Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Technics SB-C700's frequency response in the farfield; for nearfield frequency response, I used an Earthworks QTC-40, which has a ¼" capsule and thus doesn't present a significant obstacle to the sound.
The SB-C700's voltage sensitivity is specified as 85dB/2.83V/m. My estimate was usefully higher than this, at 87.5dB/2.83V/m. The speaker's nominal impedance is 4 ohms, but as fig.1 shows, the impedance magnitude drops below 4 ohms only in a narrow band in the lower midrange, reaching a minimum value of 3.5 ohms at 215Hz. I would classify the Technics as a 6 ohm load, though there is a combination of 4.9 ohms and a 41° electrical phase angle at 145Hz, suggesting that tube amplifiers would work best from their 4 ohm output transformer taps.
Higher in frequency in fig.3, the black trace shows the Technics' quasi-anechoic response on its tweeter axis at 50", averaged across a 30° horizontal window. The response on this axis is impressively even, though a slight rising trend can be seen up to 10kHz. The SB-C700's lateral radiation pattern (fig.4) and vertical radiation pattern (fig.5), both referenced to the tweeter-axis response, indicate that the speaker's dispersion is wide and even up to 10kHz in both planes. Above that frequency there are the usual peaks and dips off axis, due to the tweeter being positioned in the center of the woofer diaphragm, but these will not affect the speaker's tonal quality.
Turning to the time domain, the Technics' step response on the tweeter axis (fig.6) reveals that both drive-units are connected in positive acoustic polarity and that the tweeter's output leads that of the woofer. However, the seamless blend between the decay of the tweeter's step and the start of the woofer's step implies optimal crossover design. The cumulative spectral-decay plot (fig.7) is impressively clean.
Fig.1 Technics SB-C700, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Other than a very slight wrinkle at 600Hz, the impedance traces are free of the discontinuities that would suggest the presence of enclosure resonances. When I investigated the cabinet's vibrational behavior with a plastic-tape accelerometer, I did indeed find a low-level mode at 600Hz that was present on the top and sidewalls. However, the strongest mode on the top was at 492Hz (fig.2). Given the small radiating area, I doubt that this mode would lead to any sense of midrange congestion.
Fig.2 Technics SB-C700, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of top panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The saddle between 45Hz and 55Hz in the impedance-magnitude trace suggests that the 2"-diameter port on the rear panel is tuned to this region. The blue trace in fig.3, which shows the nearfield output of the woofer, confirms this tuning, as it has a well-defined notch at 48Hz. (This notch is due to the backpressure from the port resonance holding the woofer diaphragm stationary.) The port output itself (fig.3, red trace) peaks in textbook fashion between 40 and 60Hz, and its upper-frequency rolloff is clean. The black trace below 300Hz in fig.3 shows the complex sum of the woofer and port nearfield responses. The broad peak between 70 and 300Hz is entirely an artifact of the nearfield measurement technique, the SB-C700's low frequencies extending at full level to below 80Hz, with the output down by 6dB at the port-tuning frequency and rolling off with the usual 24dB/octave slope below that frequency.
Fig.3 Technics SB-C700, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue), port (red), and their complex sum (black), respectively plotted below 300Hz, 620Hz, and 300Hz.
Fig.4 Technics SB-C700, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.5 Technics SB-C700, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.
Fig.6 Technics SB-C700, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Technics SB-C700, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The Technics SB-C700's measured performance reveals some excellent audio engineering from an unexpected source. I am not surprised HR liked it.—John Atkinson
