B&W Nautilus 805 loudspeaker Measurements

Sidebar 3: Measurements

My estimate of the Nautilus 805's B-weighted voltage sensitivity was a little below specification at 85.5dB/2.83V/m. However, its impedance plot (fig.1) revealed it as being very easy to drive, with a minimum value of 4.8 ohms at 200Hz and a generally low phase angle, other than in the upper bass. The midbass "saddle" in the magnitude trace indicates the tuning of the unique dimpled port to lie at 42Hz, implying only moderate bass extension.

Fig.1 B&W Nautilus 805, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Other than one wrinkle, at the tweeter's ultrasonic resonance frequency, the impedance plot is free from anything that might indicate unwanted resonant problems. The waterfall plot calculated from the output of an accelerometer fastened to the curved side wall (fig.2) reveals a relatively strong side-wall vibrational mode at 477Hz. However, the complicated cabinet construction has pushed the frequency of this mode sufficiently high that it will have little subjective effect.

Fig.2 B&W Nautilus 805, cumulative spectral-decay plot of accelerometer output fastened to center of side wall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

>From left to right, fig.3 shows the individual outputs of the port, woofer, and tweeter. The reflex notch in the woofer's response lies fractionally lower than the peak of the port's bandpass, but the port tuning seems otherwise well arranged. There is a mode apparent at 900Hz in the port output, with a suspicious-looking suckout in the woofer's farfield output at the same frequency. It's hard to predict what, if any, will be the subjective effect of this behavior. The crossover appears to be set at the specified 3kHz and features steep acoustic slopes, though the woofer rolls out a little early in absolute terms. The tweeter is flat over the lower two-and-a-half octaves of its passband, but peaks up by 15dB at its resonant frequency. This will not be a problem with CD programs, but with the new wider-bandwidth SACD and DVD-A, it might become an issue.

Fig.3 B&W Nautilus 805, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz and 1kHz, respectively.