Tannoy Churchill loudspeaker Measurements

Sidebar 4: Measurements

The humongous Churchill is one of the few high-sensitivity loudspeakers that actually meets its spec: my estimated B-weighted figure was exactly 95dB/2.83V/m. This speaker will go very loud with a single watt of power. Its impedance (fig.1) varied between 5 and 8 ohms over almost the entire audioband, with only a moderate electrical phase angle above 200Hz. The magnitude saddle in the bass at 26Hz indicates the tuning of the V-shaped reflex port, implying good LF extension. However, note the peak at 125Hz and the wrinkle in the curve around 180Hz. These indicate the presence of some severe resonances at these frequencies.

Fig.1 Tannoy Churchill, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Using a simple accelerometer to investigate the cabinet's resonant behavior did reveal panel vibrational modes at 125Hz and 180Hz. These can be seen in fig.2, a waterfall plot calculated from the output of the accelerometer when it was fastened to the side panel 12" from the top. But their level is quite low, and I don't believe that it was the panel resonances that contributed to BW finding that the Churchill's upper bass/lower midrange sounded colored. The culprit must lie somewhere else in the speaker's measured behavior.

Fig.2 Tannoy Churchill, cumulative spectral-decay plot of accelerometer output fastened to cabinet sidewall 12" from the top. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

And there it is in fig.3, which shows the individual outputs of the port, woofer, and tweeter. The woofer's and tweeter's responses above 300Hz were taken on the tweeter axis at 50"; below 300Hz, the woofer's response was measured in the nearfield, as was the port's. There is a massive air-space resonance present at 125Hz, which dominates the woofer's output in the upper bass. Fig.3 shows the crossover to be placed at 1kHz, as specified, while the port covers the bandpass centered on the 20-60Hz region.

Fig.3 Tannoy Churchill, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield woofer and port responses plotted below 300Hz and 1kHz, respectively.

The Churchill's overall response, averaged across a 30 degrees window on the tweeter axis, is shown in fig.4. Plotted below 250Hz is the complex sum of the woofer and port responses. Again you can see the massive upper-bass resonance, though in a room this will be less obvious. But note that it contributes to a lack of energy in the lower midrange that might well correlate with BW's feeling that the midrange sounded "dark." Higher in frequency, the response trend is quite smooth through the upper midrange and treble, though with a loss of energy above 11kHz. This, again, might contribute slightly to the dark balance noted.

Fig.4 Tannoy Churchill, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with complex sum of the nearfield woofer and midrange responses plotted below 300Hz.

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