Shares of Nuance Communications Inc. tumbled Tuesday after the company announced a $25.8 million loss for its fiscal second quarter, pulled down by hefty acquisition and other charges.

The company's adjusted profit and revenue fell short of Wall Street predictions, and it issued a weaker-than-expected outlook for the full year. Its shares dropped 19 percent in afternoon trading.

Burlington, Mass.-based Nuance's speech-recognition tools have helped power the iPhone's virtual assistant, Siri, and the company's other technology enables different gadgets to respond to voice commands.

The companies licensing Nuance's products include smartphone makers such as Nokia and HTC, consumer electronics makers such as Samsung and Sharp and automobile makers such as Chrysler and Toyota.

For the quarter ended March 31, Nuance said its loss amounted to 8 cents per share and compared with a profit of $890,000, or less than 1 cent per share, in the same quarter a year ago.

Excluding one-time items, Nuance said it posted an adjusted profit of 34 cents per share.

Total revenue increased 16 percent to $451 million from $390.3 million. Adjusted revenue, which includes revenue lost to accounting treatment in conjunction with acquisitions, totaled $484 million, the company said.

Analysts, on average, expected a profit of 40 cents per share on $514.7 million in revenue.

"We are disappointed with our results for the second quarter, which were driven by a combination of execution issues and external factors," Paul Ricci, the company's chairman and CEO, said in a statement.

Revenue at the company's health care business jumped 53 percent to $229.3 million on an adjusted basis, helped by the addition of new customers, but those gains were partially offset by a 19 percent drop in adjusted revenue at its enterprise business to $74.5 million. Mobile and consumer adjusted revenue edged up 1 percent to $116.2 million.

Nuance warned that it expects current market trends to continue and that it will take some time for the company's profitability to improve.

The company projected an adjusted 2013 profit of $1.33 to $1.45 per share, below analysts' predictions of $1.80 per share.

It also projected revenue for the year of between $1.86 billion and $1.9 billion, or between $1.96 billion and $2 billion, after adjustments. Analysts expect $2.17 billion.

Also on Tuesday, Nuance announced plans to buy back up to $500 million of its stock.

In morning trading, Nuance shares fell $4.34 to $18.96, after dropping as low as $18.86 earlier in the session. Over the past 52 weeks, the company's shares have traded between $18 and $25.89.