For most of 2008, album sales were lower than 2007's marks but were fairly consistent. In a typical week, album sales were between 10% and 14% lower than the same week in 2007.

But in September, album sales dropped sharply. The year-over-year decline for the four full weeks of September was 20.4%. From January through August, the average decline was only 9.3%.

Part of the explanation is that album sales in September 2007 were relatively strong and reversed the slight, downward trend that existed throughout the year. Not once in the four weeks of September 2008 did album sales exceed seven million units in a week. Last year, each weeks' tally was at least seven million units and twice exceeded nine million units. (I have taken the weekly sales tallies from Billboard.biz's weekly chart recap.)

But taking just 2008 into consideration and ignoring September 2007, album sales in the month of September were still weak. The September weekly average of 6.74 million units is nearly a full million units below the 7.73 million-unit weekly average for January-August and is well below the 7.33 million-unit July-August weekly average.

September was filled with new releases. Metallica's Death Magnetic has performed very well, Young Jeezy's The Recession moved 260,000 units in its first week, and Ne-Yo's Year of the Gentleman debuted with sales of 250,000 units. The launches of many other big name releases, however, are typical of first-week sales in the latter half of this decade. The new Pussycat Dolls album moved only 74,000 units in its first week -- the lead-off single moved 1.4 million downloads by that point. Nelly's latest arrived with a whimper with sales of 84,000 units in its first week.

Not much can be read into any single four-week period. A number of factors could be involved. Since the mid-year point, album sales had been faring well compared to 2007. But the recent drop is noticeable and merits close attention. Such a sales drop usually happens over a longer period of time.