The weekly audience of all forms of online radio is now at an estimated 76 million Americans age 12 and older, up more than 30% from a year ago.

Arbitron SVP Marketing Bill Rose said that’s the largest increase ever in weekly online listening since Arbitron and Edison Research have been conducting their “Infinite Dial” research, which began in 1998.

That jump in weekly online listening is one of the findings in their latest study, “The Infinite Dial 2012: Navigating Digital Platforms.”

Online radio, defined as the streams of AM and FM stations as well as so-called “pure-play” online stations, reach 29% of the population, according to the findings.

The results were culled from more than 2,000 calls with adults 12+ earlier in the year. Among other findings: 44% of Americans own a smartphone, representing half of all cellphone owners and 61% own a portable digital media device such as a smartphone, tablet or MP3 player.

Seventeen percent of all cellphone owners have listened to online radio streamed in their cars by connecting their phone to their car stereo system, an increase of more than 50% compared to 2011.

Rose called that result “remarkable” considering in-car online listening is not yet seamless.

“Smartphones have had a profound impact on entertainment-driven media,” said Edison Research VP Strategy & Marketing Bill Webster in announcing results from this year’s Infinite Dial study conducted with Arbitron.

“People who wouldn’t have bothered filling an iPad [with music] can now watch the game on the bus,” he added. Eighty-eight percent of Americans 12+ own some kind of mobile phone, according to the findings.

That mobility gives radio more opportunity for content consumption, according to Arbitron and Edison executives.

When asked to rate how digital devices had impacted their lives, 53% of respondents said the iPhone has had a big impact, while 43% cited broadband use. A non-Apple MP3 player came in last at 14%.

“Digital platforms have made cross-platform strategies crucial to satisfy connected consumers,” said Arbitron’s Rose.

The average age of a so-called “heavy user” of the Internet is 36, compared to 42 for radio and 47 for TV.

In 2002, roughly 72% of the population could get online. Now, 85% of the population has access to the Internet. Radio still reaches 93% of Americans with some 241 million listeners a week, according to the study results.

The study can be more closely examined at either the Arbitron or the Edison Research Websites.