27.51.00 Distributed Audio-Video Communications Systems
27.51.23.50 Educational Intercommunications and Program Systems
27.51.13 Paging Systems 27.51.13.13 Overhead Paging Systems 27.51.16 Public Address Systems 27.51.19 Sound Masking Systems 27.51.23 Intercommunications and Program Systems 27.51.23.20 Commercial Intercommunications and Program Systems 27.51.23.30 Residential Intercommunications and Program Systems 27.51.23.50 Educational Intercommunications and Program Systems 27.51.23.63 Detention Intercommunications and Program Systems 27.51.23.70 Healthcare Intercommunications and Program Systems 27.51.26 Assistive Listening Systems 27.51.29 Emergency Communications Systems 27.51.29.13 Rescue Assistance Signal Systems 27.51.29.17 Two-way Radio Communication Enhancement Systems 27.51.33 Occupancy Indicator