Sidebar 3: Measurements
I used DRA Labs' MLSSA system, a calibrated DPA 4006 microphone, and an Earthworks microphone preamplifier to measure the Triangle Antal 40th's behavior in the farfield. I used an Earthworks QTC-40 microphone for the nearfield responses. This mike has a ¼"-diameter tip, so it will not obstruct the radiation from the driver diaphragms or port.
The Antal 40th's anechoic sensitivity is specified as a high 92dB/W/m. My B-weighted estimate was 3dB lower, at 89dB(B)/2.83V/m, though this is still higher than average. Triangle specifies the Antal 40th's impedance as 8 ohms, with a minimum magnitude of 3 ohms. Using Dayton Audio's DATS V2 system, I measured an impedance magnitude (fig.1, solid trace) that reached a minimum value of 2.5 ohms at 132Hz. Compounding the need for current from the partnering amplifier, the electrical phase angle (dotted trace) is occasionally high. As a result, the effective resistance, or EPDR (footnote 1), lies below 2 ohms between 66Hz and 145Hz and between 180Hz and 645Hz, and the minimum value is 1.11 ohms at 93Hz. While, to some extent, the drive difficulty will be ameliorated by the moderately high sensitivity, the Antal 40th will work best with amplifiers that don't have problems driving low impedances.
The complex sum of the Antal 40th's midrange, woofer, and port responses is shown as the black trace below 300Hz in fig.4. The peak at 200Hz will be due to the midrange unit's output, and there is no sign of the boosted midbass region typically seen as a result of the nearfield measurement technique. The Triangle speaker's low-frequency alignment is therefore overdamped, favoring articulation over ultimate bass weight.
Fig.5 shows the Antal 40th's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line. Other than some irregularities in the mid-treble region, the contour lines in this graph are evenly spaced, suggesting a steady, well-controlled narrowing of the speaker's radiation pattern as the frequency increases. The Triangle's dispersion in the vertical plane, shown in fig.6, confirms that the tweeter-axis response is maintained 5° below that axis. A suckout at 4kHz develops above the tweeter axis, which implies that this is the crossover frequency between the midrange unit and the tweeter.
In the time domain, the Antal 40th's step response (fig.7) indicates that the tweeter and midrange unit are connected in negative polarity, the woofers in positive acoustic polarity. The tweeter's output arrives first at the microphone, followed by that of the midrange unit, then that of the woofers. The positive-going decay of the midrange unit's output smoothly blends with the positive-going start of the woofers' step, but the decay of the woofers' output is disturbed by some oscillations that correlate with ridges of delayed resonant energy in the speaker's cumulative spectral-decay plot (fig.8). The plot's decay is clean, however, in the region covered by the horn-loaded tweeter.
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.
Footnote 2: Though the Antal 40th's farfield frequency response has many small peaks and dips, my experience has been that these tend to balance each other when it comes to tonal balance perception.
Fig.1 Triangle Antal 40th, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The traces in fig.1 are free from the slight discontinuities that would imply the presence of resonances of various kinds. However, when I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I did find some resonant modes. Fig.2 is a waterfall plot calculated from the accelerometer's output when it was fastened to a sidewall level with the upper woofer. (This was the liveliest of the enclosure's panels when I rapped them with my knuckles.) Modes can be seen at 336Hz and 523Hz, both of which are relatively high in level. Their Q (Quality Factor) is high, which will work against audibility.
Fig.2 Triangle Antal 40th, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel level with upper woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The saddle centered at 37Hz in the impedance magnitude trace indicates that this is the tuning frequency of the flared port at the bottom of the front baffle. The port's response, measured in the nearfield (fig.3, red trace), peaks at the tuning frequency. While the upper-frequency rolloff is disturbed by a small peak just below 200Hz, the rolloff is otherwise smooth and free from resonances in the midrange. The sum of the two woofers' nearfield outputs (blue trace; both woofers behave identically) has the expected minimum-motion notch at the port tuning frequency, which is when the back pressure from the port resonance holds the cones still. The woofers' response slopes down gently above 150Hz, with some overlap with the nearfield output of the midrange unit (green trace). The latter has a small peak at 200Hz.
Fig.3 Triangle Antal 40th, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of midrange unit (green), woofers (blue), and port (red), respectively plotted below 450Hz, 350Hz, and 550Hz.
Fig.4 Triangle Antal 40th, anechoic response on tweeter axis (black) at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange, woofer, and port responses plotted below 300Hz.
The black trace above 300Hz in fig.4 shows the Antal 40th's quasi-anechoic farfield response averaged across a 30° horizontal window centered on the tweeter axis. While the tonal balance is even from 300Hz to 10kHz, many small peaks and dips are present. The tweeter is 41" from the floor, which is 5" higher than what we have found to be the ear height of a typical listener. I queried RS about the height of his ears; he responded that sitting on his "somewhat saggy-cushioned sofa," his ears were level with the midrange unit. He noted that he didn't hear any change in tonal balance when he raised himself higher. I repeated the farfield response measurement on the midrange axis. It was identical to the tweeter-axis response in fig.4, confirming RS's observation.
Fig.5 Triangle Antal 40th, 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 Triangle Antal 40th, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.
Fig.7 Triangle Antal 40th, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Triangle Antal 40th, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The Triangle Antal 40th's frequency balance is flat and even (footnote 2), its dispersion in both planes is well-managed, and the low frequencies are extended if overdamped. The presence of resonances in the region where hearing is most sensitive might be problematic, although their high Q will make them less audible. I could hear the effect of this behavior with the noiselike MLSSA signal, but it will be less audible with music, where it might be outweighed by the loudspeaker's positive attributes.—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.
