B&W DM302 loudspeaker Measurements

Sidebar 4: Measurements

The DM302's specified sensitivity, 91dB/W/m, is high for a small speaker, but indeed I measured a B-weighted figure of 90.1dB/W/m—this mini will play quite loud with only a few amplifier watts. However, its impedance plot (fig.1) reveals that it drops below 4 ohms for much of the midrange, coupled with a moderately high phase angle in the upper bass. Wimpy amplifiers need not apply for the job of driving this speaker. The "saddle" in the magnitude trace at 55Hz indicates the tuning of the reflex slot.

Fig.1 B&W DM302, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

There is a slight wrinkle in the impedance traces just above 200Hz. Using a simple plastic-tape accelerometer to assess the cabinet's vibrational behavior did reveal the presence of a resonant mode at 207Hz (fig.2), and another an octave higher. This might be expected to make the sound slightly "chesty," but I note that WP did not remark on any lower-midrange coloration that could be placed at the door of this behavior.

Fig.2 B&W DM302, cumulative spectral-decay plot of accelerometer output fastened to cabinet sidewall. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Fig.3 shows the farfield response of the '302—averaged across a 30 degrees window on the tweeter axis—spliced to the nearfield responses of the woofer and port, as well as the complex sum of those nearfield responses. The balance is slightly swaybacked, a moderate rise in the high treble being balanced by a slight excess of upper-bass energy. This kind of measured response does tend to make a small speaker sound bigger than it actually is, as WP found in his auditioning. The bass is to specification at 6dB down at 59Hz, the approximate frequency of the port tuning. Though the slight peak at 1kHz might make the speaker a bit unforgiving on recordings that are themselves bright, on neutral-balanced CDs it will usefully highlight recorded detail.

Fig.3 B&W DM302, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with nearfield woofer and port responses plotted below 300Hz and 500Hz, and the complex sum of woofer and port responses plotted below 300Hz.

WP remarked on the B&W's excellent imaging. Fig.4 shows the speaker's lateral dispersion. Other than a slight flare at the bottom of the tweeter's passband, it is uniform, with an increasing degree of top-octave rolloff off-axis. This behavior always correlates with well-defined imaging, I have found. WP also thought that the speaker sounded a little "relentless" on high stands. This might have some connection with that off-axis tweeter flare. The vertical dispersion plot (fig.5), which shows the changes in response as the listener moves above or below the tweeter axis, reveals that sitting just above that axis pulls down the response in the same region. Sit too high, however, and the sound will become too sucked-out.

Fig.4 B&W DM302, horizontal response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90 degrees-5 degrees off-axis; reference response; differences in response 5 degrees-90 degrees off-axis.

Fig.5 B&W DM302, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45 degrees-5 degrees above-axis; reference response; differences in response 5 degrees-45 degrees below-axis.

The DM302's step response (fig.6) is absolutely normal, with both drive-units connected with the same, positive acoustic polarity. The cumulative spectral-decay plot (fig.7) is overall very clean, implying a grain-free treble. However, there is a slight ridge of resonant energy apparent at the cursor position, 2kHz, which might add a slight brightness to the sound.

Fig.6 B&W DM302, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.7 B&W DM302, cumulative spectral-decay plot at 50" (0.15ms risetime).

Overall, this is superb measured performance for a loudspeaker that doesn't cost much more per pair than a family's weekly grocery bill.—John Atkinson

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