NHT Classic Absolute Tower loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the NHT's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses. NHT specifies the Classic Absolute Tower as having a sensitivity of 86dB/W/m. My estimate was within experimental error of this figure, at 85.5dB(B)/2.83V/m. The impedance lies above 6 ohms for much of the audioband, dropping below 5 ohms in just two regions, with minimum magnitudes of 4.56 ohms at 133Hz and 4.27 ohms at 550Hz (fig.1). The speaker is a relatively easy load for the partnering amplifier.
Fig.1 NHT Classic Absolute Tower, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
The traces in the impedance graph are free from the small glitches that would imply the presence of cabinet vibrational resonances. Nevertheless, investigating the behavior of the panels with a simple accelerometer did uncover a moderate mode at 234Hz that was present on all surfaces, as well as a lower-frequency mode on the side panel level with the bottom woofer (fig.2). However, Bob Reina heard no lower-midrange congestion that might have resulted from this behavior.
Fig.2 NHT Classic Absolute Tower, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel adjacent to lower woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The single peak at 67Hz in the impedance graph indicates that this is the tuning frequency of the sealed-box woofer alignment. This is also the frequency at which the output of the woofers (which behave identically) is down by 6dB (fig.3). The Absolute Tower doesn't offer much more than a minimonitor in the way of bass extension, though BJR did comment very positively on the quality of what low frequencies were present. The blue trace in fig.3 shows the NHT's response with its grille on, the red with it removed. Bob strongly preferred the speaker's tonal balance with the grille in place; its main effect is to suck out energy at 6 and 12kHz. There is nothing in this graph, however, that would explain why Bob felt the balance without the grille to suffer from "a slight depression in the lower midrange that detracted from the speaker's normally coherent sound." Perhaps the grille reduces the audibility of the panel resonances noted earlier.
Fig.3 NHT Classic Absolute Tower, anechoic response on tweeter axis at 50" with (blue trace) and without (red) grille, averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield midrange and woofer responses plotted below 300Hz.
Measured without the grille, the NHT Tower's lateral dispersion (fig.4) is as wide and even as you'd expect from its narrow front baffle and the contouring in the vicinity of the tweeter. Though there is a slight flare off-axis at the bottom of the tweeter's passband, this is too small to add any treble character to the speaker's sonic signature. In the vertical plane (fig.5), the Absolute Tower maintains its even balance over a wide window centered on the tweeter axis, which is 35" from the floor.
Fig.4 NHT Classic Absolute Tower, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 905° off axis, reference response, differences in response 590° off axis.
Fig.5 NHT Classic Absolute Tower, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 155° above axis, reference response, differences in response 510° below axis.
Turning to the time domain, the NHT's step response (fig.6) indicates that all four drive-units are connected with positive acoustic polarity, and that the tweeter's output leads that of the midrange, which in turn leads that of the woofers. That the decay of each step smoothly blends into the start of the step of the next driver lower in frequency suggests optimal crossover implementation. I was a bit puzzled by the presence of a small reflection at 7.2ms in this graph, which didn't appear to be from any of the room boundaries. I windowed it out of the impulse response when I calculated the NHT's cumulative spectral-decay plot (fig.7), which is superbly clean.
Fig.6 NHT Classic Absolute Tower, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 NHT Classic Absolute Tower, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
That NHT's Classic Absolute Tower measures very well is especially commendable when you consider that it costs just $1000/pair.John Atkinson