Remember the last time you were trying to have a conversation in a crowded room? You probably remember that the volume of the crowd noise determined how loud you needed to talk. This is an example of a signal (your voice) to noise (the crowd) ratio. A ratio shows the relationship in size between the two noise levels. The signal-to-noise ratio can be used in electronics as well as acoustics. The comparison is usually expressed in dB (decibels). A decibel is a ratio between two levels. In a piece of electronic equipment, the signal-to-noise ratio is a comparison of the desired signal and the unwanted noise that is generated by the equipment.

Figure 1
A full sanctuary has increased noise.

For example, let's look at a typical full church, perhaps like the one in Figure 1, just prior to the beginning of the service. You may hear a "noise floor" (or level of ambient noise) of perhaps 60 dB. In order for the pastor to be understood, his voice (with the help of the sound system) must be at least 10 dB higher in level than the ambient noise in the room. Or put another way, the signal (the voice)-to-noise ratio between the sound system output and the audience noise must be at least 10 dB.

Confused? Maybe a different example will help. A cassette tape recorded on a typical cassette deck will have a nominal (normal) signal level of 0 dBu. The u in dBu is a standard reference level. The noise floor of the tape and the cassette deck electronics is typically -68 dBu. This means that the signal-to-noise ratio is 68 dB.