Magnepan Magneplanar 20.7 loudspeaker Lab Report

With any large panel speaker the principal measurement problem is positioning the microphone far enough away to be in the free-field, or at least at a realistic listening distance. Given that the 20.7 is just over 2m high, true free-field measurement requires a microphone distance of at least 6 metres, perhaps more, which is quite impractical when that measurement is made indoors in anything less than baronial circumstances. So we measured the 20.7's forward frequency response at 2m [see Graph 1] which includes a correction of +6.02dB, added to attempt to make the results comparable with our normal 1m measurements. (As this figure is appropriate to the free-field it is at best an approximation.)


While the response trend is essentially flat, panel resonances give rise to sharp peaks and dips


While free of LF 'cabinet resonances', the panel(s) still betray numerous breakup modes

Regardless, it is clear that the 20.7 has very low voltage sensitivity, significantly short of the specified 86dB, which is around 6dB optimistic according to our results. Moreover, the 20.7 is fairly tough to drive, despite its impedance being largely resistive. Impedance dips to a minimum of 2.5ohm around 450Hz, and despite the low impedance phase angles (±16° at worst) equivalent peak dissipation resistance falls to a minimum of 1.9ohm at 20kHz with more pertinent dips to 2.0ohm at both 318Hz and 646Hz. Although the trend is generally flat, the on-axis responses are highly uneven, leading to large response errors of ±8.7dB and ±9.7dB respectively. Over the same 500Hz to 20kHz frequency range, pair matching error is also poor at ±5.5dB. The response may well be somewhat smoother at larger measurement/listening distances but, as the CSD waterfall [Graph 2] makes clear, much of the high treble unevenness is due to resonance. KH

Sensitivity (SPL/1m/2.83Vrms – Mean/IEC/Music): 80.4dB / 79.7dB / 79.2dB
Impedance modulus min/max (20Hz–20kHz): 2.5ohm @ 451Hz / 4.4ohm @ 30Hz
Impedance phase min/max (20Hz–20kHz): –16° @ 232Hz / 16° @ 20kHz
Pair matching/Response Error (500Hz–20kHz): ±5.5dB / ±8.7dB/±9.7dB
LF/HF extension (–6dB ref 200Hz/10kHz): – / >40kHz / >40kHz
THD 100Hz/1kHz/10kHz (for 90dB SPL/1m): <0.1% / 0.5% / See text
Dimensions (HWD): 2007x737x52mm
Price: £18,995

Magnepan Inc.
White Bear Lake, MN, USA
Supplied by: Decent Audio, Stockton-on-Tees
05602 054669