"You are presenting a public fire education program about smoke alarms. A parent asks you this excellent question about combination smoke/carbon monoxide (CO) alarms; 'How do I know when it is the smoke part of the alarm going off?' Neither National Fire Protection Association (NFPA) 72, 'National Fire Alarm and Signaling Code' nor NFPA 720, 'Standard for the Installation of Carbon Monoxide (CO) Detection and Warning Equipment' have a requirement for a voice announcement when the alarms activate. However, the smoke alarm is equipped with a requirement to 'beep' three times with a short pause between each beep and then a slightly longer pause before the next three beeps. A CO alarm is equipped with a requirement to 'beep' four times with a short pause between each beep and then a slightly longer pause before the next four beeps."
Coffee Break Training - Fire Prevention and Public Education Series No. FM-2013-8
United States Fire Administration: http://www.usfa.fema.gov/