Acustik-Lab SR Bolero loudspeaker & Forte woofer Measurements

Sidebar 3: Measurements

I estimated the voltage sensitivity (using 1/3-octave pink noise centered on 1kHz) and measured the change of impedance with frequency, while the nearfield low-frequency response of each speaker was assessed with a sinewave sweep to get an idea of the true bass extension relative to the level at 100Hz.

The Acustik-Lab Bolero's modulus of impedance with frequency is shown in fig.1. Typical of a 4 ohm design, it stays below 8 ohms most of the time, the only exceptions being in the region of the tweeter resonance and the twin reflex humps in the bass. The fundamental box resonance lies at a high 81Hz, as is to be expected from such a small enclosure, while the port is tuned to 58Hz, giving a bass response that extends, measured in the nearfield, to 48Hz (–6dB). The sealed-box woofer is tuned to 45Hz, as can be seen in fig.2. Its impedance drops to a cruel 3.1 ohms in the upper bass, suggesting that low-powered tube amplifiers should be avoided with this combination, unless they have a very good output transformer.


Fig.1 Acustik-Lab SR Bolero, impedance magnitude vs frequency (2 ohms/vertical div.)


Fig.2 Acustik-Lab SR Forte woofer, impedance magnitude vs frequency (2 ohms/vertical div.)

Fig.3 shows the response of the Forte woofer measured in the nearfield and the complementary high-pass feed to the satellite (loaded with an 8 ohm resistor). The extension is modest for a subwoofer, the –6dB frequency being 32Hz, while the Forte still has significant output in the lower midrange above its crossover point to the Bolero. The woofer cabinet vibrates strongly at 180Hz but otherwise seems quite inert, while the Bolero itself has strong cabinet output between 365 and 385Hz (the latter strongly audible as a "hoot" centered on the G above middle C when the ear is pressed close to the side wall), and at 185Hz and 96Hz.


Fig.3 Acustik-Lab SR Forte woofer, nearfield response and the complementary high-pass feed to the satellite,

The frequency response of the speakers in the listening area was measured using pink noise and an Audio Control Industrial SA-3050A 1/3-octave spectrum analyzer. Nine sets of six averaged measurements were taken independently for left and right loudspeakers at a distance of just over 2m in a window 72" wide and varying from 27" to 45" high. This spatial averaging is intended to minimize the effect of low-frequency room standing-wave problems on the measurement, and gives a response curve that has proved to correlate reasonably well with what is perceived; it also gives an idea of the off-axis behavior of the speaker under test.

The spatially averaged, 1/3-octave, in-room responses of both the Boleros on their own and for the Bolero/Forte combination are shown in fig.4. (The responses shown are the average of these measurements, weighted slightly toward the sound heard at the listening position.) Room effects in the bass aside, the response is in general respectably flat, though the bottom two octaves of the tweeter are depressed by between 2dB and 3dB compared with the woofer. The tweeter, however, features a strong on-axis peak in its top octave, something I have noted with other versions of this drive-unit.


Fig.4 Acustik-Lab SR Bolero/Forte, 1/3-octave, spatially averaged response.

The woofer does extend the lows, by half an octave or so, but more importantly, it boosts the entire bass region by 2–3dB. The balance between sensitivity and bass extension for a constant enclosure volume is a critical one for a speaker engineer to manage, but I suspect that the Forte's designer has favored the former rather than the latter. Changing the alignment to reduce the Forte's sensitivity by 3dB would usefully extend the response by a few Hz and give slightly better integration with the Bolero satellite, I feel. Hey, a compound "Isobarik" system, using a second woofer behind the first (provided they are 8 ohm rather than 4 ohm units), would do just that. Perhaps a Forte Mk.II would be in order.—John Atkinson

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