The Pietra Model 2 series uses a nonstandard 3rd-order crossover to mitigate two problems inherent to 2-way loudspeakers: directivity mismatch caused by large ratio of woofer-to-tweeter cone radiating area, and acoustic center offset of the midwoofer. The target transfer function for the crossover was derived via numerical maximization of the width of the vertical and horizontal listening windows. Detailed cabinet diffraction and radiation analysis (pictured below) was performed with the MFS algorithm developed by Pietra [J. Audio Eng. Soc., 61 (2013) 356].
Model 2a/b radiation pattern at 2050 Hz crossover frequency
The Model 2a uses a ring-radiator tweeter to limit the high-frequency dispersion at very wide listening angles. The presence of a phase plug (silver structure in center of tweeter) creates an annular (i.e., ring-shaped) radiation pattern. This radiation pattern is more directional than from a traditional convex dome and matches better to the directivity of the midwoofer in the vicinity of the crossover frequency. The unfiltered on-axis frequency response from this tweeter is the flattest of all candidate drivers tested at Pietra. This excellent tweeter is paired with a very-high-excursion midwoofer. The midwoofer uses powerful neodymium ring magnets -- which are nearly a factor of 10 stronger than ferrite magnets -- to enable the use of an underhung voice-coil. The underhung design ensures the magnetic flux seen by the voice coil is constant over the excursion range, thereby reducing nonlinear distortion. This remarkable midwoofer, however, has a very deep acoustic center which, as mentioned above, motivated the design of the nonstandard 3rd-order filter. Like all Pietra enclosures are fabricated from slabs of solid granite by a master craftsman.
|midwoofer diaphragm diameter||14cm|
|tweeter diaphragm diameter||2.7cm|
|sensitivity||86dB @ 2.83V, 1m|
|minimum impedance||3.4 Ohms @ 130Hz|
|dimensions||251cm x 33cm x 34cm|
$6000 USD Sales