When prompted with just one cue of "Speak loudly!" or "Think loud!", Parkinson's patients automatically take in a deeper breath, open their mouth more for better resonance and articulation, and increase volume. All aspects of speech production strengthen together, says Fox, who, together with Ramig, cofounded a company called LSVT Global.

LSVT doesn't train patients to actually yell or scream, although to them it may feel disconcertingly as though that's what they're doing. "A huge piece of the treatment is to teach them that what feels very funny to them, and maybe too loud, is actually within normal limits," says Fox.

Each workout session starts with many repetitions of sustained "aaaahhhs" at normal, high, and low pitch, followed by repetitions of 10 everyday phrases or sentences. Over weeks, the therapist trains patients to build a healthy, louder voice to use first with individual words, phrases, and sentences, and then building up to continuous reading and conversational speech.

How long the benefits last:

Studies of LSVT show that in conversation, patients' voices grow louder by about 5 to 6 decibels, which makes a big difference to a listener's ability to hear them. A study published in 2001 found that some of the gains from LSVT persist for two years. Ideally, patients keep up voice exercises to maintain benefits. Occasional tune-up therapy visits may be needed.