NHT Classic Three loudspeaker Measurements

Sidebar 3: Measurements

The NHT Classic Three's voltage sensitivity was significantly lower both than average and than specified, at an estimated 83dB(B)/2.83V/m. However, its impedance was generally higher, dropping below 6 ohms only in the lower midrange and above the audioband (fig.1). The minimum value was 4 ohms between 120Hz and 170Hz, but there is also a combination of 5.5 ohms and –46° electrical phase angle at 94Hz that will demand a goodly amount of current from the partnering amplifier.

Fig.1 NHT Classic Three, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The impedance traces are free from the small discontinuities that would imply the presence of cabinet vibrational resonances, and the Classic Three's cabinet was effectively braced. The only resonant mode I could find lay at 496Hz (fig.2), which is high enough in frequency not to affect sound quality, especially considering its low level and the panels' small radiating areas.

Fig.2 NHT Classic Three, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the cabinet's rear panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The peak of 27 ohms at 61Hz in the impedance plot indicates that this is the tuning frequency of the sealed cabinet, the frequency where the anechoic response is down 6dB. However, if you look at the woofer's nearfield response, shown to the left of fig.3, the speaker's response actually appears to extend down to that frequency, due to the boost in the upper bass. Though BJR did remark on a slight upper-bass emphasis, this peaking is mainly due to the nearfield measurement technique; the Three is basically flat down to 90Hz or so. Higher in frequency, the NHT's basically even response on the tweeter axis is broken by an equal number of small peaks and dips. These should be benign when it comes to coloration, but I do note that Bob Reina was taken by the Classic Three's reproduction of vocals, which may be a result of the slight boost in output apparent at the top of the midrange in fig.2. Note how the use of a small-diameter metal-dome tweeter pushes the usual dome resonance up above 30kHz.

Fig.3 NHT Classic Three, 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 grille consists of cloth stretched across a vestigial frame, so I was somewhat surprised to find that it had a relatively large effect on the Classic Three's tweeter-axis response (fig.4). Though he didn't remark on any tonal differences, BJR did feel that the speaker's presentation was slightly more detailed without the grille; this might be due to the grille's slight suppression of mid-treble output.

Fig.4 NHT Classic Three, effect on tweeter-axis response of adding the grille (2dB/vertical div.).

Measured without its grille, the Classic Three's lateral dispersion was superbly wide and even (fig.5), with only a very slight off-axis flare in the low treble, though this will add to the audibility of the on-axis peak in the same region. Note also that the speaker's output doesn't fall off to the sides as much as usual above 10kHz. This, again, will be due to the speaker's use of a small-diameter tweeter. In the vertical plane (fig.6), the speaker is relatively unfussy regarding exact listening axis, but a big suckout develops at 5.3kHz more than 10° above or below the tweeter axis. I would have thought that this suckout was a crossover artifact, but it is a little higher than the specified frequency at which the midrange dome crosses over to the tweeter.

Fig.5 NHT Classic Three, 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 NHT Classic Three, 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.

In the time domain, the Classic Three's step response (fig.7) indicates that all three drive-units are connected with the same positive acoustic polarity, with the fact that each unit's step smoothly hands over to that of the next lower in frequency, correlating with the good integration seen in the speaker's frequency-response graph. The NHT's farfield cumulative spectral-decay plot (fig.8) is generally superbly clean, though a low-level resonant mode can be seen at 8kHz. This is most probably the primary breakup mode of the midrange dome, but it is well suppressed by the crossover.

Fig.7 NHT Classic Three, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 NHT Classic Three, cumulative spectral-decay plot at 50" (0.15ms risetime).

This superb measured performance is especially commendable given the Classic Three's price of $800/pair.—John Atkinson

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