Loudspeakers

While most of the 'reactive action' takes place through the bass (sub-200Hz) with swings in phase angle of –54°/79Hz and +46°/55Hz (all >15ohm), the Reference Four's minimum impedance is an 'easy' 6.3ohm/168Hz. Beyond this the modulus continues to rise, reaching 8ohm at 415Hz and 14.8ohm/20kHz (there's an inconsequential 'dip' to 10ohm/+11°/1.6kHz). So AVID's Reference Four is a very gentle load for the partnering amplifier, a quality traded, in part, for its slightly lower-than-average sensitivity of 85.5dB/1kHz (or 85.3dB from 500Hz-8kHz) – rather less than the optimistic 88dB specification.

The reward comes in a generally flat response, peaking slightly in the mid and high treble [see Graph 1, below], with errors of just ±1.2dB and ±1.1dB from 500Hz-8kHz (±3.2dB from 200Hz-20kHz) and a fine pair matching of 0.6dB from 200Hz-10kHz. The 160mm polymer-coned woofer reaches across 53Hz-790Hz (–6dB), augmented by a downward-firing port tuned, sharply, to 31Hz. This delivers an impressive, diffraction-corrected bass extension of 29Hz (–6dB re. 200Hz), but very low bass rolls away steeply [green infill, Graph 1]. The partnering 28mm doped silk dome tweeter extends almost precisely to 20kHz before rolling away just as steeply to ~26kHz (–6dB re. 10kHz). While this renders the Reference Four moot (mute?) to 'hi-res' audio, and while very mild midrange resonances are visible in the CSD waterfall [Graph 2], the lack of any ultrasonic dome break-up ensures the speaker's impulse response is exceptionally clean. Distortion, too, is held to a consistent, and consistently low, ~0.5% through mid and treble (all re. 90dB SPL/1m). PM