No-one could fail to be impressed by Whatmough’s P33s. They’re finished in a superb Bubinga real wood veneer (a metallic duco is also available) and stand 1.2 metres tall on a 260×430mm footprint. But more than that, they’re shaped more like pieces of modern sculpture than loudspeakers.
A true three-way loudspeaker, the P33 design calls for two 180mm diameter dual-layer fibreglass/Nomex cones to deliver the bass. These operate in tandem, to increase piston area, before crossing to a single 125mm midrange driver. Handling the high frequencies is a 35mm dual concentric tweeter with a waveguide centre plug. This type of tweeter is a relatively new design, and is sometimes called a ‘ring radiator’ to distinguish it from the ordinary ‘dome’ tweeter design. Ring radiator tweeters have far smoother frequency responses than ordinary dome tweeters, and extend high-frequency response beyond 30kHz. This would not be so unusual except that ring radiator tweeters can also handle far more power than conventional domes, so high frequencies don’t get attenuated after prolonged playing. Unlike the similar-looking tweeters on some other speakers, the P33 uses a genuine Vifa XT25—though one that’s modified to Whatmough’s own specification.
Although Whatmough rates the bass drivers as having a diameter of 170mm the all-important dimension is the effective piston area (SD), which is derived from measuring the diameter of the cone, and adding the width of the roll surround. (A distance often referred to as the Thiele/Small diameter, after Australian engineers Neville Thiele and Richard Small.) The P33 has a T/S diameter of 135mm, which gives an SD of 143cm². However, because the two drivers operate together, the SD for the system is twice this, or 286cm². This means that if Whatmough wanted to achieve the same SD using just a single driver, it would have to have a T/S diameter of 191mm, which would equate to an overall cone size of around 236mm, based on a similar frame size and roll surround.