An eighth company pulled its advertisements from The Rush Limbaugh Show on Monday, several days after the firebrand host repeatedly called law student and contraceptives advocate Sandra Fluke a “slut” on the air. Two of the departed advertisers are mattress retailers. Why are there so many mattress commercials on the radio?

Because if you’ve seen one mattress, you’ve seen them all. Advertising is, in part, about differentiating a product from its competitors. The products that are marketed most heavily on television—such as cars, clothes, pizzas, and smart phones—tend to be the ones that customers like to see in advance. That’s not the case with mattresses. Everyone knows what a mattress looks like, and they all look about the same. With that in mind, radio advertising would seem to be the better value. The spots are much cheaper to produce than those for TV, and it costs less to get them on the air. In a midsize urban market like Portland, Ore., the opportunity to run a 30-second audio spot would cost about $200. The same amount of television time for the Portland market would be nearly 10 times that much.

Companies that manufacture mattresses, like Serta and Sealy, do rely on television spots, as do a few retailers with large ad budgets, like Sleepy’s. But many mattress sellers avoid major TV ad buys. The retail mattress business is usually local or regional, and radio is a regional medium. Plus, mattresses are big-ticket items, and the average consumer buys one every seven years. The conventional wisdom among marketing strategists is that constant exposure to the brand is important for products with long buying cycles, like mattresses, cars, and furniture. Since the company gets only a few chances to make the sale over the course of a consumer’s life, and those opportunities come at unpredictable times, it’s important that customers have the name of the advertiser burned into their brains. Radio is a cheap way to get in a consumer’s head and stay there with a catchy jingle. Teaching a consumer to associate a product with a specific company is especially useful for mattress sellers, whose warehouse-style stores are sometimes in remote locations where few customers would naturally encounter them.

Another thing that appeals to mattress retailers is specificity. When advertisers buy time on a radio station, they know exactly when their spot will play. That’s not true for television: Now that 40 percent of U.S. homes have a DVR, it’s very difficult to know when a viewer might see an ad (if ever). Print advertisements are even worse in this regard. A magazine might sit on the coffee table for days or weeks before anyone begins to flip the pages. Time specificity gives an advantage to businesses that want to target consumers at particular moments in their schedule, like when they’re just waking up from a lousy night’s sleep, and it’s useful to companies that like to offer special promotions lasting only a few days.

Talk radio is particularly appealing to mattress retailers, because its listeners tend to have above-average disposable incomes. Marketing types thinks this makes them more likely to spend a little more for a mattress or to buy slightly more often.

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Explainer thanks Mike Frey of Paradux Media Group, Mark Jones of Jones Advertising, Kevin Lane Keller of Tuck School of Business at Dartmouth, and Janet Northen of McKinney.