NEW YORK (CNN/Money) - Music file sharing has seen a dramatic drop since the recording industry started threatening lawsuits against anyone who swaps songs over the Internet, according to a published report.

Trade publication Variety, citing a study by consumer research firm NPD Group, said that the number of homes acquiring music files reached a high of 14.5 million in April, then fell to 12.7 million in May and dropped again to 10.4 million households in June.

The survey of 40,000 online panelists also found the number of music files shared dropped from a high of 852 million in April to 655 million files in June.

The Recording Industry Association of America, the industry trade group, has been waging a legal battle against individuals who share copyrighted songs over the Internet. In April it won a legal decision requiring the Internet service provider operated by Verizon Corp. (VZ: Research, Estimates) to identify two of its customers that the RIAA suspected of illegally sharing hundreds of songs online.

In June the group announced it would begin gathering evidence and preparing lawsuits against individual computer users who it believed are illegally offering to "share" substantial amounts of copyrighted music over the Web, and in July it won approval for nearly 1,000 subpoenas in its efforts to identify those file swappers.

"While we can't say categorically that the RIAA's legal efforts are the sole cause for the reduction in file acquisition, it appears to be more than just a natural seasonal decline," NPD vice president Russ Crupnick told Variety.

The IFPI, the international trade group, estimates that pirated music is now costing the recording industry $4.6 billion annually, and that doesn't count the value of CD's burned by individuals.