Sidebar 3: Measurements
I used DRA Labs' MLSSA system, a calibrated DPA 4006 microphone, and an Earthworks microphone preamplifier to measure the Quad Revela 1's quasi-anechoic frequency response and time-domain behavior in the farfield. I used an Earthworks QTC-40 microphone for the nearfield responses and Dayton Audio's DATS V2 system to measure the impedance magnitude and phase.
The Quad Revela 1 offers excellent measured performance, but its current-hungry impedance will require care in system matching.—John Atkinson
Footnote 1: 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.
Fig.1 Quad Revela 1, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Quad specifies the Revela 1's sensitivity as 86dB/2.83V/m; my estimate was higher, at 87.6dB(B)/2.93V/m, which is similar to what Paul Miller found in his measurements for Stereophile's sister magazine Hi-Fi News. The Revela 1's nominal impedance is specified as 6 ohms, with a minimum value of 4.1 ohms. The impedance magnitude (fig.1, solid trace) stayed above 6 ohms except in the midrange, and the minimum impedance was 3.71 ohms at 210Hz. The electrical phase angle (fig.1, dotted trace) is high from the bass through the low treble; as a result, the effective resistance, or EPDR (footnote 1), drops below 4 ohms from 81Hz to 1kHz and below 3 ohms from 260Hz to 860Hz. The minimum EPDR values are 1.74 ohms at 130Hz and 2 ohms at 469Hz, frequencies where music can have a lot of energy. The Revela 1 needs to be partnered with an amplifier that doesn't have a problem delivering current into low impedances.
Fig.2 Quad Revela 1, cumulative spectral-decay plot calculated from output of accelerometer fastened to the center of a sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The impedance traces are free from discontinuities that would imply the presence of resonances. Nevertheless, using a plastic-tape accelerometer I found two resonant modes, at 340Hz and 379Hz on the top panel, the rear panel, and the sidewalls (fig.2). These modes are relatively high in level, but their high Q (Quality Factor) will reduce the possibility of them having audible consequences.
Fig.3 Quad Revela 1, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the woofer (blue), port (red), and their complex sum (black) respectively plotted below 300Hz, 400Hz, and 300Hz.
The saddle centered on 42Hz in the impedance magnitude trace implies that this is the tuning frequency of the port on the Quad's rear panel. The woofer's nearfield response (fig.3, blue trace below 300Hz) had the expected reflex tuning notch at this frequency. The port's nearfield response (fig.3, red trace) peaks at the tuning frequency before rolling off cleanly at lower and higher frequencies. The complex sum of the woofer's and port's nearfield responses (black trace below 300Hz in fig.3) doesn't have the usual "bump" in the bass due to the nearfield measurement technique, which assumes the baffle extends to infinity in both planes. This suggests that the Revela 1's low-frequency alignment is overdamped, which in turn suggests that the speaker can be positioned close to the wall behind it without sounding boomy.
Fig.4 Quad Revela 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.5 Quad Revela 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.
The Quad's farfield response without the grille, averaged across a 30° horizontal window centered on the tweeter axis, is shown as the black trace above 300Hz in fig.4. The response is even through the midrange and treble, with a slight depression in the presence region and a slight boost in the top octave. The Revela 1's horizontal radiation pattern, normalized to the response on the tweeter axis (which therefore appears as a straight line in fig.4), indicates that the lack of energy in the presence region fills in to the speaker's sides. The dispersion is generally well-controlled, though the radiation pattern narrows in the top octave. As is often the case, the Quad's treble balance can be optimized by experimenting with toe-in. Fig.5 shows the Revela 1's dispersion in the vertical plane, again normalized to the response on the tweeter axis. A suckout in the crossover region develops 5° above the tweeter axis and more than 10° below that axis. This speaker needs to be used with a stand that places the listener's ears on or just below the tweeter axis.
Fig.6 Quad Revela 1, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Quad Revela 1, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Turning to the time domain, the Quad's step response on the tweeter axis (fig.6) shows that the tweeter and woofer are both connected in positive acoustic polarity. The tweeter's output arrives first at the microphone, followed by that of the woofer, with the decay of its step smoothly blending with the start of the woofer's step. This implies an optimum crossover topology. The Revela 1's cumulative spectral-decay, or waterfall, plot (fig.7) is superbly clean in the tweeter's passband, and while there is some delayed energy present in the midrange, this is low in level.
Footnote 1: 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.
