The nature of noise
What is noise? This is one of those questions like ‘What is time?’, to which one might answer: ‘I know exactly what it is until someone asks me’. But it’s worse than that, for ‘noise’ means two quite difierent things. For a scientist, noise is extraneous acoustic or electromagnetic energy. When used for communication, noise is whatever is not the signal, in other words does not carry information (whether the information is a voice down a wire, or a structural element in an ultrasonically scanned foetus). The larger the amount of noise, the more diffcult it becomes to unweave the signal from it—hence the concept of signal to noise ratio.
Once such noise has been identifled, it can be fairly easy to remove. Noise in a circuit that occurs at particular frequencies (a 50 Hz hum from a mains electricity supply, for example) can often be dealt with by fllters, so that only frequencies of interest remain. Unfortunately, in electronics, a very common form of noise is white noise, a signal that fllls all frequencies of interest and sounds like a hiss when heard through a loudspeaker. This cannot be flltered out so easily, but one may instead fllter all but the frequency bands that carry the signal; there will still be white noise in those bands, but the overall signal to noise ratio will improve.
For most of us most of the time though, ‘noise’ is any sound which is unwanted by the person who is exposed to it. A trumpeter may be very pleased with his output and certainly not think of it as noise, but a neighbour might well disagree, however technically excellent it may be. And both are right—one person’s music is another’s noise. This disagreement goes to the very heart of why noise is so diffcult to quash. But fortunately, while almost every sound has at least a few people who love it, and there are those to whom almost anything but silence is anathema, it is also true that sounds which many of us think of as noise do tend to share three characteristics: suddenness, loudness, and tunelessness. To flnd out why this is so, we need to consider why we can hear anything at all.