Castle Windsor Duke Loudspeaker Lab Report

Lab Report

With Karl-Heinz Fink consulting on this loudspeaker, expectations are necessarily high, so it's reassuring to see the Duke delivering a 'clean' forward response [see Graph 1], free of notches or peaks and, aside from a shallow depression at 1kHz-2.5kHz near the top of the 200mm woven polypropylene cone's range, an essentially flat, perhaps slightly uptilted, response. Breakup modes are also craftily distributed and at a low level [see CSD waterfall, Graph 2] while conventional harmonic distortion is held to within ~0.3% from 100Hz-5kHz (re. 90dB/1m).

Surprisingly, the magnetic frame grille exerts little adverse influence – there are no obvious cancellation notches [blue trace, Graph 1] – instead almost uniformly reducing the 28mm fibre dome's output by ~1.5dB and, in effect, 'flattening' the forward response still further. Sans grilles the response errors amount to just ±2.1dB and ±2.5dB, respectively, while pair matching is a creditable 0.7dB (re. 200Hz-20kHz). Note that the HF output falls rapidly above 30kHz [pink shaded area, Graph 1].

Sensitivity is rather lower than Castle's optimistic 90dB at 87.6dB/1kHz/1m, and 87.1dB across 500Hz-8kHz, but then the Duke does not punish its partnering amplifier with an especially tough load. Although the minimum impedance of 4.3ohm/165Hz suggests the Duke warrants a nominal 4ohm rating, rather than the suggested '8ohm', the biggest swings in impedance phase occur through the bass at +52°/22Hz and –59°/79Hz and contribute to a moderate EPDR of 1.9ohm/106Hz. The 38Hz port tuning and 59Hz-450Hz/–6dB woofer bandwidth produce a corrected LF extension of 43Hz (–6dB re. 200Hz). PM


Response inc. nearfield summed driver/port [green], freefield corrected to 1m at 2.83V [yellow], ultrasonic [pink]. Left, black; right, red; w. grille, blue


Cabinet resonances are well controlled while all cone modes are held to an impressively low level

Sensitivity (SPL/1m/2.83V – 1kHz/Mean/IEC) 87.6dB / 87.1dB / 85.9dB
Impedance modulus: minimum & maximum (20Hz–20kHz) 4.25ohm @ 165Hz, 35.7ohm @ 69Hz
Impedance phase: minimum & maximum (20Hz–20kHz) –59° @ 79Hz, +52° @ 22Hz
Pair matching/Resp. error (200Hz–20kHz) 0.7dB/ ±2.1dB/±2.5dB
LF/HF extension (–6dB ref 200Hz/10kHz) 43Hz / 31.8kHz/31.4kHz
THD 100Hz/1kHz/10kHz (for 90dB SPL/1m) 0.20% / 0.15% / 0.7%
Dimensions (HWD) / Weight (each) 470x280x310mm / 10kg
Price £4500 (£500 stands)

IAG/Castle Acoustics
Huntingdon, Cambs
Supplied by: IAG House, Huntingdon, Cambs
01480 447700