Loudspeakers

Lab Report
While the heavy foam bung in the rear reflex port has direct impact on bass roll-off, extension and low frequency ease-of-drive it has no bearing on the B38’s 85.9dB/1kHz (re. 2.83V/1m) sensitivity, a figure that matches Burmester’s own 86dB for this floorstander. Averaged between 500Hz-8kHz, sensitivity is a little lower still at 85.6dB, courtesy of the gently concaved mid/presence response [see Graph 1, below] that then picks up beyond 4kHz (crossover is 2.4kHz) once the AMT tweeter gets into its stride. The peak at 13.9kHz is still very mild and is reflected in the marginal ±2.35dB and ±2.2dB response errors, respectively, just as Burmester’s fine QC is seen in the tight 0.9dB pair matching (all re. 200Hz-20kHz). The custom AMT tweeter reaches out to a very respectable ~34kHz (–6dB re. 10kHz) and the CSD waterfall [see Graph 2] indicates it is free of any significant resonances, unlike the bass/mid (see below). Conventional harmonic distortion is reassuringly low across the board at 0.2-0.35% (100Hz-10kHz, re. 90dB SPL/1m).

Despite the B38’s modest sensitivity its impedance still dips below 4ohm from 85Hz-350Hz with a minimum of 3.39ohm/115Hz. The speaker’s deep bass also coincides with a big swing in phase angle to –61o/49Hz, contributing to a fairly tough minimum EPDR of 1.3ohm/88Hz. Moreover, the phase plot shows some ‘noise’ from 50-600Hz and there are mild response ripples at ~550Hz and 830Hz, possibly emanating from the 32cm ‘sandwich’ woofers, that punch through the woven fibre-glass mid unit’s response [Graph 1] and are revealed in the CSD waterfall [Graph 2]. PM

Response (port-shorted/‘–ve’ toggle) inc. nearfield woofer [green], freefield corrected to 1m at 2.83V [yellow], ultrasonic [pink]. Left, black; right, red

Ripples seen in the response at 550Hz/830Hz [Graph 1] are revealed as possible LF driver modes