As previously stated, the three common obstacles to a clear speech signal reading the person with hearing loss are: Distance, Reverberation and Noise. Let’s take a quick look at each of these and how FM systems mitigate these effects.
Distance. By wearing a microphone (transmitter), the teacher’s voice is made to sound as if it were only 6 inches from the student’s ear – instead of 10 feet away. The teacher’s voice is then ‘closer’ to the student’s hearing device and ear than the other noise around that student (such as coughing, chairs moving, noise from a heating unit, noise in the hallway, talking, etc.) and therefore the teacher’s voice sounds louder than the ambient or background noise. New technology also has the ability to automatically raise the level of the teacher’s voice over any increase in the background noise in the classroom. This has been a huge advancement in FM systems.
Reverberation. Another issue to combat in any listening environment is reverberation. Reverberation is caused when a sound is produced in an enclosed space and bounces off the walls and other hard spaces until it is absorbed. These ‘echoes’ of the teacher’s speech and the ambient noise in a classroom interfere with a single direct delivery of the message. When using an FM system, the teacher’s voice is delivered directly to the personal hearing device and is not subject to reverberation.
Noise. Even though state of the art personal amplification devices (such as digital hearing aids, cochlear implants, and BAHS) deliver high quality sound to a person in an ideal listening situation, classrooms are not ideal. The average ambient noise level in classrooms has been measured as 61dB! This is louder than the sounds of speech. Students with hearing loss need a signal-to-noise ratio (e.g. speech louder than the background noise) of 15-20 dB – again, the average classroom teacher speaks only 1 to 5 dB louder than the ambient noise in her classroom.