Fifteen-inch woofers have a long history. Before the days of transistor amplifiers, the limited electrical output power of tube amplifiers made it necessary to use drive units with high efficiency. The easiest way of increasing a driver’s efficiency is to increase its radiating area, and the 15-inch driver proved to be a good compromise between size and efficiency. At the same time 15-inch woofers became a de facto standard for pro audio applications, whether in sound reinforcement systems or bass guitar amplifiers.
When transistor amplifiers eroded the need for large drivers, the 15-inch hi-fi-woofer fell out of fashion. It survived in the pro audio arena where 15-inch drivers still play an important role in numerous applications, but the requirements for this type of driver are somewhat different to those in the hi-fi-world. A typical pro audio 15-inch woofer provides high power handling, high excursion and is compatible with quite a small cabinet. It is optimised to run most of the time at 20 to 50 per cent of its rated power handling. On the hi-fi side, design priorities would be low distortion and low mechanical losses while cabinet size is not as restricted.
Having screened the market for possible candidates for the WM-4 it became apparent to us that none of the 15-inch pro audio woofers is suitable for a home hi-fi application, so a new 15-inch woofer had to be developed from scratch. The woofer we designed features a stiff corrugated straight sided cone made from glassfibre-reinforced non-pressed paper pulp with a thickness of 3mm to prevent sound leaking through it from inside the cabinet. The cone is rather deep to make it as stiff as possible, with the result that the first cone breakup mode occurs at a high 1kHz, well outside the woofer’s operating range.
Hi-fi woofers generally use half-roll rubber surrounds but the WM-4’s is a triple-roll fabric type which, with the paper cone, gives it the appearance of a pro woofer. The difference is that we have co optimised the stiffness of the surround and spider (suspension) to achieve the linearity of a half-roll surround but without its high hysteresis. Subjectively this ensures ‘quick’, ‘snappy’ bass.
The motor uses a neodymium magnet within the 102mm diameter voice coil. Its excursion capability was determined by the requirements of normal listening levels. It was modelled on the premise of loud hi-fi use rather than for use as a PA system. Thus the voice coil’s winding height is 20mm, combined with a 10mm magnet gap height. Within the motor system multiple aluminium demodulation rings compensate for voice-coil induced eddy currents, while a copper shield on the pole piece reduces voice coil inductance. Together these measures assure extraordinary low distortion figures across the whole woofer passband, and together with the low inductance change with excursion.
When we first built the driver with a dust cap, a cavity resonance in the air volume trapped beneath it caused a severe artefact in the frequency response. This was subsequently eliminated by replacing the dust cap with a phase plug. This also further reduced mechanical losses caused by non-laminar airflow in the magnet gap and through the magnet’s vent hole.
The die-cast aluminium basket features precision-milled functional dimensions (eg at the spider and surround gluing areas).
A corrugated straight-sided cone, supplied by Dr Kurt Mueller in the UK is used made from glassfibre-reinforced non-pressed paper pulp. The cone thickness is 3mm to help prevent sound migrating from inside the cabinet.
The spider and surround geometries have been optimised to match the force versus excursion (Bl(x)) characteristic of the motor. Spider and surround have nearly identical compliance values, so the woofer in effect features a double suspension system which prevents instability and cone rocking at high excursions. The spider is made from pure Nomex fabric which guarantees low mechanical losses and minimal degradation due to ageing.
Unlike a typical single-roll foam or rubber surround, the triple-roll poly cotton surround generates a stiffness versus excursion (Kms(x)) characteristic comparable to that of the spider.