Attendance at Tanglewood, the summer home of the Boston Symphony Orchestra, fell by 11% this year, with BSO staff blaming the decline on lousy weather and a cutback in the number of free tickets available. It was an unusually rainy summer in the Berkshires – 20 rain days in July, and 22 in August – which can directly affect Tanglewood concerts, since many patrons sit outdoors, and even the main Shed, which is covered, has no walls to insulate concertgoers from the rain. Still, more than 320,000 people attended a concert at the famous venue over the ten weeks of the festival.