May / June 2021

Editor's Lead In
Good Noise Versus Bad Noise
The signal-to-noise ratio is exactly what its name implies.
Editorial By Greg Borrowman

  You don't have to know much about hi-fi in order to be aware that having unwanted noise in your hi-fi system is a bad thing. If there's too much noise, you can't hear the finer detail in the music. And even if you have only a little noise in your system, it can be intensely annoying.

Recordings made using magnetic tape were renowned for having excessive noise, so much so that a term was coined specifically for the type of noise produced — tape hiss. It was present even on high-priced professional open-reel machines, but reached its peak nuisance value with the invention of the compact cassette. It was so intrusive that systems were developed to eliminate it, the most popu- lar of which was developed by Ray Dolby and named after him — Dolby Noise Reduction — which came in three flavours — Dolby A, Dolby B and Dolby C.

Noise is so intrusive that there's even an electrical test specifically designed to quantify it, the S/N ratio. The signal-to-noise ratio is exactly what its name implies, the amount of noise there is as a ratio to a defined signal level. The higher the ratio, the lower the noise. But because the human ear is more sensitive to noise at some frequencies than others, this measurement is often adjusted to reflect this, which is called "weighting" and the reason you'll see specifications such as "S/N Ratio: 90dB (A)." (Weighting also comes in different flavours: A-weighted, B-weighted, C-weighted and so on... )

But it appears that noise can also be a good thing... at least for birds.

Because most birds have eyes at the sides of their heads, they can't see in front of them when they're flying, and instead look down at the terrain and if it's a long way down, assume they won't hit anything. Which worked well until humans started building tall communications towers in rather large numbers. These towers are now killing birds at a great rate as a result of them flying into them.

To counter this scientists are now placing loudspeakers on the towers that play band-limited noise as a warning for birds that there's an object in front of them. At present, they're using two different types of noise, one that's band-limited to 4–6kHz and a noise the scientists say "sounds like a fast-running stream." The other warning noise is higher in frequency, up in the 6–8kHz range, which they say "sounds like the hiss of a large gas leak."

So far, the noise project has been quite successful, with a reduction in bird deaths overall and an indicator that the lower-frequency noise is the more effective of the two, but it's thrown up a twist, which is that, like humans, different bird species are more sensitive to noise in some frequency bands than in others, so noise that works for one species might not work for another. So if your hi-fi system suddenly develops a hiss, my advice would be not to look at your turntable, your CD player or your amplifier, but instead to look out your window. If you see a communications tower that's newly sprouting some loudspeakers, you will have found your problem.

— Greg Borrowman

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