Technics SB-7000 Loudspeaker Lab Report

Lab Report

With B&W's DM6 and the Dahlquist DQ-10, the mid/late '70s saw the SB-7000 as Technics' contribution to the 'phase coherent' speaker debate. The use of staggered drivers to achieve aligned acoustical centres, and passive, maximally flat group/phase delay crossovers are concepts now firmly incorporated into the speaker design cook book – witness everything from KEF's Uni-Q drivers to the sloped baffles of many modern designs. However, the pursuit of 'phase linearity' in the SB-7000 is not without consequence in its 'notchy' amplitude response [see Graph 1] where narrowband peaks and dips through mid and treble are superimposed on a broad, U-shaped trend. These contribute to high ±6dB response errors, coinciding with strong driver resonances at 800Hz, 1.1kHz and 1.5kHz [see CSD waterfall, Graph 2] – and at lower 108Hz, 155Hz and 640Hz frequencies seen on the impedance/phase spectra. The mean 500Hz-8kHz output suggests a sensitivity closer to 88dB than the rated 93dB.

Additional diffraction effects from the grille 'hood' are visible above 2kHz [cyan trace, Graph 1] as is the impact of the –2dB and –4dB crossover pads between 2-5kHz [red and green traces], but THD remains <1% at 90dB SPL. Technics' 6ohm rating is also upheld by the 4.6ohm/2.4kHz minimum, although the load remains sub-8ohm from 85Hz-8.6kHz where the impedance phase swings through a symmetrical ±50o. The 'slot' bass loading is arguably closer to a resistive 'leaky' sealed-box alignment than a true 4th-order reflex, the 350mm woofer peaking at 75Hz [green shaded area, Graph 1] before rolling away to deliver a (diffraction correction) extension of 46Hz (–6dB/200Hz). PM

221vinttech.lab1

Response inc. nearfield summed driver/slot [green], corrected to 1m/2.83V [yellow], ultrasonic [pink]. 0dB, black; –2dB, red; –4dB, green; grille, blue

221vinttech.lab2

Numerous driver resonances correlate with undulations in midrange and treble response [Graph 1]

Sensitivity (SPL/1m/2.83V – 1kHz/Mean/IEC) 88.0dB / 88.3dB / 87.4dB
Impedance modulus: minimum & maximum (20Hz–20kHz) 4.56ohm @ 2.4kHz, 26.2ohm @ 59Hz
Impedance phase: minimum & maximum (20Hz–20kHz) –56° @ 69Hz, +55° @ 9.5kHz
Resp. error 0dB/–2dB/–4dB (200Hz–20kHz) ±6.1dB / ±5.9dB / ±7.6dB
LF/HF extension (–6dB ref 200Hz/10kHz) 46Hz / 27.4kHz
THD 100Hz/1kHz/10kHz (for 90dB SPL/1m) 0.35% / 0.65% / 0.55%
Dimensions (HWD) / Weight (each) 845x480x410mm / 36kg
ARTICLE CONTENTS

X