Stenheim Alumine loudspeaker Measurements
Sidebar 3: Measurements
I measured the Stenheim Alumine using DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. The Alumine's B-weighted sensitivity on its tweeter axis was 89.5dB/2.83V/m, which is both higher than average and within experimental error of the specified 90dB. The Stenheim's plot of impedance magnitude and electrical phase angle (fig.1) suggests that it is an easy speaker to drive, the impedance remaining above 8 ohms for much of the audioband and dropping to 5.5 ohms in the lower midrange. The phase angle is also fairly benign, meaning that, in conjunction with its high sensitivity, this speaker will work well with low-powered tube amplifiers.
There is a sharp discontinuity in the impedance traces just above 500Hz, and I did find some very high-Q resonances present in the enclosure's panels in the same region. Fig.2, for example, is a cumulative spectral-decay plot calculated from the output of a simple plastic-tape accelerometer fastened to the center of one of the side panels. Two fairly severe resonances can be seen, at 527 and 645Hz, with a third at a lower level and lower in frequency, 406Hz; these were also present, at lower levels, on the top panel. Art Dudley didn't comment on any midrange coloration that might have resulted from this behavior; it's possible that the resonances are of sufficiently high frequency and Q not to be excited by musical signals. (In general, the higher a resonance's Q, or Quality Factor, the longer it needs to be stimulated with sound at precisely the same frequency as the resonance to be fully excited.)
The saddle centered on 52Hz in the Alumine's impedance-magnitude trace suggests that this is the tuning frequency of the large reflex port on the front baffle. However, in the plot of the woofer's nearfield response (fig.3, blue trace), the minimum-motion notch is a little lower in frequency, at 48Hz. (This is the frequency at which the back pressure from the port resonance holds the woofer cone stationary and all the acoustic output comes from the port.) The port output itself (red trace) peaks sharply between 40 and 60Hz, and rolls off smoothly at higher frequencies. Although there is a slight interruption of the rolloff between 600Hz and 1kHz, there are no pipe resonances of the sort I often find in small, reflex-loaded loudspeakers. Though there is a slight bump in the upper bass, this is entirely an artifact of the nearfield measurement technique; the Alumine's low-frequency alignment appears to be somewhat overdamped, with the output down by 6dB at the port tuning frequency. Nevertheless, I note that AD found the Alumine's bass response "satisfyingly deep."
Higher in frequency, there is a suspicious-looking peak in the Stenheim's farfield response between 700 and 900Hz. This is a little low in frequency to be the result of a termination problem in the cone surround, but I do suspect that this behavior correlates with Art's finding that, with piano recordings, "the right hand sounded brighterand thus louder and more forwardthan the left." The Alumine's treble is impressively even, though the top octave rolls off earlier than the norm. Together with the usual narrowing of the tweeter's dispersion above 10kHz (fig.4), this will tend to make the Stenheim sound a little dark or lacking in "air." This graph also indicates that the dispersion of the relatively large-diameter woofer narrows above 800Hz or so, giving rise to the often-found off-axis flare at the bottom of the tweeter's passband. Vertically (fig.5), a severe suckout centered at 2.2kHzthe crossover frequency?develops more than 10° above the tweeter axis, with, as AD noted, a severe rolloff in the top octaves. The Alumine must be used with its dedicated, 28.75"-tall stand, which places the listener's ears close to the tweeter axis.
In the time domain, the Stenheim Alumine's step response on the tweeter axis (fig.6) suggests that the tweeter is connected in inverted acoustic polarity, the woofer in positive polarity, with the tweeter's output leading that of the woofer by a greater time difference than is usual in a two-way design. Though there is a slight amount of delayed energy at 800Hz, the Alumine's cumulative spectral-decay plot on the tweeter axis (fig.7) is remarkably clean.
There is much to admire in the Stenheim Alumine's measured performance, though that somewhat lively cabinet raised my eyebrows. When I examine a speaker's cabinet-resonant behavior, I support it with three upturned cones at the edges of the speaker's base, which allows resonances to develop to their fullest. I didn't have the Alumine's dedicated stands when I performed the measurements; it's possible that the stand modifies the cabinet's behavior.John Atkinson