PBN Montana EPS loudspeaker Measurements

Sidebar 3: Measurements

The big Montana was among the more sensitive speakers I have measured, at an estimated 92.5dB(B)/2.83V/m. As BW found, it won't need many amplifier watts to play loud. However, the partnering amplifier needs to have good current capability, as the impedance (fig.1) drops to 3 ohms or below for much of the bass and midrange regions. Thankfully, the phase angle is low over most the audioband.

Fig.1 PBN Montana EPS, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Despite its large panels, the well-braced cabinet is extremely rigid and free from panel resonances. For example, calculating a cumulative spectral-decay plot (fig.2) from the output of a simple plastic-tape accelerometer fastened to the cabinet side wall level with the tweeter gave a graph with just one clearly identifiable mode visible, at 730Hz. Though this resonance could be picked up at many places on the EPS's enclosure and was stronger at some places than others, 730Hz is far above the frequency region where cabinet resonances will have any major effect on sound quality.

Fig.2 PBN Montana EPS, cumulative spectral-decay plot of accelerometer output fastened to side wall level with tweeter. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 shows the individual responses of the two 6"-diameter ports, the two 8" woofers, and the central midrange/tweeter/midrange array. (Although PBN refers to this as a "D'Appolito" array, note that the MTM configuration as specified by Joe D'Appolito uses odd-order crossover filters, not the EPS's fourth-order ones.) The notch at 25Hz in the woofers' output indicates the tuning of the ports, but there is some odd behavior in the ports' response between 70Hz and 100Hz, coincident with a slight notch in the woofers' response in the same region. The crossover to the MTM array is set approximately at 240Hz, with symmetrical 18dB/octave slopes.

Fig.3 PBN Montana EPS, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz and 700Hz, respectively.

The upper-region response, as can also be seen in fig.4, is dominated by a peak between 1kHz and 2kHz, this around half an octave wide. Here is BW's "almost prominent" midrange—while this kind of on-axis behavior might not sound unpleasant and will make the speaker sound very detailed, it does add a "forwardness" to the balance that might make the speaker a bit fussy with respect to amplifier and source quality. Other than that, the midrange and treble regions are quite flat.

Fig.4 PBN Montana EPS, anechoic response on tweeter axis at 50", averaged across 30º horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange, woofer, and port responses plotted below 300Hz.

To the left of fig.4 is shown the complex sum (amplitude and phase) of the woofer and port outputs. As BW found, the big Montana EPS doesn't actually extend as low in frequency as you might expect. While its measured –6dB point lay at 23Hz, the alignment is quite overdamped, which will make the speaker sound lean in large rooms. However, in smaller rooms, and with placement quite close to the wall behind the EPS, the speaker will actually work well in the low bass.

The lateral dispersion (fig.5) is superbly well controlled, with very little evidence of beaming throughout the midrange and low treble. However, the tweeter does roll off a little more quickly to its sides than is usually the case with a 1" dome, and this will make the perceived balance rather mellow in large rooms. Note that there is actually a slight excess of energy off-axis in the very same upper-midrange region where there is an on-axis peak, which will reinforce the impression of the EPS's midrange-forward balance in all but very dead rooms. Vertically (fig.6), the balance doesn't change much over quite a wide window centered on the 35"-high tweeter axis, as BW found. But if you sit with your ears more than 45" from the floor, a suckout starts to develop in the low treble.

Fig.5 PBN Montana EPS, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90º–5º off-axis, reference response, differences in response 5º–90º off-axis.

Fig.6 PBN Montana EPS, vertical response family at 50", from back to front: differences in response 20º–5º above HF axis, reference response, differences in response 5º–10º below HF axis.

In the time domain, the EPS's step response (fig.7) reveals that the speaker is not time-coherent, the tweeter output arriving at the microphone before that of the midrange units, which in turn arrives before that of the woofers. All of the drive-units are connected with positive acoustic polarity, however. Other than a broad ridge of delayed energy associated with the on-axis peak between 1kHz and 2kHz, the Montana EPS's cumulative spectral-decay plot (fig.8) is free from resonant problems.—John Atkinson

Fig.7 PBN Montana EPS, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 PBN Montana EPS, cumulative spectral-decay plot at 50" (0.15ms risetime).

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