DALI Zensor 1 loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the DALI Zensor 1's frequency response in the farfield; and an Earthworks QTC-40, with its small, ¼" capsule, for the nearfield responses. (The small capsule doesn't interfere with the airflow.)
My estimate of the Zensor 1's voltage sensitivity was 88dB(B)/2.83V/m, which is both surprisingly high for such a small speaker and 1.5dB higher than specified. The plot of the DALI's impedance magnitude and electrical phase (fig.1) suggests that the speaker is relatively easy to drive. The minimum impedance is 5 ohms, and though the phase angle is quite large around 100Hz, the impedance magnitude is also high, mitigating the adverse effect of the phase angle.
There are small wrinkles in the impedance traces at just under 400Hz and between 500 and 600Hz, suggesting the presence of enclosure resonances. However, when I examined the cabinet's vibrational behavior with a plastic-tape accelerometer, I found resonances of relatively high Quality Factor (Q) at 266, 441, and 1400Hz (fig.2). This graph was taken with the accelerometer on the DALI's top panel; the 441Hz mode was also present on the side panels (not shown). These resonances are sufficiently high in frequency and low enough in level that, given their high Q, they should have no subjective consequences.
The saddles centered on 55Hz in the impedance-magnitude trace suggest that this is the tuning frequency of the small reflex port on the Zensor 1's rear panel. Confirming this, the woofer's nearfield response (fig.3, blue trace) had a deep notch at the same frequency, this due to the back pressure from the port resonance holding the cone stationary. The output of the port itself (fig.3, red trace) peaks between 40 and 100Hz, and its upper-frequency rolloff is smooth, unmarred by resonant peaks. The complex sum of the nearfield responses (fig.3, black trace below 300Hz) peaks by 4dB, but this will be almost entirely an artifact of the nearfield measurement technique. The Zensor 1 appears to be flat down to the upper bass, with then the usual 24dB/octave reflex rolloff giving a response down by 6dB at the port tuning frequency of 55Hz. This is approximately the frequency of the note A, the third string of the four-string double bass and electric bass guitar.
Higher in frequency in fig.3, the DALI speaker's balance, measured with its grille removed, tilts up toward the treble, which is probably why my estimate of the speaker's sensitivity was higher than specified. With the well-damped bass alignment, the Zensor 1's response reminds me of the classic ProAc Tablette minimonitor from the mid-1980s. However, the perceived balance will also depend on the speaker's radiation pattern.
Fig.4 shows the DALI's lateral dispersion. The speaker's output to its side is wide and even below 8kHz, but the Zensor 1 quickly becomes quite directional above that frequency, which, in small rooms, will compensate for the exaggerated top-octave response on the tweeter axis. In the vertical plane (fig.5), deep suckouts centered on the crossover frequency of 3.5kHz develop more than 5° above or below the tweeter axis. Stands of the optimal height, at least 24", should be used with these speakers.
In the time domain, the Zensor's step response on the tweeter axis (fig.6) indicates that both drive-units are connected with positive acoustic polarity. The decay of the tweeter's step smoothly bends with the rise of the woofer's step, which suggests optimal crossover design. The cumulative spectral-decay plot (fig.7) is remarkably clean for such an inexpensive speaker. The single resonant mode present, at 1465Hz, is well suppressed.
This is impressive measured performance for a budget loudspeaker.John Atkinson