Depending on the medium, it can take some very clever creative to inspire a positive reaction from a simple advertisement.

Usually, it's a challenge just make any kind of impression, but when a company's single fixed billboard can endure for 40 plus years and develop an actual cult following, you know you've hit PR gold.

Such is the legacy of the famous Koss billboard, on the east side of I-43 between Hampton Avenue and Capitol Drive.

Since 1972, that one board has served as the palette for dozens of original ads, some which could legitimately be called works of commercial art.

For decades, the Koss billboard has been a Milwaukee institution.

According to Haley Landsman, public relations coordinator at Koss, the first billboard displayed a yellow smiley face with the caption, "Ever wonder why he's smiling?" After about a month, they put headphones on the smiley face.

That "reveal" resonated with consumers, and Koss hasn't looked back.

Landsman says the billboards rotate every three to six months, and a number of people weigh in on the decision on the creative, which is usually at least a little light-hearted. Fans of the billboard also write in with suggestions, too.

Koss outsources the actual design work to a number of agencies, says Landsman. Over the years, several boards have been created by Matt Zumbo and Steve Eichenbaum. The designers have used several icons over the years, too, from the Mona Lisa to George Washington to Brett Favre.

"We have about 30 of them back in storage," says Landsman. "Those are either the more recent ones or reusable ones, like the graduation one, which is a common theme."

Koss, a maker of high-end audio equipment and headphones, also runs billboards in other markets, but in Glendale, its corporate home, this kind of branding is more important, Landsman says.

It's some permanent good PR for the company that has lately been in the news because of an alleged embezzling scam by one of its former corporate executives.

"It's a nice reminder that we're located here," says Landsman of the billboard, "and with the recent push to buy local, it points right down to our outlet store -- we do sell our products right here."

Given how unique it is, the billboard series has been the focus of numerous trade publication articles, and even a documentary about advertising.

Said Koss Corp. CEO Michael J. Koss in one recent video, "The people in the community feel as though it is their board, which is kind of nice. They like to see what is coming up next ... it really shows what a fun business it is to work at."

For her part, Landsman says her personal favorite board is the Mona Lisa board.

"I have wondered why she's smiling, and it's pretty clever. I like the ones where 'Koss' is substituted for another word."