Naim NBL loudspeaker Measurements part 3
Fig.7 Naim NBL, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-10 degrees below axis.
The Naim's step response on the tweeter axis (fig.8) indicates that all of its drive-units are connected with positive acoustic polarity, although, unlike the Vandersteen 2Ce Signature (also reviewed in this issue), the Naim NBL is not time-coherent. Note that the output of the twin woofers (the rise starting at the 6.5ms mark) arrives about 1ms late to integrate smoothly with the tail end of the midrange unit's step. Though this usually correlates with less-than-optimum crossover integration, the NBL's boundary-adjacent mounting of the woofers makes it hard to predict what the overall effect will be.
Fig.8 Naim NBL, on-axis step response at 50" (5ms time window, 30kHz bandwidth).
Finally, the Naim's cumulative spectral-decay plot (fig.9) reveals some delayed energy present in the mid-treble, which might interfere with ultimate treble transparency and result in the "wearing" nature of high-level playback that PM noted.
Fig.9 Naim NBL, cumulative spectral-decay plot at 50" (0.15ms risetime).
Manufactured to a very high standard, and with heroic and successful efforts made to control cabinet resonances, the NBL's idiosyncratic measured performance strongly mandates careful auditioning.
One final note: It is not part of my usual brief to mention a speaker's documentation in this section, but the Naim's user's manual was so well-written, especially regarding the unpacking and setup of the speaker and the adjustment of the various suspensions, that I just had to let you know how highly I thought of it.—John Atkinson