Spendor Classic SP100R2 loudspeaker Measurements
Sidebar 3: Measurements
The Spendor Classic SP100R2 loudspeaker was measured with DRA Labs' MLSSA system, using a calibrated DPA 4006 microphone to measure the speaker's frequency response in the farfield, and an Earthworks QTC-40 mike for the nearfield responses. The Spendor's voltage sensitivity is specified as 89dB/W/m, a little higher than my estimate of 88.2dB(B)/2.83V/m. The impedance is specified as 8 ohms with a minimum of 5.5 ohms; the solid trace in fig.1 indicates that the impedance remains above 8 ohms for almost all of the audioband, with a minimum magnitude of 5.95 ohms at 171Hz. Though the electrical phase angle (dotted trace) sometimes reaches extreme values, this is always when the impedance is high, ameliorating the increased demand for current. With its slightly-higher-than-average sensitivity and benign impedance, the Spendor should work well with low-powered tube amps.
The traces in fig.1 are free from the small discontinuities in the midrange that would indicate the presence of panel resonances. Nevertheless, the response of the SP100R2's cabinet to the knuckle-rap test was very lively, and testing with an accelerometer found a number of high-level resonances. Fig.2, for example, taken at the center of the rear panel, shows strong modes at 324 and 258Hz, and a weaker one at 211Hz. All three resonances could be detected on all the speaker's panels, and I must admit to some surprise that Art Dudley noted no specific coloration that could be laid at the feet of this behavior. I wondered, however, if these modes contributed to the speaker's warm tonal balance, especially as AD did note some overhang with low-bass notes.
The saddle centered at 24Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the two ports on the front panel. There is the expected notch in the woofer's nearfield output at that frequency (fig.3, blue trace), which is where the back pressure from the port resonance holds the woofer cone stationary. However, the ports' output (red trace), scaled in the ratio of the square root of the radiating areas to that of the woofer, is a little lower in level and extends over a wider bandwidth than I anticipated. The rise in the woofer's response in the midbass is primarily an artifact of the nearfield measurement technique; the woofer alignment appears to be somewhat overdamped, which is why AD found the speaker to sound a little lean until he could optimize the placement in his room.
The woofer's upper-frequency rolloff is well controlled, and the crossover to the midrange unit appears to be set between 500 and 600Hz, with symmetrical third-order slopes. The midrange unit's and tweeter's outputs (green trace) are basically flat from 700Hz to 21kHz, though with three small peaks visible at the bottom of that range. These peaks are also visible in the overall response on the tweeter axis, averaged across a 30° horizontal window (fig.4). While small peaks and dips in a speaker's response tend not to be noticed as such when of equal amplitude and width, I could hear the lowest-frequency peak with the MLSSA pseudo-random noise signal as a touch of squawkiness. With the small peak in the bass due to the nearfield measurement technique, the low frequencies will actually be flat down to 40Hz or so, but without full reinforcement from the ports in the octave below that frequency. This is sensible design, given that there will be boundary reinforcement of the low frequencies in rooms of small to medium size.
The Spendor's treble is actually commendably flat, and not shelved down as I was expecting from AD's description of the speaker's balance as "warm." Again, I wonder if the warmth stems more from the cabinet's behavior than from the anechoic response, but the SP100R2's lateral dispersion (fig.5) reveals a lack of energy off axis that coincides with the presence-region notch in the tweeter-axis response. I imagine this is a result of the limited dispersion of the 5" midrange drive-unit in the region below its crossover to the tweetera high 5kHz, according to the specification. The vertical-dispersion plot (fig.6) indicates that the SP100R2 is relatively fussy about listening axis, though the balance doesn't change significantly over a ±5° window centered on the tweeter axis. The stands used should be tall enough to place the listener's ears between the top of the woofer and the midrange unit's dustcap.
Turning to the time domain, the Spendor's step response on the tweeter axis (fig.7) reveals that its tweeter is connected in inverted acoustic polarity, its midrange and woofer in positive polarity. The decay of each drive-unit's step blends smoothly with the start of that of the next lower in frequency, suggesting optimal crossover design. The SP100R2's cumulative spectral-decay plot (fig.8) is clean in the region handled by the tweeter, but a ridge of delayed energy is evident at 2.3kHz.
I must admit to some puzzlement over the Spendor SP100R2's measured performance, especially as, other than having tighter, better-controlled low frequencies, it doesn't appear to perform as well as Spendor's S100 from 20 years ago. Certainly, these measurements don't indicate why AD liked the speaker as much as he did. An enigma.John Atkinson