Don’t be surprised if an elderly person sleeps through a high-pitched smoke detector alarm.

Elderly aged 65 and older are much more likely to waken to a mixed-frequency signal, a new Fire Protection Research Foundation study indicates. A pure high-frequency signal used in most smoke alarms is less likely to produce arousal, even less than a male voice saying, “Danger, fire, wake up,” which also scored poorly in the tests, the report found.

The study also examined how elderly react to being awakened suddenly by an alarm. They estimated a 10% to 17% reduction in physical functioning during the first five minutes after waking up, but they also noted “no important” effects on simple or complex cognitive functioning.