AAD Reference Silver-1 loudspeaker Measurements

Sidebar 3: Measurements

The AAD Reference Silver-1 was of below-average sensitivity, at an estimated 84dB(B)/2.83V/m, which, all things being equal, will demand more watts than usual from the partnering amplifier. However, the Silver-1's impedance modulus (fig.1, solid trace) remains high over almost the entire audioband, with a minimum value of 5.4 ohms at 284Hz. And although the electrical phase angle (fig.1, dotted trace) reaches ±45°, this occurs only when the impedance is high, meaning that the Silver-1 will not demand much current from the amplifier. It should work well with tube designs.

Fig.1 AAD Reference Silver-1, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

There is a glitch in the impedance traces at 280Hz, and investigating the cabinet panels' vibrational behavior with a simple accelerometer did reveal, on all surfaces, a resonant mode at this frequency (fig.2). This mode was strongest on the top panel, but the limited radiating area of this surface will work against audibility. Though impossible to see at the scale fig.1 is printed, there is a hint of a discontinuity in the impedance traces around 700Hz—but there are no panel resonances present at that frequency.

Fig.2 AAD Reference Silver-1, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

However, there is a strong, high-Q resonance present at that frequency in the port's output, as well as a milder mode just above 400Hz (fig.3, left-hand trace). The fact that the port faces away from the listener will reduce any audible effects of these resonances. While I wondered if the existence of the higher-frequency mode correlated with the occasional confusion I noticed in the midrange, I could hear this resonance only if I hit it with a signal more or less at its exact frequency, and then only if I crouched down behind the speaker. The audible congestion probably has more to do with the behavior of the cabinet around Middle C.

Fig.3 AAD Reference Silver-1, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the port and woofer plotted below 900Hz and 300Hz, respectively.

Lower in frequency, the peak in the port's response is slightly offset from the minimum-motion notch in the woofer's nearfield response, which coincides with the saddle centered on 42Hz in the fig.1 impedance-magnitude trace. Fig.3 reveals that the crossover between the woofer and tweeter occurs just below 3kHz, but also that there is a discontinuity in the woofer's output at the top of its passband. The crossover's ultimate acoustic rollout slopes are steep, with the tweeter's output down by more than 40dB at 1kHz. The tweeter's on-axis output rises gently throughout its passband, and is still strong at the 30kHz upper limit of this graph.

A similar circular flat-diaphragm tweeter, but without the horn loading, is used by Seattle-based Genesis. When I reviewed Genesis's 5.2 floorstander in February 2006, I found that the tweeter suffered from subharmonic distortion at the bottom of its passband; ie, it produced lower-frequency tones half the frequency of the fundamental. Listening to pure tones, I didn't find that the AAD tweeter behaved in this manner. The combination of the fast crossover slope, the crossover frequency, and the increase in sensitivity gained from the horn-loading must keep the tweeter diaphragm out of trouble.

Fig.4 shows how these individual responses add up in the farfield, averaged across a 30° horizontal window on the tweeter axis. The slightly overdamped bass alignment results in an output that shelves down slightly between 40 and 100Hz, while the bump in the upper bass is entirely due to the nearfield measurement technique. The upper frequencies are superbly flat, broken only by a narrow notch at the top of the woofer's passband, and small peaks at 5 and 10kHz. Even with the tweeter's limited dispersion above 20kHz, there is still full-level energy evident at 30kHz.

Fig.4 AAD Reference Silver-1, 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 Silver-1's horizontal radiation pattern (fig.5) is wide and even below 10kHz, with the on-axis notch just below 2kHz filling in to the speaker's sides. In the vertical plane (fig.6), suckouts in the crossover region develop not much above or below the tweeter axis, indicating that any stands with which the speaker is used be high enough to place the listener's ears level with the tweeter. In-room, the AAD's 1/6-octave, spatially averaged response at the listening position (fig.7) is broken by some residual room effects in the lower midrange and bass. There is also a slight lack of presence-region energy and an excess around 10kHz—the same frequency as the larger of the on-axis peaks—but the balance is smooth overall.

Fig.5 AAD Reference Silver-1, 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 AAD Reference Silver-1, 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 AAD Reference Silver-1, spatially averaged, 1/6-octave response in JA's listening room.

Turning to the Silver-1's time-domain performance, its step response on the tweeter axis (fig.8) indicates that both drive-units are connected with the same positive acoustic polarity, with the individual outputs smoothly integrated, suggesting excellent crossover design. The AAD's cumulative spectral-decay plot (fig.9) demonstrates a very clean initial decay throughout the midrange and treble, correlating with the speaker's grain-free sound quality, but a ridge of delayed energy develops at 2kHz, the frequency of the on-axis suckout. It is possible that this behavior is due to some kind of interaction between the cone and its surround. There is also some low-level hash evident in the mid-treble, perhaps the result of some woofer cone modes that are not totally suppressed by the crossover's low-pass filter.—John Atkinson

Fig.8 AAD Reference Silver-1, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 AAD Reference Silver-1, cumulative spectral-decay plot at 50" (0.15ms risetime).

AAD by Phil Jones, American Acoustic Development LLC
8559 Page Avenue
St. Louis, MO 63114
(314) 814-4660
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