Pioneer SP-BS41-LR loudspeaker Measurements
Sidebar 3: Measurements
I measured the Pioneer SP-BS41-LR's frequency response in the farfield with DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. For the nearfield measurements I used an Earthworks QTC-40 microphone, whose ¼" capsule provides only a minimal obstacle to the flow of air through the port. The Pioneer speaker's B-weighted sensitivity on the tweeter axis was 85.8dB/2.83V/m, which is within experimental error of the specified 85dB. The electrical impedance remained above 6 ohms at all frequencies (fig.1, solid trace). The average was closer to 9 ohms, which both means that the SP-BS41-LRwho thought of that name?will be an easy load for the partnering amplifier to drive, and suggests that the specified 6 ohms figure is too pessimistic.
The impedance traces have glitches evident at 180, 470, and 700Hz, which implies the existence of some kind of enclosure resonances at these frequencies. Investigating the vibrational behavior of the enclosure panels with a simple plastic-tape accelerometer revealed a rather lively cabinet. The side panels (fig.2) have ridges of resonant energy evident at 570Hz, as anticipated, but also at 470Hz; and while a lower-frequency ridge is visible, this is just above 200Hz, rather than at the 180Hz I had expected from the impedance plot. I would expect some lower-midrange congestion to stem from this behavior.
Turning to the Pioneer's acoustic behavior, the low-frequency saddle in the impedance-magnitude trace suggests that the rear-panel port is tuned to 60Hz. The red trace in fig.3 shows the output of the port, measured in the nearfield. It peaks between 40 and 90Hz, but its smooth upper-frequency rolloff is marred by two high-Q resonances, at 470 and 900Hz. The blue trace in this graph is the nearfield output of the woofer. As expected, it has a sharp notch at the port tuning frequency of 60Hz. The black trace below 300Hz in fig.3 is the sum of the nearfield woofer and port responses. The peak apparent in the upper bass is actually an artifact of the nearfield measurement technique; the SP-BS41-LR's low frequencies will actually be close to flat before rolling off to reach 6dB at the port tuning frequency.
Higher in frequency, the Pioneer's farfield output (fig.3) is respectably even before featuring a small peak between 10 and 15kHz. The plot of the speaker's horizontal dispersion (fig.4) shows that the small suckout below the top-octave peak fills in to the speaker's sides, but also that the tweeter's output above 10kHz falls off rapidly, which will ameliorate the audibility of that peak. Lower in frequency, the Pioneer's dispersion is broad and even, with no sign of the usual off-axis flare at the bottom of the tweeter's passband. In the vertical plane (fig.5), suckouts in the crossover region develop more than 5° above and below the tweeter axis, which suggests that the stands used be tall enough to place the listener's ears close to the tweeter axis.
In the time domain, the SP-BS41-LR's step response on the tweeter axis (fig.6) suggests that both drive-units are connected in positive acoustic polarity. The decay of the tweeter's step smoothly blends into the start of the woofer's step, which implies optimal crossover design. The cumulative spectral-decay plot (fig.7) is impressively clean, with only a very slight amount of delayed energy apparent at 1.7kHz.
The measured performance of Pioneer's SP-BS41-LR would not be out of place in a $1000/pair speaker, let alone one costing just $150/pair. Color me impressed.John Atkinson