Loudspeakers

Achieving a smooth treble response is a familiar issue with concentric drive unit arrays. The cone of the midrange section forms a horn for the tweeter, and finding a cone and roll surround profile which provides good control of cone breakup behaviour while also loading the tweeter optimally is a challenge. It's one that KEF has largely met with its latest-generation of Uni-Q drivers, and convincingly demonstrated in its £4000 R11 floorstander [HFN May '19]. Our tests on the Navis ARF-51 suggest that ELAC is also well on the road to meeting this challenge as its treble response is nothing like the rollercoasters we've seen in the past from Tannoy's dual concentric drivers.

However, as the forward frequency responses, measured at 1m on the tweeter axis show [Graph 1, below], it could be smoother. In fact the treble fluctuations account for a big chunk of the ±3.4dB and ±3.9dB response errors and, because of their misalignment in the two speakers, to the higher than normal pair matching error of ±2.1dB over the same 300Hz-20kHz frequency range. Not until the upper frequency limit is reduced down to 7.5kHz does the pair matching fall to within a ±1dB range.

Also notable is that ultrasonic output declines rapidly beyond 20kHz, although that might be by design. This all assumes that the ARF-51's HF and MF controls are set to their middle positions. The HF control allows the treble output to be cut or boosted by about 1dB over the tweeter bandwidth (above 2.2kHz), and the midrange control provides the same cut/boost over the midrange passband (down to 260Hz). The diffraction-corrected summed nearfield bass response shows LF output to be pretty flat to 45Hz, below which output falls away rapidly. The CSD waterfall [Graph 2] shows a cluster of resonances in the low treble, and another associated with the ~8kHz response dip. KH