Signal-to-noise ratio is the measure used in science and engineering to compare the level of a desired signal, to the level of background noise. In schools, the signal is the teacher's voice, and the background noise is all the interfering noise, including that created by chairs and desks scraping on the floor. The signal-to-noise ratio (or speech-to-noise ratio, referred to as SNR) is basically a number value assigned to describe how much louder the teacher is over the background noise. For example, if a teacher's voice is 65 decibels and the background noise (chairs, heaters, students) is 55 decibels, the SNR is +10. In order for typically-hearing students to understand what they hear, they require an SNR of +15 decibels. At risk students, students with hearing loss and English as a Second Language students need an even greater speech-to-noise ratio. In a school classroom, spoken language is the tool most often used. Typically 60% of classroom learning activities involve verbal communication and interactions. Teachers lecture classes, students ask questions, instructions are given. A learning environment that facilitates clear communication is critical. SNR is particularly important in classrooms because of it's effects on speech intelligibility which involves not only the student's ability to hear, but his/her ability to understand and process what is said. Intelligibility increases when the teacher's voice is much louder than the background noise. Classrooms need to be acoustically healthy to allow all students full access to clear channels of communication.   Sources: The Institute for Advanced Classroom Hearing American National Standard: Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools