Diapason Adamantes II loudspeaker Measurements
While my estimate of the Adamantes' B-weighted sensitivity was below specification at 88.5dB/2.83V/m, this is still high for a small speaker. Its plot of impedance magnitude and phase (fig.1) dropped to 3.75 ohms at 230Hz, but the speaker is otherwise a moderate load for an amplifier to drive. The tuning of the rear-mounted port can be seen from the saddle in the bass to lie at 49Hz.
Note that a couple of wrinkles between 300Hz and 500Hz can be seen in the fig.1 traces, indicating some resonance problems in this region. This is confirmed by fig.2, which is a cumulative spectral-decay or waterfall plot calculated from the output of a simple plastic-tape accelerometer fixed to the Adamantes' rear panel. As well as some motion in the port-tuning region, two strong panel resonances can be seen at 477Hz (the cursor position) and 360Hz. The latter mode could also be found on the cabinet's side and top panels—while it is relatively high in frequency, it might be strong enough to reduce midrange clarity (although JE didn't remark on any such aspect to the Diapason's sound).
Fig.1 Diapason Adamantes II, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Fig.2 Diapason Adamantes II, cumulative spectral-decay plot of accelerometer output fastened to back of enclosure near the top (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 shows the individual responses of the tweeter, woofer, and port, taken with the DRA Labs MLSSA system. The port's output is the sharp bandpass centered on 50Hz. It looks as though the fundamental reflex tuning is underdamped—this would account for JE's finding the Adamantes' bass to sound "slow" and somewhat disconnected from the midrange. The woofer's output, however, is very flat within its passband, rolling out very steeply but smoothly above 2.5kHz, the specified crossover frequency. The tweeter, however, doesn't reach its full output until about an octave higher in frequency.
Fig.3 Diapason Adamantes II, acoustic crossover on HF axis at 50", corrected for microphone response, with nearfield woofer and port responses plotted below 300Hz and 900Hz, respectively.
The overall response is shown in fig.4 and pretty much ties in with JE's description of the Diapason's character: rather underdamped bass, with a steep rolloff below the port tuning frequency; a smooth, flat, neutral midrange; but a lack of energy in the mid-treble which contributes to, if it does not totally explain the speaker's distant soundstaging.
Fig.4 Diapason Adamantes II, 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.
Looking at the Adamantes' lateral dispersion plot (fig.5), another reason for the laidback balance and distant soundstage suggests itself: The relatively large woofer becomes somewhat beamy at the top of its passband, which will lead to a lack of presence-region energy in the room's reverberant field. (Note that only the off-axis response differences are shown in this graph, which means that the on-axis reference response appears to be a straight line.) Overall, however, the changes in the Diapason's response as you move to the side are smooth and well-controlled, something that always correlates with good, well-defined imaging—as JE found.
Fig.5 Diapason Adamantes II, horizontal 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.
In the vertical plane (fig.6), the dispersion plot reveals that the Adamantes is quite critical when it comes to listening axis. Suckouts appear at the bottom of the tweeter's passband much above or below the tweeter axis, which will add to the lack of in-room presence-region energy. However, this graph does reveal that the best mid-treble balance will be obtained with the listener's ears just below the tweeter axis.
Fig.6 Diapason Adamantes II, 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.
In the time domain, the Diapason's step response (fig.7) is typically non–time-coherent in that the tweeter's output arrives at the microphone before the woofer's. Both drive-units are connected with the same positive acoustic polarity, however. The related waterfall plot (fig.8) reveals a superbly clean decay throughout the upper midrange and treble. No wonder JE used the adjective "clean" throughout his auditioning comments.—John Atkinson
Fig.7 Diapason Adamantes II, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Diapason Adamantes II, cumulative spectral-decay plot at 50" (0.15ms risetime).