Naim NBL loudspeaker Measurements
The Naim NBL's specified sensitivity is very high at 92dB/W/m, though this will be boosted 3dB by the speaker's 4 ohm nominal impedance. (That "W" in the specification is therefore actually equivalent to 2W). I measured approximately 89.8dB(B)/2.83V/m, which is still high. Fig.1 plots the Naim's impedance magnitude and phase against frequency. The former varies little, staying within 4 and 6 ohms over most of the audioband, with only a moderate phase angle. A good 4 ohm-rated amplifier will have no problems driving this speaker. A couple of small wrinkles can be seen in these traces, at 250Hz and 1.2kHz, but, as fig.2 reveals, the NBL has the least resonant cabinet I have ever experienced. No wonder PM could not feel no vibrations!
Fig.1 Naim NBL, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.2 Naim NBL, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
The magnitude peak centered at 42Hz in fig.1 indicates the tuning of the woofers' sealed enclosure and implies only moderate low-frequency extension. However, as this tuning doesn't allow for the effects of the adjacent room boundaries on the floor-adjacent woofer mounting, not too much should be read into it. The nearfield output of the woofers can be seen to the left of fig.3, which shows the NBL's individual drive-unit responses. (The relative level of the woofers with respect to the midrange unit in this graph was estimated using real-time analysis of the speaker's output in a quasi-anechoic space—my back yard—with the speaker mounted on a 3'-high stand.) The bass extends to about 33Hz, -6dB (this extension will be increased in-room). There is some unevenness in the woofers' passband; the woofers then cross over to the midrange unit at around 300Hz, which should appropriately be below the Allison Effect "floor dip" resulting from their mounting. The midrange unit basically covers the decade between 300Hz and 3kHz, with again some unevenness apparent in its passband output. The acoustic crossover slopes all appear to be second-order, 12dB/octave.
Fig.3 Naim NBL, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and midrange responses plotted below 300Hz and 500Hz, respectively.