Stand Loudspeaker Reviews

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the Audio Note AN-E/SPx Ltd. Field Coil's farfield frequency behavior and dispersion. I couldn't raise the loudspeaker as far off the ground as usual for the quasi-anechoic measurements, which meant that the first reflection from the floor occurred relatively early. I therefore placed the microphone at 1m rather than my usual 50". I used an Earthworks QTC-40 mike for the nearfield and corner-positioned responses. As instructed in the manual, I made sure that the speaker's serial number, 42994, matched that of the field-coil power supply.

Audio Note specifies the AN-E/SPx Field Coil's sensitivity as a very high 97dB/W/1m. While my B-weighted estimate was lower, at 91dB(B)/2.83V/1m, this is still usefully high. Audio Note recommends that this speaker be placed in a room's corner, where the reflections from the closely spaced boundaries will increase the level, particularly in the bass (see later). I suspect that Audio Note's sensitivity was taken in this condition. By contrast, my measurement was taken with the loudspeaker well away from room boundaries, my long-term practice. In addition, the B-weighting I use reduces the effect on the estimated figure of the speaker's high- and low-frequency extension. (A series of blind listening tests I performed in the 1990s indicated that the B-weighted sensitivity correlated well with a loudspeaker's perceived loudness.)

The AN-E/SPx Field Coil's impedance is specified as 6 ohms. Measured with Dayton Audio's DATS v2 system, the impedance magnitude lies above 6 ohms for most of the audioband (fig.1, solid trace), with a minimum value of 3.96 ohms at 230Hz. As the electrical phase angle (dashed trace) is occasionally high, the effective resistance or EPDR (footnote 1) drops below 4 ohms for several regions between 28Hz and 724Hz and between 2.8kHz and 6.1kHz. The minimum EPDR values are 3 ohms at 40Hz, 2.2 ohms at 362Hz, and 3 ohms between 3.8kHz and 4.7kHz. Although the AN-E/SPx Ltd. Field Coil is a moderately demanding amplifier load, this will be ameliorated by the speaker's high sensitivity. Audio Note notes that this speaker works best with tube amplifiers; I recommend using such an amplifier's 4 ohm output transformer tap.

There is a slight discontinuity just below 200Hz in the traces in fig.1. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found a resonant mode at 191Hz on the back panel and sidewalls (fig.2). However, this mode is both very low in level and has a high Q (Quality Factor), which will work against it having audible consequences.

The saddle centered at 36Hz in the magnitude trace in fig.1 indicates that this is the tuning frequency of the port at the base of the loudspeaker's rear panel. The red trace in fig.3 shows the nearfield response of the port. It reaches its maximum level at the tuning frequency and the upper-frequency rollout is clean, other than a resonant peak at the frequency of the cabinet vibrational mode and another peak lower in level between 400Hz and 500Hz. The blue trace below 350Hz in fig.3 shows the nearfield response of the woofer; it has the expected minimum-motion notch at the port tuning frequency, which is when the back pressure from the port resonance holds the cone stationary.

The blue trace above 350Hz in fig.3 shows the woofer's quasi-anechoic farfield response, the green trace shows that of the tweeter. Audio Note recommends in the speaker's manual that the tweeter be positioned at roughly ear height. Both the responses in fig.3 were taken on the tweeter axis, which, with the speaker sitting on its 10"-high stand, is 38.5" from the floor; this is close to what a survey performed by Thomas J. Norton for Stereophile found is a typical listener's ear height. The crossover between the two drive units appears to be set at 2.7kHz, though the tweeter's average level is 2–3dB lower than that of the woofer.

The usual rise in the midbass of the complex sum of the woofer and port responses (black trace below 310Hz in fig.4), which will be due to the nearfield measurement technique, is absent. As with the Audio Note AN-E Lexus Signature loudspeaker that Art Dudley reviewed in June 2006 (footnote 2), the AN-E/SPx Ltd. Field Coil's woofer alignment is overdamped in order that the speaker doesn't boom when it is placed in the corner of the room. (The manual states that the AN-E/SPx's ported enclosure has been designed to be placed close to room boundaries, where the bass performance is augmented significantly by the additional reinforcement from the nearby walls.)

The AN-E/SPx Ltd. Field Coil's quasi-anechoic response at 1m, averaged across a 30° horizontal window centered on the tweeter axis, is shown as the black trace above 310Hz in fig.4. The response is very similar to that of the Lexus Signature, with the high frequencies balanced a little too low in level compared with the midrange. There is also a small suckout between 3kHz and 4kHz.

The plot of the AN-E/SPx Ltd. Field Coil's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line (fig.5), shows that this suckout fills in more than 10° to the speaker's sides. In an email, Audio Note's Adrian Ford-Crush noted that as a general rule of thumb, the AN-E/SPx Ltd.s should be toed-in approximately 45° to point at the listening position; experimenting with toe-in will optimize the speaker's balance. However, the dispersion narrows considerably above 10kHz, which will lead to a lack of "air" if the speakers are not toed-in to the listening position.

The Audio Note speaker's radiation pattern in the vertical plane, again normalized to the response on the tweeter axis, is shown in fig.6. A suckout in the crossover region develops above the tweeter axis, so, for optimal results, don't listen to this speaker while standing.

In addition to my usual quasi-anechoic, free-field measurements, I examined how the speaker's response was affected by the recommended corner placement. Fig.7 shows the AN-E/SPx Ltd. Field Coil's 1/3 octave-smoothed response on the tweeter axis at 2m, taken in my listening room with the Fuzzmeasure app. There was only one corner in my listening room that I could use to make this measurement, and I wasn't able to take a meaningful measurement at a greater distance due to interference from the room's furnishings. (In contrast to my quasi-anechoic measurements with MLSSA, this measurement isn't windowed to eliminate reflections.) This graph shows that the Audio Note's low frequencies are indeed boosted by the corner placement, and the treble now smoothly slopes down in textbook fashion. The response in the midrange is uneven.

In the time domain, the AN-E/SPx Ltd. Field Coil's step response (fig.8) indicates that the tweeter and woofer are both connected in positive polarity. The tweeter's output arrives first at the microphone, followed around 0.4ms later by that of the woofer. The AN-E/SPx Ltd. Field Coil's cumulative spectral-decay plot (fig.9) is generally clean, though some low-level delayed energy is present at the top of the woofer's passband. (As always with my waterfall plots, ignore the ridge of delayed energy close to 16kHz, which is due to interference from the MLSSA host PC's video circuitry.)

The Audio Note AN-E/SPx Ltd. Field Coil's measured performance is somewhat idiosyncratic. It confirms that the speakers need to be placed in the room's corners, with the appropriate care taken in setup.—John Atkinson

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Footnote 1: See EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.

Footnote 2: See fig.4 here.