Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Elac Carina BS243.4's frequency response in the farfield, and an Earthworks QTC-40 mike for the nearfield responses. The measurements were taken without a grille.
Though Elac specifies the BS243.4's sensitivity as 85dB/2.83V/m, my estimate was closer to 82.5dB(B)/2.83V/m. The impedance is specified as 6 ohms with a minimum value of 4.8 ohms. The solid trace in fig.1 shows that while the impedance magnitude does briefly drop below 6 ohms, in the midbass and lower midrange, the average impedance in the treble is closer to 8 ohms. The minimum magnitude is 4.6 ohms at 190Hz, and while the electrical phase angle (dotted trace) reaches –57.4° at 101Hz, the magnitude at that frequency is 12.65 ohms. The BS243.4 is therefore not a particularly difficult load for an amplifier to drive.
Higher in frequency in fig.3, the farfield outputs of the BS243.4's woofer (blue trace) and tweeter (green trace) are impressively flat within their passbands, with the crossover occurring at the specified 2.7kHz. The aluminum-cone woofer does have some resonant peaks present at 5kHz and above, but the high-order crossover suppresses these by close to 20dB. The Elac's farfield response above 300Hz, averaged across a 30° horizontal window centered on the tweeter axis, is shown in fig.4. The midrange and treble are superbly flat. The black trace below 300Hz in fig.4 shows the sum of the nearfield woofer and port outputs, taking into account acoustic phase and the different distance of each radiator from a nominal farfield microphone position. Again the nearfield bump can be seen, but the BS243.4's low frequencies are tuned to be maximally flat and down by 6dB at the port tuning frequency.
Fig.1 Elac BS243.4, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
There don't appear to be any small discontinuities in the impedance traces that would suggest the presence of cabinet vibrational resonances. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, however, I found two high-level modes, at 453Hz and 566Hz, on the sidewalls (fig.2), as well as on the top panel. Both of these modes have a high Q, meaning that they need to be excited for a while to be fully developed, and, as they also have relatively high frequencies, it is unlikely that they would lead to audible congestion in the midrange.
Fig.2 Elac Carina BS243.4, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The impedance-magnitude plot has a saddle centered on 54Hz, which implies that this is the port's tuning frequency. The blue trace in fig.3 shows the woofer's nearfield response, which does indeed have its minimum-motion notch at 54Hz. (This is the frequency at which the back pressure from the port resonance holds the cone stationary.) The nearfield response of the downward-facing port (red trace) peaks at the same frequency; while its upper-frequency rolloff is clean, there are a couple of small peaks present between 400Hz and 550Hz. The small rise in the upper bass is due to the nearfield measurement technique, which assumes the woofer is mounted in a baffle that extends to infinity in both horizontal and vertical planes.
Fig.3 Elac Carina BS243.4, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield woofer (blue) and port (red) responses respectively plotted below 350Hz and 700Hz.
Fig.4 Elac Carina BS243.4, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with their complex sum of the nearfield woofer and port responses plotted below 300Hz.
Fig.5 shows the Elac's horizontal dispersion and reveals that there is a very slight lack of energy off-axis at the top of the woofer's passband, which might make the speaker sound polite in large rooms. The BS243.4 rapidly becomes directional above 7.6kHz to its sides, and as the on-axis response doesn't have the usual top-octave peak, this speaker's balance will lack air in overdamped, medium-to-large rooms. In the vertical plane (fig.6), a suckout develops in the crossover region more than 5° above and 10° below the woofer axis. These speakers should be used on stands that are sufficiently high that the listener can't see their top panels.
Fig.5 Elac Carina BS243.4, 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 Elac Carina BS243.4, 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.
Turning to the time domain, the Carina BS243.4's step response (fig.7) indicates that both the tweeter and midrange unit are connected in positive acoustic polarity. (I checked this by looking at which way the woofer moved when I applied a 2V DC voltage to its terminals.) The decay of the tweeter's step, which arrives first at the microphone, smoothly blends with the start of the woofer's step, suggesting optimal crossover implementation. The Elac's cumulative spectral-decay plot (fig.8) is extremely clean, with the high-frequency resonances in the woofer cone well-suppressed.
Fig.7 Elac Carina BS243.4, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Elac Carina BS243.4, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
For a fairly expensive minimonitor to justify its price, it should offer excellent audio engineering. The Elac Carina BS243.4 does so.—John Atkinson
