By Sam Oliver

Published: 03:00 PM EST

Apple saw its Mac sales growth decelerate last month, according to preliminary figures from market research firm NPD -- a sign that a tepid consumer environment is impacting the company's business.

In a research report to clients Monday afternoon, Piper Jaffray analyst Gene Munster said the NPD data suggests Mac unit sales rose 23 percent year-over-year during August, down from the 43 percent growth rate reported the month earlier.

Munster believes the "weaker than expected" directional trend is a result of the Mac product family being in need of a refresh and a "weak consumer environment [that] is negatively impacting Apple's business."

When combined, NPD data for the first two months of Apple's September quarter suggests a yearly Mac growth rate of 32 percent on sales of 2.8 million - 2.9 million units, according to the analyst, which is still above Wall Street's consensus estimates of 25 percent growth on sales of 2.7 million units.

At the same time, sales of the company's iPod digital media players appear to have accelerated. Munster said iPod units were up 19 percent year-over-year during the month of August, compared to 12 percent growth in July. Revenues from the players were up sharply, however, rising 27 percent.

The analyst estimates that Apple is on course to report sales of approximately 11 million units for the September quarter, representing 8 percent growth compared to consensus estimates of 6 percent growth.

"Given concerns regarding iPod weakness, we believe the segment's outperformance relative to Street expectations is a positive," he said.

Munster estimates Apple will earn $1.19 per share on revenues of $8.5 billion for the September quarter should it achieve unit sales of 2.9 million Macs, 11 million iPods, and 4.1 million iPhones at a gross margin of 32 percent.