Thumper: One of the types of stills used to accomplish the second distillation of American whiskey. It effectively removes impurities and concentrates the alcohol even further. “Low wines” go in; “high wines” come out. Thumpers differ from doublers in that the low wines enter a thumper as vapors that are bubbled through water, causing the stills to make a thumping sound; a doubler makes no distinctive noise since the low wines enter in condensed, liquid form.