MEASUREMENTS
Iused DRA Labs’ MLSSA system, a calibrated DPA 4006 microphone, and an Earthworks microphone preamplifier to measure the quasi-anechoic frequency- and time-domain behavior of the 805 D4 Signature in the farfield. I used an Earthworks QTC-40 microphone for the nearfield responses and Dayton Audio’s DATS V2 system to measure the impedance magnitude and phase. I used a plastic-tape accelerometer with a preamplifier of my own design to examine the enclosure’s vibrational behavior.
Bowers & Wilkins specifies the 805 D4 Signature’s voltage sensitivity as 88dB/2.83V/m. My B-weighted estimate, measured on the tweeter axis, was within experimental error of that figure, at 88.4dB(B)/2.83V/m. The 805 D4 Signature’s nominal impedance is specified as 8 ohms, with a minimum value of 4.6 ohms. The impedance magnitude (fig.1, solid trace) varied considerably but had an average value close to 8 ohms. The minimum impedances I measured were 4.68 ohms at 175Hz and 4 ohms at 20kHz. The electrical phase angle (fig.1, dotted trace) is occasionally high, with the result that the effective resistance, or EPDR,1 drops below 3 ohms in several places between 33Hz and 574Hz and above 4.5kHz. The minimum EPDR values were 2.54 ohms at 56Hz, 2.34 ohms at 108Hz, 2.71 ohms at 373Hz, and 2 ohms between 8.9kHz and 10kHz. The 805 D4 Signature is a relatively current-hungry load for the partnering amplifier. This is probably why the sound hardened somewhat with the NAD M10 amplifier when I turned up the volume.
The enclosure’s sidewalls seemed inert when I rapped them with my knuckles. While I found resonant modes at 855Hz and 1100Hz on the aluminum back wall and the center of each sidewall (fig.2), these modes are very low in level and have a high Q (Quality Factor). It is extremely unlikely they will have audible consequences.
The saddle centered on 42Hz in the impedance magnitude trace suggests that this is the tuning frequency of the port mounted below the woofer on the front baffle. The blue trace below 300Hz in fig.3 shows the woofer’s response measured in the nearfield; it has the expected notch at the reflex tuning frequency. The port’s nearfield response (fig.3, red trace) peaks at the tuning frequency before rolling off 1 EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier’s output devices as the loudspeaker. See “Audio Power Amplifiers for Loudspeaker Loads,” Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/ index.html.