WhisperPhone Duet is an acoustical telephone that enhances student-to-student and teacher-to-student reading activities. Duet keeps classrooms quieter by creating an acoustically-clear connection between two readers. Duet also helps develop reading skills and encourages expressive reading teachers want. Kids will love Duet for the fun and adventure! WhisperPhone Duet comes with 2 handsets linked by an expandable (up to 6 feet) voice tube. The BioFilter inline bacterial viral filter is designed to minimize the spread of germs with a minimum loss of air flow; allowing students to still hear each other. Each unit is sonically sealed so there are no chemical adhesives or unwanted material residue. The inline filter is easy to install and does not require any tools for assembly. Sold as a ClassPak of 4, each with BioFilter.