of generally vertical transversely spaced triangularly shaped side walls 98 with upper and lower margins 100 102 converging at the subassembly downstream end 96 and an upstream margin 104 which intersects the upper margin 98 at an angle of approximately seventy two degrees each vibration coupling subassembly 92 includes an upstream end panel 106 extending transversely between the upstream sides 104 and a lower panel 108 extending transversely between the side wall lower sides 102 the aforementioned angle of approximately seventy two degrees formed by the side wall upper and upstream margins 98 and 104 determine the slope in a downstream direction from top to bottom of the upstream end panel 106 which slopes approximately eighteen degrees from the vertical each vibrator assembly 91a 91c includes a pair of vibrator units 110 mounted in transversely juxtaposed relation on the coupling subassembly end panel 106 the vibrator units 110 can comprise for example heavy duty electric vibrators such as those available from vibco inc of wyoming rhode island e g model no 4t1000 suitable vibrator units could employ either eccentric rotary or reciprocating principles of operation the aforementioned vibco vibrator units 110 employ an eccentric or off center imbalanced rotary principle of operation and can be adjusted with respect to speed and vibratory force amplitude the vibratory units 110 are attached to the end panels 106 in vibration coupling relationships.