Audio Note AN-E Lexus Signature loudspeaker Measurements

Sidebar 3: Measurements

The Audio Note AN-E Lexus Signature was one of the more sensitive dynamic speakers I have measured, at an estimated 92.5dB(B)/2.83V/m, though it should be noted that this is well below the specified 98dB figure. The impedance remains above 6 ohms for most of the audioband (fig.1), though it does drop to 3.9 ohms in the lower midrange and to 4.8 ohms in the upper treble, and there is a combination of 6.2 ohms magnitude and –40° electrical phase angle at 112Hz. Even so, I don't think the speaker will make significant demands for current on the partnering amplifier, which, in combination with the high sensitivity, makes it very suitable for use with flea-powered single-ended amplifiers.

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Fig.1 Audio Note AN-E Lexus Signature, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance traces are broken by some slight discontinuities between 200Hz and 900Hz, though they'll be hard to see at the scale this graph is printed in the magazine. The Audio Note's plywood cabinet was very lively when subjected to the "knuckle-rap" tests: investigating the panels' vibrational behavior with a plastic-tape accelerometer revealed several major resonances. Fig.2, for example, is a cumulative spectral-decay plot calculated from the accelerometer's output when it was fastened to the enclosure's sidewall 8" from the top. A prominent ridge of resonant energy can be seen at 484Hz, with two other strong modes present at 230Hz and 270Hz. These last two modes were also present on the front baffle and the rear panel, while another strong mode was present at 550Hz lower down on the sidewall. The AN-E's high sensitivity will reduce the audibility of this resonant behavior—for a given input signal, the Audio Note will put out a significantly higher sound-pressure level than a conventional speaker—but it will introduce audible coloration, in my opinion. I suspect that it is this behavior that accounts for AD finding the piano's left-hand register to sound too prominent.

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Fig.2 Audio Note AN-E Lexus Signature, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's side panel 8" from the top (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Because the Audio Note has an external crossover, I could examine the behavior of the unfiltered drive-units. The woofer is around 6dB more sensitive than the tweeter, and while it has a basically flat response throughout the midrange and low treble (not shown), its high-frequency output is disturbed by strong breakup modes at 1kHz, 4.8kHz, and 7.1kHz. Once the speaker is driven via its crossover, the upper two woofer modes are suppressed by 15dB and more (fig.3), though the LF driver still seems balanced a couple of dB too high in level compared with the tweeter. The crossover to the tweeter appears to be set at 3kHz, which is both higher than specified and a bit high for a woofer of this relatively large diameter.

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Fig.3 Audio Note AN-E Lexus Signature, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the woofer and port plotted below 350Hz and 700Hz, respectively.

The optimistic specification lists the AN-E's –6dB point as 17Hz, implying that this is the port tuning frequency. However, the saddle centered on 29Hz in the impedance graph (fig.1) suggests that this is the tuning frequency of the rather small port at the base of the cabinet's rear. The port's response does indeed peak at 29Hz (fig.3, left-hand trace), with the woofer's output showing the usual notch at this frequency. However, the woofer rolls off a little early for the port to extend the speaker's output down to the port tuning frequency.

Fig.4 shows the Lexus Signature's response on the tweeter axis at 50", averaged across a 30° horizontal window, spliced at 300Hz to the sum of the nearfield woofer and port responses, taking into account both acoustic phase and the distances of the two radiators from a nominal farfield position. The speaker's bass is well extended, but does shelve down below 120Hz. Bearing in mind that this trace includes the usual 2pi boost in the upper bass that results from the nearfield measurement technique, it looks as if the AN-E really doesn't produce as much bass as you might expect. I'm not surprised that Audio Note recommends placement close to the wall behind it. This will both extend the low frequencies, as AD found, and bring up the midbass level, though at the expense of lower-midrange smoothness.

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Fig.4 Audio Note AN-E Lexus Signature, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses.

Higher in frequency in fig.4, there is a distinct step down in the speaker's output at 1kHz, something that I would have thought would add some nasality to the tonal balance. The treble, however, is pretty flat overall. The AN-E's lateral dispersion is shown in fig.5, with the off-axis traces, spaced at 5° intervals, normalized to the tweeter-axis response. What's interesting about this graph is that it suggests, at least in a room of small to medium size, that the discontinuities in the speaker's on-axis response will, to some extent, be compensated for by its off-axis behavior. The dispersion has a flare just above 1kHz, where the on-axis response has that step down. Similarly, the slight off-axis flare in the octave at the base of the tweeter's passband coincides with a slight lack of on-axis energy, while the on-axis peak centered on 10kHz will, to some extent, compensate for the 1" tweeter's increased directionality in the top audio octave.

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Fig.5 Audio Note AN-E Lexus Signature, 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.

The plot of the Audio Note's vertical dispersion (fig.6) reveals severe suckouts between 2kHz and 3kHz immediately above and more than 10° below the tweeter axis. The AN-E needs to be used with stands that place the listener's ears at or just below the tweeter axis. Note the filling in of the on-axis suckout at 3.5kHz between the 5° and 10° axes, suggesting that the optimal integration of the tweeter and woofer outputs occurs just below the tweeter axis.

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Fig.6 Audio Note AN-E Lexus Signature, 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.

In the time domain, the AN-E's step response on the tweeter axis (fig.7) indicates that both drive-units are connected with the same, positive, acoustic polarity, though the slight discontinuity that can be seen just after the 4ms mark again suggests that the optimal listening axis is just below the tweeter axis. There is a reflection apparent in this graph just before the 7ms mark. I place speakers on a high stand for the acoustic measurements, so I'm not sure what this reflection is from. I windowed it out when I calculated the AN-E's farfield cumulative spectral-decay plot (fig.8), which is why the valid area in this graph extends only to 2.6ms. The decay of the speaker's sound in the region covered by the tweeter is commendably clean. However, some delayed energy can be seen in the octave at the top of the woofer's passband.

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Fig.7 Audio Note AN-E Lexus Signature, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

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Fig.8 Audio Note AN-E Lexus Signature, cumulative spectral-decay plot at 50" (0.15ms risetime).

In its use of a fairly large woofer in a large cabinet, the Audio Note AN-E Lexus Signature is a somewhat old-fashioned design for a two-way. It didn't measure as badly as I expected it to, its designer obviously having worked hard to produce a neutral balance in-room. But I was disappointed by the very lively cabinet and by the discontinuity at the top of the woofer passband, the effects of both of which I could hear with the sample that I measured. And its specified sensitivity and bass extension are misleadingly optimistic, in my opinion.—John Atkinson

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