Classroom Audio Reinforcement
Sometimes it pays to take a step back and really think about all the noise in a classroom. Is the heating or air conditioning loud? How about the dull drone coming from the classroom next door? Can you hear traffic from nearby roads? Many of these things may seem insignificant, or certainly quieter than your voice, but they can affect your ability to clearly hear and be heard. The severity of noise can be difficult to perceive, because when you are exposed to the same noise for a long time, your brain ignores it. Often you don’t notice just how loud a noise is until it goes away.
A classroom reinforcement system can help overcome these limitations. It often doesn’t take much to make a difference between being heard and fading into the background. This is doubly important with young people, who have a lower tolerance for noise than adults. To show you why, let’s dig a little deeper into how we measure sound.
Sound is measured in decibels or dB. A dB is a way of expressing a ratio; in other words, it compares the loudness of two different sounds. There are standard reference points, such as dBA, that allow us to make measurements we can all compare, but thanks to the power of the human ear to hear both very loud sounds and very quiet sounds, we are mostly interested in how loud things are relative to another. This should be fairly intuitive: if you are in a quiet environment, you don’t need to speak very loud to be heard clearly, but if you are in a noisy space, it may be difficult to speak loud enough to be heard at all. Of course you are louder in the second case than the first, but we aren’t interested in absolute loudness, only in how loud we are in comparison. When we compare our voice or another sound source that we want to hear against a background of steady noise, we call that measurement a signal-to-noise ratio, or SNR.
It should also be intuitive that if we have a very large difference between the signal and the noise, it should be easy to hear the signal. We will only have trouble hearing the signal when the noise is nearly as loud as the signal, or when the SNR is small. So what we’re probably interested in is the smallest possible SNR to ensure that we are reliably heard over the Noise. For adults with good hearing, this ratio can be as small as 6 dB, which means the noise has about one-quarter the sound power of the signal. Children aren’t as good at filtering noise as adults. Numerous studies suggest that in order to be effective the classroom, students require a SNR as high as 15 dB or more.
Now, the human voice at conversation is usually between 60 and 70 dBA. Soft-spoken individuals may be quieter and a teacher with a strong voice can certainly speak louder; however, 60 dB is a good, safe assumption. If we can make sure a teacher speaking at 60 dB is heard over the background then we have a good environment for learning. This means we need background noise to be less than 45 dBA, or about as loud as an office or a new refrigerator.
Achieving this same SNR of +15 dB can be much more difficult to achieve in a real classroom. Outside of test-taking, have any of us ever heard a classroom as quiet as an office? Of course not. Classrooms are full of energetic children fidgeting, chattering, laughing, coughing, sneezing, and dropping pencils, books, or paper. And then there are noises coming from outside the classroom: playgrounds, hallways, highways, and neighboring classrooms. It doesn’t take a scientist or measurement equipment to see that teachers struggle just to match the level of ambient noise in the classroom, let alone best it by a margin of 15 dB. And as a teacher, your voice is your greatest ally: shouting all day is a recipe for losing your voice altogether.
What does all this mean? Teachers need help. They need a way to operate in noisy environments and ensure that they are heard clearly above the noise in all corners of the room. The simplest way to do this is a voice amplification system. Typically this will include a pendent-style microphone worn around the neck to pick up the teacher’s voice, one or more speakers mounted in the ceiling to play the voice back and a receiver to pass the audio signal to the speakers. Often this receiver will offer additional functionality to the classroom, such as the ability to amplify audio from other sources, or even send the teacher’s voice back into the computer.
Next week, see Bob Jackman’s companion piece on SMART’s classroom audio solution, how it can help overcome issues of noise in the classroom, and for applications beyond voice amplification.
Noisy noise is noisy. Der