Snell Type C/IV loudspeaker
Despite not offering design aid, many loudspeakers created with the NRC's testing and listening laboratories share some common philosophies. Chief among these is the belief that flat amplitude response is far and away the most significant factor in listener preferences and thus should be the paramount design goal. Many NRC-influenced loudspeakers share steep crossover slopes, wide dispersion, smooth off-axis response, and pay considerable attention to the way the loudspeaker interacts with the listening room.
The Snell Type C/IV is no exception: it features fourth-order crossover slopes, a very flat frequency response that varies very little off-axis, and a rear-firing tweeter to compensate for the front tweeter's increasing directionality with rising frequency.
The Snell Type C/IV is a large ported floorstanding system employing a 10" woofer, 5" cone midrange, and a 1" metal-dome tweeter. An additional rear-firing ¾" dome tweeter is mounted on the enclosure back, but can be turned off with a toggle switch. Despite being a box 46" tall and 14½" wide, the Type C/IV projects a slim, streamlined, elegant appearance. My review samples were finished in oak veneer (other finishes are available) with a black grille cloth stretched over a wooden frame that completely covers the front baffle. A 1"-thick base, slightly smaller than the Type C/IV's dimensions, lifts the loudspeaker off the floor. Four carpet-piercing spikes (supplied) thread into the base.
The input terminals are about halfway up the cabinet rear. The recessed terminal block has two pairs of five-way binding posts for bi-wire or bi-amp operation. The terminal plate also holds a tweeter fuse, rear tweeter on/off switch, and a front-tweeter level control. Although I don't usually like recessed terminals (it's often hard to connect thick bi-wired cable), the Type C/IV's terminals were quite accessible.
Rather than mount the drivers on a flat front baffle, the Type C/IV's drivers are sited on a panel that protrudes ¾" from the enclosure. The protruding baffle has the same shape and thickness as the grille frame. Consequently, the grille frame fits around rather than in front of the baffle, and is thus invisible to the drivers. Snell calls this technique the "Zero Diffraction grille." Designer Kevin Voecks tried a variety of baffle shapes, including curved baffles, before settling on this design. He found that even obstructions as small as 1/16" affected the off-axis response. To further reduce diffraction, the drivers are rebated in the front baffle.
The Type C/IV's three forward-firing drivers are all Vifas. The custom-made treated pulp woofer is mounted midway up the cabinet and features a cast basket frame. The 5" midrange was chosen for its flat response both on- and off-axis. The rear-firing ¾" dome tweeter, by virtue of its small size, has excellent dispersion characteristics.
Crossover slopes are quite steep at 24dB/octave (fourth-order), with crossover frequencies of 275Hz and 2.7kHz. The rear-firing tweeter comes in gently at about 6kHz with a first-order slope. This additional driver compensates for the reduced power response in the room as the front tweeter becomes more directional. It isn't designed to contribute to the front tweeter's energy, but rather to provide a sense of air and spaciousness lost by the front tweeter's increasingly narrow dispersion as frequency rises. The fourth-order slopes were chosen for several reasons: least driver interaction and lobing effects (constructive and destructive interference caused by two drivers reproducing the same frequency), higher power handling, and lower distortion. After experimentation some years ago, Kevin Voecks decided that trying to improve the phase response by using first-order filters wasn't worth trading off the higher-order filters' advantages.