Joseph Audio Pulsar loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Joseph Audio Pulsar's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. As Michael Fremer conjectured, the Pulsar is relatively insensitive, my estimate of its voltage sensitivity coming in at 83.5dB(B)/2.83V/m. However, as indicated by its plot of impedance magnitude and electrical phase against frequency (fig.1), the Pulsar is an easy load for the partnering amplifier to drive. Not only is the phase angle relatively low, the impedance remains above 8 ohms for most of the audioband, dropping below that value only in the lower midrange, to reach a minimum magnitude of 6.5 ohms at 210Hz.
There is a small wrinkle between 400 and 500Hz in the impedance traces. Investigating the enclosure panels' vibrational behavior with a plastic-tape accelerometer, I did uncover a very mild resonant mode at 473Hz on the top panel (fig.2), and another at 395Hz that was most pronounced on the side panel (not shown). However, both modes were well down in levelthe Pulsar's cabinet is commendably inert, which is probably why MF could get away with not filling the speaker stands' pillars with damping material.
The saddle centered between 40 and 50Hz in the impedance-magnitude trace suggests that the tuning frequency of the large port on the speaker's rear lies in this region. Indeed, the minimum-motion notch in the woofer's nearfield output (fig.3, blue trace), which is where the woofer cone is held stationary by the back pressure from the port resonance, occurs at 45Hz. The port's output (fig.3, red trace) peaks between 30 and 70Hz, with a smooth upper-frequency rolloff marred by a resonant spike at 800Hz. Fortunately, the fact that the port faces away from the listener will work against the audibility of this behavior. Higher in frequency, the crossover between the woofer and the tweeter (fig.3, green trace) occurs at 1700Hz, and both drive-units are well behaved within their passbands. A couple of resonant modes disturb what would otherwise be the woofer's smooth low-pass rolloff, but these are well suppressed by the crossover.
Fig.3 implies that the tweeter is 12dB more sensitive than the woofer, this idea reinforced by the top two octaves of the overall response on the tweeter axis (fig.4)but, as MF commented, the Pulsar's overall tonal balance was indeed "remarkably flat from the upper midrange up." He also noted that the upper bass was slightly elevated, but as the rise in the bass response due to the nearfield measurement technique is mild, the Joseph's low-frequency alignment actually appears to be maximally flat, with a 6dB point around 40Hz.
The Pulsar's plot of lateral dispersion (fig.5) is uniform, other than a slight off-axis flare in the tweeter's bottom octave. As expected, the speaker's output drops off to the sides above 8kHz or so, which, in a room of typical size, will balance the slightly hot tweeter level in the same region. In the vertical plane (fig.6), a suckout develops in the crossover region more than 15° above or 20° below the tweeter axis, which suggests that the speaker's balance will be relatively immune to listener ear height.
I measured the Pulsar's spatially averaged in-room response in my usual manner, taking 20 spectra for the left and right speakers individually in a vertical rectangular grid centered on the positions of the listener's ears when sitting down. The result is shown in fig.7. The Pulsar produces a relatively flat balance from the midrange through the treble, though there is a slight excess of energy at the base of the tweeter's passband. Having auditioned the Pulsars in MF's room, I can confirm that this does not make them sound bright, but I suspect it does add to the sense of good information retrieval. Below the midrange, the reflections from the room boundaries produce a lack of energy in the upper bass and a peak between 40 and 65Hz, which is where the lowest-frequency mode in MF's room coincides with the tuning frequency of the Pulsar's port. I could hear the effect of this peak as an overwarm midbass; but, as MF noted, this elevated midbass wasn't accompanied by "the baggage of obvious bloat," presumably due to the critically damped reflex alignment.
Turning to the time domain, the Pulsar's step response on the tweeter axis (fig.8) indicates that both drive-units are connected with positive acoustic polarity, and that the decay of the tweeter's step smoothly blends into the rise of the woofer's step. This suggests optimal crossover design. The cumulative spectral-decay plot (fig.9) is commendably clean, with the woofer's cone-breakup modes very well suppressed by the crossover. (The ridge of delayed energy just below 16kHz is due to the usual leakage from the computer's video circuitry.)
Looking over the Joseph Audio Pulsar's measured performance, I keep coming back to that superbly flat on-axis response and the well-damped, relatively inert enclosure. This is definitely a high-performance loudspeaker.John Atkinson