Aperion Audio Intimus 533-T loudspeaker Measurements
The Intimus 533-T was surprisingly sensitive, at an estimated 90dB(B)/2.83V/m on its tweeter axis, which is within experimental error of its specified 89dB. The 533-T's impedance was close to 6 ohms from 100Hz to 10kHz; it rose above that range, due to the tweeter's voice-coil inductance. Below 100Hz, the impedance curve (fig.1) shows the twin peaks typical of a reflex design, with the minimum value of 4.3 ohms at 44Hz indicating the port-tuning frequency.
Fig.1 Aperion Intimus 533-T, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The impedance traces are commendably free from the small wrinkles that would indicate the presence of cabinet panel resonances, though there is a peculiar bump between 100 and 200Hz. Investigating the enclosure panels' vibrational behavior with a simple accelerometer, I did find a strong resonance at 719Hz present on all surfaces (fig.2). Though quite high in level, this is sufficiently high in frequency, and the areas of the affected panels are sufficiently small, that I don't think it will have any subjective consequences.
Fig.2 Aperion Intimus 533-T, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's top panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The 533-T's frequency response, averaged across a 30° horizontal window centered on the tweeter axis, is plotted above 300Hz in fig.3. There is a gentle rising trend evident from the lower midrange to the high treble, broken by a discontinuity at 1kHz. The colored traces indicate the nearfield responses of the port (red), the lower woofer (green), and the upper woofer (blue). As explained by Bob Reina, the lower woofer does indeed roll off earlier than the upper, so that only the upper unit is handling any significant energy at the crossover frequency to the tweeter.
Fig.3 Aperion Intimus 533-T, anechoic response without grille on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the port (red), upper woofer (blue), and lower woofer (green), and their complex sum (black), plotted below 1kHz, 500Hz, 700Hz, and 300Hz, respectively.
Both woofers have the expected minimum-motion notch in their outputs at the port tuning frequency, but all three low-frequency radiators (woofers and port) show a second, less extreme notch in their outputs around 150Hz. I suspect that this behavior is related to the impedance anomaly in the same region; the end result is a small suckout at this frequency in the calculated sum of the individual responses (fig.3, black trace below 300Hz), which takes into account acoustic phase. Even with the usual nearfield boost in this measurement, the 533-T's bass response is 6dB down around 45Hz, which is only modest low-frequency extension. As BJR said, you shouldn't expect earthshaking low-bass performance from the Intimus 533-T. But I also couldn't see any measured evidence for the midbass warmth he mentioned. Perhaps this character is associated more with the measured anomaly in the upper bass.
All things being equal, I would have expected the discontinuity at 1kHz in the 533-T's farfield response to have resulted in some nasal-sounding coloration, but BJR noted that the speaker was free from any midrange or treble coloration. However, as he mentioned, Bob auditioned the Aperions with their rather bulky grilles on, and I did my primary measurements with them off. Fig.4 shows the difference made to the tweeter-axis response in fig.3 by adding the grille. Eyeballing the sharp peaks and dips due to reflections, you can see that, as well as slightly depressing the top two octaves, the grille boosts the speaker's low-treble output by up to 1dB, which will reduce the influence of the response discontinuity just below this region.
Fig.4 Aperion Intimus 533-T, effect on tweeter-axis response of adding grille (1dB/vertical div.).
The plot of the Aperion's horizontal dispersion (fig.5) shows a slight flare at the bottom of the tweeter's passband, as well as the usual 1" dome's increase in directivity in the high treble. Other than that, the contour lines in this graph are quite uniformly spaced, which generally correlates with stable, accurate stereo imaging. In the vertical plane (fig.6), the 533-T's response doesn't change significantly over quite a large angle, suggesting a useful tolerance of listener ear height.
Fig.5 Aperion Intimus 533-T, 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 Aperion Intimus 533-T, 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 5–10° below axis.
The Intimus 533-T's step response on the tweeter axis (fig.7) suggest that all three drive-units are connected with positive acoustic polarity, the tweeter's step smoothly handing over to that of the woofers, which correlates with the good integration seen in the frequency domain. A ripple with a period of around 1 millisecond can be seen in the tail of the step, however, this associated with the on-axis peak around 1kHz. This delayed energy can be seen in the speaker's cumulative spectral-decay plot (fig.8), but other than that and a small resonant ridge at 5kHz, this graph is superbly clean.
Fig.7 Aperion Intimus 533-T, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Aperion Intimus 533-T, cumulative spectral-decay plot at 50" (0.15ms risetime).
Though there were a couple of anomalies in its measured performance, given the Aperion Intimus 533-T's very affordable price, I was impressed by how it behaved in the test lab.—John Atkinson