Lena Baby Monitor
The LENA Baby Monitor measures the number of words your child hears per day. LENA stands for Language Environment Analysis -- it uses digital signal processing to to parse conversation into words.
The hardware is a small, pocket-sized unit that the child keeps in a pocket that logs words heard per day.
For a small fee, you can get a "developmental snapshot" -- showing how your child's development compares to others in terms of expressive and receptive language output.