Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Wharfedale Diamond 225's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield responses. Wharfedale specifies the Diamond 225's voltage sensitivity as 87dB/2.83V/m; my estimate was lower than that, at 85dB/2.83V/m. The speaker's impedance is specified as being "8 ohm compatible," with a minimum value of 4.2 ohms. My measurement is shown in fig.1—an 8 ohm rating would be fair, particularly given the generally moderate phase angle, but the minimum magnitude was 3.87 ohms between 190 and 200Hz.
Fig.3 shows the individual responses of the tweeter (green trace), the woofer (blue), and the slot-loaded port (red). The crossover appears to occur at the specified 2.3kHz, with steep acoustic slopes. The rise in the woofer's output in the upper bass will be due to the nearfield measurement technique adopted below 350Hz and the port is tuned to 42Hz, the frequency of the lowest open string of the four-string bass guitar and double bass. The port's output extends a little higher than usual, not rolling off until above 100Hz.
Ah. Fig.4 shows the Wharfedale's farfield response, averaged across a 30° horizontal window centered on the tweeter axis, with (blue trace) and without (red) the grilles. With the grilles, which is how I measured the sensitivity, there is a relative lack of energy between 2 and 6kHz, which goes some way toward explaining why the sensitivity I measured was 2dB lower than the specified figure. But without the grilles, the Diamond's entire treble region is now in better balance with its midrange.
Footnote 1: Following some failed experiments with home-brewed speakers in the late 1960s, I bought a pair of Wharfedale's two-way Super Lintons, the model with the "purple jellyfish" tweeter, with which I lived happily for several years.
Fig.1 Wharfedale Diamond 225, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
There is a suspicious-looking discontinuity just above 400Hz in the impedance traces, and I did find a strong vibrational mode on the cabinet sidewalls at 418Hz (fig.2). This resonance was also present on the top panel, at a lower level, but there was also a strong mode at 1040Hz. However, I note that Herb Reichert didn't comment on any midrange congestion that could be laid at the feet of the lower-frequency mode.
Fig.2 Wharfedale Diamond 225, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 Wharfedale Diamond 225 with grille, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer (blue) and port (red), plotted in the ratios of the square roots of the radiating areas below 350 and 500Hz, respectively.
This response, which was taken with the vestigial woofer and tweeter grilles in place, doesn't look as good as I was expecting from Herb's positive comments—I would have expected the boost in the tweeter's output between 6 and 10kHz to make the sound rather bright. When I e-mailed him to ask if he had reached his conclusions with the speakers' grilles in place, he responded that "with every speaker I always start with the grilles, then remove them. If they sound better with the grilles, I put them back on. With the 225s, all my observations were made without the grilles."
Fig.4 Wharfedale Diamond 225 with (blue) and without (red) grille, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses plotted below 300Hz.
The Wharfedale's lateral dispersion, taken with the grilles (fig.5), reveals that there is more presence-region energy off axis, and that the peak in the tweeter's output is suppressed more than 25° to the speaker's sides. In the vertical plane (fig.6), a suckout develops in the crossover region 10° below the tweeter axis, which means the Wharfedales should not be used on high stands. However, the step response on the tweeter axis (fig.7) has a very small discontinuity between the decay of the tweeter's step and the start of the woofer's, which suggests that the optimal listening axis will be very slightly below the tweeter axis. The cumulative spectral-decay plot (fig.8), taken without the grilles, is superbly clean.
Fig.5 Wharfedale Diamond 225 with grille, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.6 Wharfedale Diamond 225 with grille, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.
Fig.7 Wharfedale Diamond 225, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Wharfedale Diamond 225 without grille, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Overall, the Wharfedale Diamond 225's impressive measured performance lives up to what I expect from this classic British brand (footnote 1).—John Atkinson
Footnote 1: Following some failed experiments with home-brewed speakers in the late 1960s, I bought a pair of Wharfedale's two-way Super Lintons, the model with the "purple jellyfish" tweeter, with which I lived happily for several years.
