As streaming gathers momentum, the U.S. music industry keeps breaking sales milestones -- the wrong kind.
 
This week's 3.97-million album sales tally is the smallest weekly sum for album sales since Nielsen SoundScan began tracking data in 1991. It's also the first time weekly sales have fallen below four million in that time span.
 
Last week was fairly slow for the top releases. The top album, Wiz Khalifa's Blacc Hollywood, debuted with sales of 90,000 units, a figure below the first-week sales of many other top debuts of 2014. Three other albums debuted inside the top 10 but averaged only 31,000 units apiece. And the Frozen soundtrack is no longer moving in excess of 100,000 units per week.

To compare, a year ago this past week (ending Aug. 25, 2013), 4.88 million albums were sold. But sales have been losing steam all year. The weekly average number of album sales fell from 4.75 million units in the first quarter to 4.55 million units in the second quarter. In the first 8 weeks of the third quarter, the average has fallen further to 4.2 million.