B&W John Bowers Silver Signature loudspeaker Measurements part 2

So how do the crossover filters compensate for the basic drive-unit behavior? Their electrical responses, measured at the speaker terminals, can be seen in fig.5. The woofer drive signal gently slopes down above 150Hz to compensate for the driver's rising on-axis response, with the upper midrange shelved down by 7dB compared with the level at 100Hz. The electrical crossover point to the tweeter is high, at 4.2kHz, with then a second-order, 12dB/octave rollout. The tweeter drive signal is peaked up in the top audio octave to compensate for the unit's dip in this region. Its high-pass slope is steep, at an initial measured 21dB/octave below the crossover point.

Fig.5 B&W Silver Signature, electrical crossover responses, measured at speaker terminals.

The effect on the drive-units' acoustic responses is revealed in fig.6. The tweeter is now flat in its passband, with the crossover suppressing its peakiness below 2kHz by 40dB. The woofer features a mild rise in the lower midrange, with then a flat upper midrange and low treble, before featuring a steep low-pass rollout, due to the second-order electrical slope adding to the intrinsic acoustic rollout. There are two points of interest in the woofer curve, however: the resonant peak at 3kHz remains visible, as does the suckout at 800Hz. Note that the latter is associated with a slight peak in the port output at the same frequency, as well as the wrinkle in the impedance phase trace. The port can be seen to cover a broad passband centered on the 30-50Hz region. (The plotted levels of the nearfield woofer and port responses in this graph have been weighted in the ratio of their diameters.)

Fig.6 B&W Silver Signature, individual quasi-anechoic responses of woofer, and tweeter on tweeter axis at 45" and corrected for microphone response, with nearfield woofer and port responses below 400Hz and 1kHz, respectively.

Fig.7 shows the Silver Signature's overall response on the tweeter axis at a 45" microphone distance, spliced at 300Hz to the complex sum—magnitude and phase—of the woofer and port nearfield responses. The balance is commendably flat, broken by the three things mentioned earlier: the slight peak at 3kHz, the slight lack of energy at 800Hz, and the broad hump in the upper bass. I would suggest that the slight "chalkiness" or sibilance noted in the auditioning is probably due to the residual woofer peak at 3kHz. The Silver Signature's surprisingly subjective bass extension is due to a combination of an overdamped woofer alignment, which gives a relatively slow low-frequency rolloff rate between 100Hz and 40Hz, coupled with the slight equalization noted in fig.5.

Fig.7 B&W Silver Signature, anechoic response on tweeter axis at 45" averaged across 30 degrees horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses below 300Hz.

Looking at the manner in which the B&W's response changes to the speaker's sides (fig.8), the relatively large-diameter woofer becomes directional in its top octave, resulting in a suckout off-axis at the exact frequency where it peaks on-axis. Though the discontinuity between the woofer and tweeter dispersions at crossover is not nearly as marked as with the Velodyne DF-661, the room reverberant field will lack presence-region energy. This will reduce the audibility of the already slight on-axis peak at 3kHz; but, all things being equal, it will also tend to make the speaker sound rather polite, lacking in treble bite. This, I suspect, is the root cause for some listeners remarking on a lack of dynamic contrast. Some UK critics do consider the Silver Signature to sound too bland, as mentioned by Alvin Gold in Stereophile a few issues back (footnote 1). This off-axis behavior is possibly a contributing factor.

Fig.8 B&W Silver Signature, horizontal response family at 45", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis on opposite side from tweeter; reference response; differences in response 5 degrees-90 degrees off-axis on tweeter side of baffle.

In the vertical plane (fig.9), a deep crossover suckout develops for microphone positions even slightly above the tweeter axis—there's that hollowness. Below the tweeter, the response doesn't change too much until the listener is more than 10 degrees off-axis. Nevertheless, I found listening height to be quite critical, sitting on or just below the tweeter giving seamless integration between the drive-units. If you sit so that you can see the cabinet top, or if you stand, you won't hear the magic you've paid for!

Fig.9 B&W Silver Signature, vertical response family at 45", normalized to response on tweeter axis, from back to front: differences in response 20 degrees-5 degrees above tweeter axis; reference response; differences 5 degrees-20 degrees below tweeter axis.



Footnote 1: "Industry Update," Vol.16 No.11, November '93, p.43.
Share | |

X
Enter your Stereophile.com username.
Enter the password that accompanies your username.
Loading