the wall behind them will add some low-frequency reinforcement, as he found when he played the warble tones from my Editor's Choice CD.
Fig.4 Trenner & Friedl Sun, 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 Trenner & Friedl Sun, 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.
Fig.6 Trenner & Friedl Sun, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Trenner & Friedl Sun, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Other than that single cabinet resonance in the upper midrange and the misleading use of the Golden Ratio in its promotional materials, the Trenner & Friedl Sun offers excellent measured performance.—John Atkinson
Footnote 1: The SEAS driver is specified as being 4.7" in diameter, not 3.5", but this is because it is usual to specify the diameter of the drive-unit's chassis, not that of the actual cone.
