Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the TAD CE1TX's behavior in the farfield, and an Earthworks QTC-40 mike for the nearfield responses. The mesh that covers the coaxial drive unit can't be removed, but the measured behavior was taken without the grille covering the woofer.
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 Tad CE1TX, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Tad specifies the CE1TX's voltage sensitivity as 85dB/2.83V/m, which is 2dB lower than average; my B-weighted estimate was within experimental error of that figure, at 84.7dB(B)/2.83V/m. The speaker's nominal impedance is specified as 4 ohms. My measurement, taken with Dayton Audio's DATS V2 system, indicates that the impedance magnitude (fig.1, solid trace) drops slightly below 4 ohms in the lower midrange, with a minimum value of 3.61 ohms at 124Hz. The magnitude is higher than 8 ohms in the very low bass and for almost the entire treble, however. 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 from the midbass region through the upper midrange and in the top audio octave. The minimum EPDR values are 1.45 ohms at 35Hz, 2 ohms at 60Hz, and 2.03 ohms at 861Hz. Its low effective resistance and lowish sensitivity mean that the CE1TX needs to be paired with amplifiers that can deliver both voltage and current.
Fig.2 Tad CE1TX, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of metal plate that covers one of the sidewall (measurement bandwidth, 2kHz).
The impedance traces are free from the small discontinuities in the midrange that would indicate the presence of panel resonances in the enclosure. The wooden cabinet did seem extremely inert when I rapped it with my knuckles, and when I investigated these panels' vibrational behavior with a plastic-tape accelerometer, I didn't find any resonances. However, a fairly strong resonant mode at 588Hz was present on the metal plates that cover the reflex vents on the sides of the enclosure, with a lower-level mode slightly lower in frequency (fig.2). As these modes are both relatively high in frequency and have a high Q (Quality Factor), their effect on music should be minimal.
Fig.3 Tad CE1TX, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield midrange (green), woofer (blue), and port (red) responses respectively plotted below 400Hz, 800Hz and 500Hz.
The Tad speaker's impedance-magnitude plot has a low-frequency saddle centered just below 40Hz, suggesting that this is the tuning frequency of the reflex-loading slots between the wooden side panels and the metal plates. The woofer's nearfield response (fig.3, blue trace) has the expected notch at this frequency, and the port's output (red trace) peaks sharply between 30Hz and 60Hz. The port's upper-frequency rolloff is clean overall, though some low-level peaks are present at 150Hz and between 200Hz and 500Hz. The woofer's output rolls off above 150Hz with what appears to be an 18dB/octave slope, and the midrange unit's output, measured in the nearfield (green trace), rolls off below 300Hz with the same third-order slope. The crossover frequency between these two drivers appears to be close to the specified 250Hz.
The complex sum of the midrange, woofer, and port responses is shown as the black trace below 300Hz in fig.3. The usual boost in the upper bass, which will be due to the nearfield measurement technique, is absent, which suggests that the Tad speaker's reflex alignment is tuned for articulation and low-frequency clarity rather than bass weight. The black trace above 300Hz in fig.3 shows the CE1TX's farfield output, averaged across a 30° horizontal window centered on the tweeter axis. The balance is even but with a slight lack of energy in the presence region and some small peaks and dips above 7kHz, the latter due to the coaxial mounting of the tweeter.
Fig.4 Tad CE1TX, 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 Tad CE1TX, 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 Tad speaker's horizontal dispersion, normalized to the response on the tweeter axis (fig.4), is similarly uneven in the same HF region but is otherwise well-controlled. The lack of presence-region energy in the on-axis response tends to fill in to the speaker's sides, which suggests that the CE1TX's treble balance will be neutral in all but very small rooms. As expected from use of a coaxial drive unit, the vertical dispersion (fig.5) is very similar to that in the horizontal plane.
Fig.6 Tad CE1TX, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Tad CE1TX, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
In the time domain, the CE1TX's step response on the tweeter axis (fig.6) shows that the tweeter's output arrives first at the microphone, followed by that of the midrange unit, then that of the woofer. All three drivers are connected in positive acoustic polarity, and the decay of each unit's step blends smoothly with the start of that lower in frequency, which suggests optimal crossover implementation. Other than some small ridges of delayed energy in the top octave and some ripples in the upper midrange, the speaker's cumulative spectral-decay ("waterfall") plot (fig.7) is superbly clean.
When I reviewed Tad's Compact Reference CR1 standmount, which employed a similar array of drive units, in January 2012, I was impressed both by its sound quality and by its measured behavior. I wait to read what Herb Reichert thought about the Tad CE1TX's sound quality but, as with the original CR1, this loudspeaker's measured performance is superb.—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.
