Loudspeakers

If there was ever a correlation between subwoofer size and infra-bass extension then KEF’s relatively compact KC92 has many of its ‘big box’ competitors out cold. For example, set to ‘LFE’ – the bypass mode – the KC92’s working bandpass is a maximum 17Hz-215Hz (–6dB re. 100Hz) with a very short <1msec latency. From the peak of the impulse, the (ETC) decay time is 20msec (re. –20dB) – quick, but slightly longer, perhaps, than might be anticipated from such compact drivers. This must be weighed against the extension achieved by the KC92’s DSP and drivers so, if you are of the persuasion that limiting in-band group delay (a measure of phase distortion) has subjective consequence, then the maximum 32msec/20Hz (80Hz crossover) incurred by the very gentle 3rd-order acoustic/electronic roll-off is a ‘good thing’.

With the crossover (upper cut-off frequency) set to highest 140Hz/24dB roll-off, the response tightens up to 15Hz-128Hz [orange trace, see Graph 1], latency increases to a mere 1.5msec, the group delay is 25msec/20Hz while the decay time increases to 34msec. The lowest 40Hz crossover yields a tight 18Hz-46Hz bandpass [purple trace], an increased 38msec group delay and a far longer 60msec decay time. With the typical 80Hz crossover, bandwidth is 16.5Hz-75Hz [red trace], group delay, as previously mentioned, is 32msec and decay 39msec. Here the ‘Wall’ eq. attenuates output by –2.7dB/20Hz [dashed trace] and ‘Corner’ by –5.5dB [dotted], these modes compensating for the two- and three-surface boundary gain, respectively. Finally, distortion holds to a low <1% (60Hz-100Hz), increasing to ~6%/20Hz as the drivers reach their excursion limit (all re. 90dB SPL/1m). For a given output, distortion will fall with multiple KC92s. PM