Usher Audio Technology Be-718 loudspeaker Measurements

Sidebar 3: Measurements

The Usher Be-718 was both less sensitive than average and less sensitive than specified, at an estimated 85dB(B)/2.83V/m. Offsetting the speaker's need for voltage, however, is an impedance modulus that remains above 6 ohms throughout the bass and midrange, and drops below 6 ohms only briefly in the treble (fig.1). Only a combination of 5.7 ohms magnitude and –41° electrical phase angle at 2.5kHz keeps the Usher from being rated an easy load for the partnering amplifier to drive.

The traces in fig.1 are free from the small wrinkles that would hint at the existence of cabinet resonances of various kinds. Investigating the panels' vibrational behavior with a plastic-tape accelerometer indicated that the Be-718's cabinet was well-braced. Though I detected a fairly strong mode at 340Hz on the rear panel (fig.2), this was much lower in amplitude on the side and top panels. I doubt it will have significant subjective consequences.

Fig.1 Usher Be-718, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 Usher Be-718, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the rear panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle at 40Hz in the impedance-magnitude trace implies that this is the tuning frequency of the slot-shaped reflex port at the base of the Usher's front baffle. The woofer's nearfield output did indeed have the expected minimum-motion notch at that frequency, but the port's output actually peaked a little higher in frequency (both shown at the left of fig.3). The woofer has a slight peak evident at 1kHz in its farfield response (fig.3, middle trace), with then a smooth rolloff with an average 18dB/octave slope, disturbed only slightly by some residual, low-level breakup modes in the paper cone. The tweeter (fig.3, right-hand trace) also rolls in with an 18dB/octave acoustic slope and is relatively flat within its passband. The crossover is set as specified to just above 2kHz. The low mass and high rigidity of the beryllium diaphragm pushes the fundamental dome resonance up to greater than 30kHz. (You can just see the start of the rise in response at the far right of fig.3.)

Fig.3 Usher Be-718, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer and port, plotted in the ratio of the square roots of their radiating areas below 350Hz and 1kHz, respectively.

Fig.4 shows how these individual responses add up in the farfield. The broad rise in output in the upper bass will be partially due to the nearfield measurement technique, but the summed output of the woofer and port does extend to about 38Hz, confirming WP's positive impression of the Usher's bass performance. The Be-718's midrange and treble are overall impressively flat, though a slight energy excess centered on 1kHz might—might—correlate with WP's noting an occasional upper-midrange hardness to the speaker's sound.

Fig.4 Usher Be-718, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.

The Usher's horizontal dispersion (fig.5) was smooth and even, though with the usual off-axis flare at the base of the tweeter's passband. The tweeter wasn't quite as directional above 10kHz as is usual for a 1" dome. In the vertical plane (fig.6), a large suckout developed at the crossover frequency of 2.1kHz at extreme off-axis angles. The speaker's response doesn't change significantly over a wide (±10°) range above and below the tweeter axis, suggesting that the Usher will be relatively tolerant of stand height.

Fig.5 Usher Be-718, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.6 Usher Be-718, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

Fig.7 shows the average of ten 1/6-octave spectra taken in WP's smaller room for the left and right speakers individually in a vertical rectangular grid centered on the position of WP's ears while he sat in his listening chair. The spatial averaging minimizes the effect of room acoustics on the measured response; even so, the usual lower-frequency "room gain" extends the Ushers' output to below 30Hz. The midrange is impressively flat, while the treble above 3kHz smoothly rolls off due both to the increasing absorptivity of the room's furnishings and the tweeter's increasing directivity in this region. The only significant departure from perfection in this graph is the lack of in-room energy between 1 and 3kHz, which is more than I expected from the dispersion plots. This kind of in-room balance will accentuate the presentation of recorded detail, but will also tend to make the sound a little hard at times. The degree of this effect will depend very much on the size of the listening room and how far away the listener sits from the speakers.

Fig.7 Usher Be-718, spatially averaged, 1/6-octave response in WP's smaller listening room.

Regarding the Usher's behavior in the time domain, fig.8 shows the speaker's step response on the tweeter axis. Both drive-units are connected with positive acoustic polarity, and the tweeter's step is smoothly integrated with that of the woofer, correlating with the good frequency-domain integration of their outputs on this axis in the crossover region. The cumulative spectral-decay plot (fig.9) is impressively clean, especially in the region covered by the tweeter.

Fig.8 Usher Be-718, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 Usher Be-718, cumulative spectral-decay plot at 50" (0.15ms risetime).

As I have come to expect from Joe D'Appolito designs, the Usher Be-718 offers excellent measured performance.—John Atkinson

COMPANY INFO
Usher Audio Technology
US distributor: MusikMatters
1303 Motor Street
Dallas, TX 75207
(214) 638-3500
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