BBC LS3/5a loudspeaker Stirling Measurements

Sidebar: Stirling Measurements

You can find the measurements of my 1978 samples of the LS3/5a here, and a comparison of their measurements with those of a 1996 KEF sample here. The Stirling LS3/5a V2 offered the same low sensitivity as the original, 82.5dB(B)/2.83V/m, but its impedance (fig.1) is closer to the final KEF version than the original. The peak of 34.8 ohms at 81Hz reveals the tuning frequency of the sealed enclosure, and implies only modest LF extension. Other than dips to 7.5 ohms at 171Hz and 9.7 ohms at 20kHz, the Stirling's impedance stays well above 10 ohms at all audio frequencies, meaning that it will be exceptionally easy for the partnering amplifier to drive, despite its low sensitivity.

Fig.1 Stirling Broadcast LS3/5a V2, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Other than a fairly strong vibrational mode at 227Hz on the rear panel that was also detectable at a lower level on the sidewalls, the small cabinet is relatively inert.

The red and blue traces in fig.2 show, respectively, the farfield responses of the tweeter and woofer on the tweeter axis, while the black trace shows the overall response averaged across a 30° horizontal window centered on the tweeter axis. (All measurements were taken with the grille in place.) The crossover appears to feature third-order acoustic slopes, and while the tweeter is pretty flat within its passband, the woofer has a significant peak visible in its upper-midrange output. There appears to be a small notch visible at the crossover frequency, but this is actually due to a suckout at this frequency in both the tweeter and woofer outputs. The nearfield measurement technique I use for frequencies below 300Hz in this graph tends to exaggerate the upper-bass level, but even so, much of the boost you see between 70Hz and 300Hz is real. As I heard, this upper-bass bloom tends to compensate for the speaker's lack of true low-bass extension.

Fig.2 Stirling Broadcast LS3/5a V2, anechoic response on drive-unit axis at 50", averaged across 30° horizontal window and corrected for microphone response (black trace), with the individual responses of the tweeter (red) and woofer (blue), along with the nearfield response of the woofer plotted below 300Hz (black).

How close does the Stirling V2's measured response get to that of true LS3/5as? A dig into my archives produced the response curves shown in fig.3. The red trace is the Stirling's response, duplicating the black trace in fig.2. The slightly shelved-up treble can again be seen, as can the notch between 3.9kHz and 4.9kHz. The blue trace is the response of the 1996 KEF LS3/5a sample, again averaged across a 30° horizontal window on the tweeter axis. It is broadly similar in shape, but its T27 tweeter is a couple of dB less sensitive than that used by the Stirling, and the B110 woofer is flatter at the top of its passband. The green trace in fig.3 is the averaged tweeter-axis response of one of my 1978 Rogers LS3/5as. Its treble is flatter than that of both the KEF and Stirling speakers, but its LF is less extended, and the discontinuity between 1kHz and 2kHz gives rise to an audible nasal coloration compared with the other two.

Fig.3 Stirling Broadcast LS3/5a V2, anechoic response on drive-unit axis at 50", averaged across 30° horizontal window and corrected for microphone response (red trace), with the similarly derived responses of a 1996 KEF LS3/5a sample (blue, offset by –5dB) and a 1978 Rogers LS3/5a sample (green, offset by –10dB).

The Stirling's horizontal dispersion is very similar to that of the original, with a lack of off-axis energy at the top of the woofer's passband compensating for the on-axis boost in the same region. In the vertical plane, crossover suckouts quickly develop above and below the tweeter axis, suggesting that the stands used should be tall enough to place the listener's ears level with the V2's tweeters. Fig.4 shows the Stirling V2's spatially averaged response taken in a grid centered on the position of my ears in my listening room using SMUG Software's FuzzMeasure 2.0 program on my PowerBook. The peak at 32Hz and the suckouts at 200 and 360Hz are residual room effects that have not been eliminated by the averaging, but the curve is impressively even overall. The slight boost apparent between 4 and 8kHz correlates with the slightly peaky mid-treble I noted in my auditioning, while the bass extends in-room to around 60Hz.

Fig.4 Stirling Broadcast LS3/5a V2, spatially averaged, 1/6-octave response in JA's listening room.

The LS3/5a V2's step response (fig.5) indicates that the tweeter is connected in inverted acoustic polarity, the woofer in positive. The cumulative spectral-decay plot (fig.6) shows a generally clean decay, but with a discontinuity and some delayed energy visible at the frequency of the peak in the woofer's output.—John Atkinson

Fig.5 Stirling Broadcast LS3/5a V2, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.6 Stirling Broadcast LS3/5a V2, cumulative spectral-decay plot at 50" (0.15ms risetime).

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