Plantronics, a manufacturer of headsets, has a wall of ears, a physical database of human ear models, posted on a wall in its Santa Cruz, Calif., headquarters.

There's a huge discrepancy among the quality of the human-machine interfaces of various products we use. Some, like the Apple iPod, are so smooth that people are attracted in droves. Others, like various cell phones, evoke cries of displeasure -- even disdain. Why is there such a big discrepancy?

I recently brought up that question with Darrin Caddes, vice president of corporate design at Plantronics, a fast-growing producer of headsets. Mr. Caddes joined Plantronics two years ago after a successful career designing products for the automobile and motorcycle industries, including a stint at BMW. His initial interaction with headset products was by necessity. After a motorcycle accident put him in a wheelchair without use of his limbs, he had to find a way to communicate, and headsets offered him the opportunity.

Mr. Caddes told me he thinks of headsets as empowering devices; although in some cases, such as his, they're actually a necessity. After discovering what they could do for him, he wanted to bring the concept to others.

Yet he found when he started that people are creatures of habit -- they were used to the neck pinch that came from holding a phone handset between your ear and shoulder. And while the population is finally at home in public talking on cell phones, there was still a stigma attached to headsets. He used his own experience to point out that he never used a headset until joining Plantronics. (I could easily understand his initial concern. Although I have been using headsets almost exclusively in my office for 10 years, many people around me are hesitant to start using them, fearing that they'll mess their hair or make them look silly. They're willing to give up the advantages because of their fears.)

His design challenges are similar to those who design other products. He needs to manage a trade-off between contradictory goals -- in his case, balancing size, lightness, power consumption, sound quality, market trends and comfort.

The comfort goal is among the most challenging because there are many types of comfort, including simply feeling good on your ear and the comfort involved in fashion. Mr. Caddes hopes that in the future, people will choose headsets for fashion, much the way they do eyeglasses and perhaps have different headsets for different occasions.

To help him and his team with his comfort and design challenge, Plantronics has put together a wall of ears. This is a physical database of human ear models, posted on a wall in the company's Santa Cruz, Calif., headquarters, with each set of ears made out of molded silicon and representing a cross section of the population of the United States.

The Plantronics design team also employs more widely used methods, such as creating sketches of model ideas, creating models -- often starting with pipe cleaners and paper -- and asking human testers to try out the new design concepts before committing them to production. That's also helped them build their wall of ears, which now includes more than 2,000 models.

According to Mr. Caddes, his team uses about 100 ears daily, because they represent the largest segment of the population in size, shape and rigidity. He calls the human ear "the forgotten organ" even though it is heavily used throughout the course of every day. It's just not as visible as the features on the front of your face or as noticeable as other organs throughout your body.

Yet, he and his team continue to pay attention to it -- to create products that are small and discrete and fashionable, that help workers and cellular telephone users talk more easily without using their hands.