Tetra 505LTD loudspeaker Measurements

Sidebar 3: Measurements

I performed a complete set of measurements on sample 00155 of the Tetra 505LTD, then repeated some of the measurements on the other sample, serial number 00156, to make sure that the measurements were representative.

The 505LTD is of average voltage sensitivity, at an estimated 86.9dB(B)/2.83V/m—significantly lower than the specified 91dB. However, its impedance plot (fig.1) reveals the Tetra to be a kind load for an amplifier to drive, the magnitude dropping below 8 ohms only in the lower midrange, with a minimum value of 7.1 ohms at 150Hz. The electrical phase angle is also generally mild. But note the sharp spike at 200Hz in this graph, as well as a series of smaller-amplitude spikes higher in frequency. These indicate the presence of some high-Q (quality factor) resonant modes, either in the cabinet walls or in the enclosed air space.

Fig.1 Tetra 505LTD, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Despite the benefits claimed for the primary enclosure's tetrahedral shape, its panels were lively, with strong resonances detectable at the frequencies of the impedance spikes in fig.1. The rectangular pedestal, which acoustically loads the woofer, was also marred by some resonant modes. Fig.2, for example, calculated from the output of a plastic-tape accelerometer fastened to the center of the raised wooden trim on the front panel, reveals a mode at 297Hz, as well as some lower-level modes higher in frequency.

Fig.2 Tetra 505LTD, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the front of the pedestal, in the center of the raised trim (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Pausing the noise stimulus used to generate this graph proved interesting, in that a reverberant honk could be heard dying away. The question then becomes: Is this effect due to panel resonances, to internal air-space resonances, or to a combination of both? Shown to the left of fig.3 are the individual outputs of the woofer and port, scaled in the ratio of their radiating diameters. The port's response peaks as expected at the tuning frequency of 29Hz, which is also the minimum-motion point in the woofer's response. However, the midrange outputs of both woofer and port are marred by vicious-looking resonances, again at the frequencies of the impedance anomalies. (Both samples of the 505LTD were affected in the same manner.) Hit play on the sound file below to hear the effect of the resonances on the woofer's output.

Fig.3 Tetra 505LTD, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 350Hz and 1kHz, respectively.

These resonances are of high Q, which works against their audibility. (The higher the Q of a resonance, the less likely it is to be excited unless the signal is very close in frequency.) But in the case of the Tetra 505's midrange resonances, I found them very audible in my own auditioning of the speakers. Male speaking voice—Richard Lehnert's spoken introductions on my Editor's Choice CD (Stereophile STPH016-2), for example—acquired a distinct midrange honk; again, if I paused the CD player, the honk could be heard as a reverberant die-away. On instruments with a lot of energy in the region of the resonances, such as the solo clarinet on my Mosaic CD (Stereophile STPH015-2), some notes leapt forward in the soundstage while others were suppressed almost to the point of inaudibility.

Higher in frequency in fig.3, the woofer's output rises through the midrange before peaking in the 1–2kHz region, with a shelved-down output before it crosses over to the tweeter just below 5kHz. This is almost an octave higher than is usual with a relatively large-diameter midrange unit such as the Tetra's. The crossover appears asymmetrical, with a fast woofer low-pass rollout but a slower roll-in for the tweeter. The latter starts to roll off in the top audio octave, due to the fact that the sloped-back baffle puts the listening position significantly below the main tweeter axis and the 1" soft-dome tweeter is starting to become directional in this region.

Fig.4 shows how the individual low-frequency responses sum at a nominal farfield point level with the tweeter, taking into account acoustic phase and the physical offset of the rear-firing port. Below 150Hz, the response follows a textbook reflex alignment with a –6dB point at the port tuning frequency. But higher in frequency those resonances again make their presence known, and again the upper midrange is shelved up by 3dB, followed by the entire treble region being suppressed by 3–5dB. In addition, the lack of energy between 3kHz and 8kHz suggests that the crossover is not optimized for this axis, which is 38" from the floor. Certainly in my own auditioning, I found the Tetra to sound mellow.

Fig.4 Tetra 505LTD, anechoic response on tweeter axis at 50", averaged across 30º horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses, taking into account acoustic phase and distance from the nominal farfield point, plotted below 350Hz.

The 505LTD's horizontal dispersion, assessed on the tweeter axis (fig.5), is rather more uniform than I was expecting from the woofer's relatively large diameter, though the speaker is more directional than usual over most of the treble. In the vertical plane (fig.6), the crossover-region suckout fills in above the tweeter axis but worsens below it, suggesting that a high listening chair—a director's chair rather than a typical easy chair—would give the best mid-treble balance.

Fig.5 Tetra 505LTD, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5º off axis, reference response, differences in response 5–90º off axis.

Fig.6 Tetra 505LTD, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5º above axis, reference response, differences in response 5–15º below axis.

As I had set up the Tetra 505LTDs in my listening room to be auditioned, I used pink noise and an AudioControl SA3050A 1/3-octave analyzer to assess the speakers' spatially averaged response in my room. The result is shown in fig.7. The treble and lower midrange are shelved down, while the region covered by the port peaks up. The port output did indeed sound disconnected with the speaker's higher-frequency output. I experimented with room positioning, but, unlike Paul Bolin, I couldn't get the Tetra's low bass to integrate with the rest of the low frequencies.

Fig.7 Tetra 505LTD, 1/3-octave, spatially averaged response in JA's listening room.

In the time domain, the 505LTD's step response (fig.8) shows a time-coincident right-angle shape, though there is a slight discontinuity in the step's leading edge and the tail of the step is disturbed by the regular undulations typical of a resonance. Looking at the individual steps of the woofer (fig.9, blue trace) and tweeter (fig.9, red trace), it can be seen that the leading-edge discontinuity in fig.8 is due to the HF unit, connected in inverted acoustic polarity, working against the woofer's positive-going output. This correlates with the on-axis crossover-region suckout noted in fig.4. The low-frequency undulations seen in fig.8 can now be seen to be associated with the woofer's output.

Fig.8 Tetra 505LTD, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 Tetra 505LTD, step response of tweeter (red) and woofer (blue) on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Finally, the Tetra 505LTD's cumulative spectral-decay plot (fig.10) reveals the existence of a strong resonant mode at 1.6kHz. (Note that the speaker was placed on a high stand for all the measurements other than the in-room response. Therefore, no boundary reflections affected what you see in these graphs.)

Fig.10 Tetra 505LTD, cumulative spectral-decay plot at 50" (0.15ms risetime).

While Paul Bolin apparently managed to work around this speaker's limitations to some extent, its disappointing measured performance precludes any recommendation for the Tetra 505LTD from this magazine, its stunning appearance notwithstanding. From my own auditioning, I feel this speaker is unacceptably colored in the midrange and, in my opinion, suboptimally engineered.— John Atkinson

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