It is one of the most common and hard-to-fix speech errors: making the “r” sound.

Researchers and speech therapists say the use of an unlikely tool—an ultrasound probe—could help children who have difficulty saying the letter “r” correctly. Instead of red, these children might say wed. Or buhd instead of bird.

About 10% of preschool children have some sort of speech or sound disorder, experts say. Many children naturally outgrow these, or get help correcting the problem with conventional speech therapy. But when the problem is pronouncing “r,” speech errors can persist; studies have estimated that 2 to 3 percent of college-age people still have trouble with the sound.

“We actually have a lot of 12-year-old boys who say, ‘Can you teach me to say the word girl?’ ” said Tara McAllister Byun, an assistant professor at New York University’s Steinhardt School of Culture, Education and Human Development. Dr. Byun is part of a group of researchers from several institutions who plan to begin studying how best to implement the ultrasound technology in clinical practice.

Conventional speech therapy is often effective at helping to resolve speech errors from sounds that are made with the lips, such as “p,” “b,” “m” and “v.” Children can look in a mirror and imitate a therapist’s lips. But more complex sounds like “s,” “l” and “ch” are harder to fix because they involve movements of the tongue hidden inside the mouth.

Experts say “r” has a particularly complex tongue shape. Using ultrasound biofeedback allows children to see and visualize the tongue as it moves, something not possible in traditional speech therapy. Also, unlike other speech sounds, “r” isn’t always produced the same way; there are many different tongue variations that produce the same sound.

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