Energy Connoisseur CB-10 loudspeaker Measurements
Sidebar 3: Measurements
My estimate of the Energy CB-10's B-weighted sensitivity on its tweeter axis was 88dB/2.83V/m, which is slightly higher than that of the Boston Acoustics A 25. The electrical impedance (fig.1) drops below 6 ohms for most of the midrange, reaching a minimum value of 3.8 ohms at 200Hz. Even so, the speaker should present no drive difficulties to the partnering amplifier or receiver.
Although SM wrote that "knocking on the cabinet's side panels produced a clearly audible resonance," the impedance traces are free from the small wrinkles in the midrange that would imply the existence of such resonances. There is a discontinuity between 900Hz and 1kHz, however, as well as a small peak at 25kHz that is due to the aluminum-dome tweeter's "oil-can" resonance. I was surprised, therefore, when I looked at the cabinet's vibrational behavior with a simple plastic-tape accelerometer, to find a very strong resonant mode at 280Hz on all surfaces (fig.2). I would expect this to add some congestion in the midrange. There is also a lower-level mode just below 1kHz, and the cabinet sidewalls appear to be flexing in the upper bass.
The mode at 280Hz is strong enough to give rise to a small glitch in the woofer's nearfield output (fig.3, blue trace), though the impedance anomaly between 900Hz and 1kHz appears to be associated with a massive pipe resonance in the port's output (red trace), this suspiciously close to a peak in the CB-10's farfield output (black trace above 300Hz). The minimum-motion notch in the woofer's output occurs at 60Hz, suggesting that this is the tuning frequency of the rear-facing portthe same as the frequency of the Boston A 25's port, though at 2" the Energy's port is twice as wide. The complex sum of the CB-10's nearfield woofer and port responses, taking into account acoustic phase and the different distances of the radiators from a nominal farfield point (fig.3, black trace below 300Hz), peaks higher than that of the Boston speaker; though most of this rise is due to the nearfield measurement technique, which assumes a 2pi acoustic environment. This trace does suggest that the Energy CB-10's woofer alignment is on the underdamped side. SM noted that "the bass guitar sounded bigger, fatter, and looser than I would have liked, and at times overpowered the rest of the music," and asked, "Was this due to the tuning of the CB-10's big rear port?" I suspect that it was.
Higher in frequency in fig.3, what is basically a quite flat response for an inexpensive speaker is disturbed by the peak around 1kHz. It's possible that this peak led to SM's finding that some "bright notes rang out with great presence, but were a touch uncontrolled, diffuse, and ragged at the edges." The tweeter's ultrasonic resonance gives rise to an alarming-looking 20dB peak at 25kHz, but this will not be excited by CD material.
Other than in the region of the upper-midrange peak, the CB-10's lateral dispersion (fig.4) is wide and even up to the top of the tweeter's passband, where it becomes more directional. The vertical-dispersion plot (fig.5) suggests that the Energy speakers be used with stands high enough to place the listener's ears on or just below the tweeter axis.
The Energy CB-10's step response on its tweeter axis (fig.6) reveals that both drive-units are connected in positive acoustic polarity, though the slight discontinuity where the decay of the tweeter's step blends into the start of the woofer's step suggests that perhaps the optimal listening axis is just below the tweeter (though this discontinuity could also be the residual effect of that ultrasonic tweeter-dome resonance). The decay of the woofer's step is disturbed by low-frequency undulations associated with the peak just below 1kHz in the tweeter-axis response. The cumulative spectral-decay plot (fig.7) has a ridge of delayed energy just below 1kHz, as well as one at the frequency of the tweeter's dome resonance, but is otherwise clean.
The Energy Connoisseur CB-10's measured performance is dominated by that port resonance in the upper midrange and the adjacent peak in its on-axis response. So while the speaker's excessive upper-bass output might be balanced by the small rise in its upper-octave outputand too much upper-bass energy is one way a small speaker can try to persuade the listener that it is larger than it isthe midrange problems for me hobble this speaker's competitiveness when Boston Acoustics, Infinity, Pioneer, and PSB are producing more neutrally balanced speakers in the same price region.John Atkinson