Isophon Europa II loudspeaker Measurements
The Europa II is of average voltage sensitivity, at an estimated 86.5dB(B)/2.83V/m, which is to specification. Its plot of impedance magnitude and phase against frequency (fig.1) reveals it to be a relatively easy load for the partnering amplifier to drive, its impedance resembling a 6 ohm resistor above 200Hz. This graph is more complex in the bass, which is to be expected from the bandpass woofer topology, with a minimum value of 3.4 ohms at 25Hz. The electrical phase angle gets extremely capacitive at subsonic frequencies, due to the presence of a series capacitor in the woofer feed. This will not be a problem for amplifiers unless they are used in a vinyl-based system with an underdamped resonance resulting from the tonearm mass and the cartridge compliance.
Fig.1 Isophon Europa II, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)
Fig.1 is free from the discontinuities that would indicate the presence of cabinet resonances. Examining the vibrational behavior of the Isophon's cabinet with a simple accelerometer revealed the efficacy of the metal-sheathed construction and the internal bracing at controlling resonances. Other than some slight flexing at 60Hz (the center of the region covered by the bandpass woofer), the only mode I could find was at 328Hz (fig.2). This was strongest on the top panel, but should have no negative subjective consequences.
Fig.2 Isophon Europa II, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's rear panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)
Fig.3 shows the individual outputs of (from left to right), the bandpass woofer, the midrange units, and the tweeter. The crossover frequencies are as specified, with steep slopes, and the series capacitor in the woofer feed results in a 30dB/octave rolloff below 40Hz. As LG commented, the Isophon "filtered out the very deepest bass notes." Commendably, given that the inevitable problem with bandpass woofers is the presence of high-frequency resonances in the port output, the high-frequency rollout of the Europa's vertical slot is very clean. The three midrange units cover a wide passband, from 140Hz to 3.4kHz, yet their output is smooth and flat within that range. There is a slight peak present in their response at 6.6kHz, but this is well-suppressed by the crossover. The tweeter's output is also flat within its passband, though it starts to roll off on-axis above 16kHz.
Fig.3 Isophon Europa II, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of the midrange units and bandpass woofer plotted below 500Hz.
How these individual responses sum on the tweeter axis is shown in fig.4. The treble region is lifted by a couple of dB. This is not enough to make the speaker sound bright, but is certainly enough to make it sounded detailed and vibrant in the highs, as LG found in his auditioning. Somewhat surprisingly, the midrange units didn't reach down far enough in frequency to achieve a smooth blend with the bandpass woofer, the result being a relative lack of energy in the upper bass. However, predicting the subjective effect of this behavior is difficult, as this region is dominated by the room's acoustics.
Fig.4 Isophon Europa II, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the bandpass woofer and midrange nearfield responses, taking into account acoustic phase and distance from the nominal farfield point plotted below 300Hz.
Laterally, the Europa's dispersion is wide and fairly even below the top audio octave (fig.5), though with a slight flare evident at the bottom of the tweeter's passband that, in conjunction with the on-axis treble plateau, will make the speaker sound a touch bright in lively rooms. Conversely, the tweeter gets very directional above 7.5kHz, which will make the speakers sound lacking in air and HF extension in large or well-furnished rooms. In the vertical plane, the speaker's dispersion plot (fig.6) shows very little change in balance over a ±15 degrees angle centered on the tweeter axis—just as well, given that the latter is 45" from the floor.
Fig.5 Isophon Europa II, lateral 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.6 Isophon Europa II, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15 degrees-5 degrees above axis, reference response, differences in response 5 degrees-15 degrees below axis.
In the time domain, the Europa's step response (fig.7) reveals that the tweeter (the sharp down/up spike just before the 4ms mark) is connected in inverted acoustic polarity, while the three midrange units (the slower up/down step exactly at 4ms) are connected in positive polarity. Despite the designer's claims, the speaker is not time-aligned. The corresponding waterfall plot (fig.8) is very clean, apart from some delayed energy just above 1kHz, this associated with a small suckout in the speaker's on-axis response.
Fig.7 Isophon Europa II, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 Isophon Europa II, cumulative spectral-decay plot at 50" (0.15ms risetime).
All things considered, the Europa II offers excellent measured performance, suggesting the presence of some speaker engineering skill at Isophon.—John Atkinson