SIGNAL TO NOISE RATIO

The scene below depicts a classroom setting in which the instructors voice signal is determined to be approximately 65 dBA at a distance of 3 feet from the teacher. The background noise level is 45 dBA and assumed to be fairly constant through out the room. The Inverse Square Law teaches us that the sound level of the instructors voice signal will be 59 dBA at a 6-foot distance, 53 dBA at a 12-foot distance and 47 dBA at a 24-foot distance.

A good Signal to Noise Ratio should be not less than +15 dB, that is to say the signal strength should be at least 15 decibels above the background noise level in order to achieve good Speech Intelligibility by the listener. From (Figure 12) it can be seen that beyond a 12-foot distance from the instructor the S/NR is far below what it should be.

In a typical classroom the ambient or background noise level in the unoccupied classroom should not exceed 35 dBA with the HVAC system running.

Figure A

The sound level at the listener’s ear, above the background noise level.  If the signal at the listener’s ear is 47 decibels and the background noise level is 45 decibels, the Signal to Noise Ratio = +2 dB.

Signal-to-Noise-Ratio