B&W Nautilus 801 loudspeaker Measurements
The big B&W's sensitivity was basically to specification, at an estimated 90dB(B)/2.83V/m. However, the plot of its impedance magnitude and phase (fig.1) reveals that it is a demanding load, dropping to 3 ohms through the midrange and to 4 ohms in the high treble. In addition, a punishingly high capacitive phase angle in the midbass, coupled with a low magnitude, will demand a good, current-worthy amplifier. It's no surprise that WP found biamping the 801s with two Levinson No.332s to be a worthwhile exercise. The saddle in the impedance magnitude curve indicates that the big flared port is tuned to 21Hz, which in turn implies superb low-frequency extension.
Fig.1 B&W Nautilus 801, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Other than the wrinkle at 26kHz, caused by the tweeter's oil-can resonance, the impedance plot is free from any signs that the enclosure has resonant problems. Investigating the cabinet walls' vibrational behavior with a simple plastic-tape accelerometer did show that the bass enclosure's "matrix" construction reduced the levels of almost all resonant modes to very low levels. Fig.2, for example, is a cumulative spectral-decay plot calculated from the output of the accelerometer when it was attached to the side wall of the woofer enclosure.
Fig.2 B&W Nautilus 801, cumulative spectral-decay plot of accelerometer output fastened to woofer-cabinet sidewall 12" from the top. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
But there were a few places where more lively behavior could be found. Fig.3, for example, is a plot calculated from the accelerometer's output when it was fastened to the curved rear surface. At 78Hz, however, the mode shown will probably be too low in frequency to lead to coloration. A similar low-frequency mode could be found on the midrange enclosure---perhaps this is due to the mass/spring action of the enclosure and the isolation compound used. Again, I doubt this will have any subjective consequences.
Fig.3 B&W Nautilus 801, cumulative spectral-decay plot of accelerometer output fastened to curved woofer-cabinet back surface 12" from the top. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)