Sound and light in signalling

Sound is a pressure wave in the air; light is an electromagnetic wave. Different as they are, they have one thing in common: when they spread out in all directions from a small source, their energy is distributed over an ever larger spherical surface. That area grows with the square of the distance. Double the distance therefore means only a quarter of the intensity, triple the distance a ninth. This is called the inverse square law.

Damping adds to this: air, foliage, fog and obstacles absorb and scatter part of the energy. Sound travels through air at about 340 metres per second, light so fast that on Earth it arrives practically instantly. Light travels in straight lines; it cannot reach what lies behind a hill. Sound, on the other hand, can bend a little around obstacles, low tones better than high ones.

Human hearing is most sensitive in the range of a few thousand vibrations per second, that is a few kilohertz. That is exactly where the tones of a signal whistle lie. It produces a loud, piercing tone with little breath that stands out clearly from wind, water and birdsong. Shouting, by contrast, costs a lot of energy, quickly makes you hoarse, and the voice's basic tones are lower, where hearing is less sensitive. The drawback of high tones: obstacles shadow them more than low ones.

A signal mirror uses the sun as its light source. It does not send the sunlight in all directions but directs it in a narrow beam one way. For someone standing right in that beam it therefore appears very bright. But the beam has to be aimed deliberately at the helpers – waved around at random, it usually misses them.

At night even a small lamp is enough, because the contrast with the darkness is great and eyes have adapted to the dark. Flashing signals attract more attention than steady light, because our perception reacts particularly to change.

Location matters a great deal. From open, raised spots, sound and light carry further than from a hollow, a narrow valley or dense forest. A rushing stream or waterfall can drown out your whistle. Wind carries sound with it: downwind it is heard further, upwind it can be strongly weakened.

The temperature layering of the air also plays a part. Sound is bent when the air is at different temperatures at different heights. On calm, cool nights or over a still lake, sound therefore often carries remarkably far, while on hot days it sometimes carries less well.

Patterns turn a noise into a distress signal. Rhythmic signals, such as groups of three or the alpine distress signal, stand out from random noise and are understood as a call for help. Which patterns exist and how to give them is covered in the signalling entries.

Signals complement an emergency call but do not replace it. If at all possible, first call the local emergency number (112 in the EU), give your location, and then use signals to help rescuers find you on site. Save energy and battery: signal at regular intervals rather than non-stop.

When using a signal mirror, never look directly at the sun; that can damage your eyes. And distress signals are serious: anyone giving them for fun can trigger unnecessary rescue operations and endanger helpers.

This entry deliberately gives no ranges in kilometres. How far a signal carries depends on weather, terrain, background noise, daylight and whether someone is attentively looking or listening in your direction. Figures from advertising or stories usually apply only to ideal conditions.

For visual signals by day, size and contrast with the surroundings count more than colour alone. A large, bright area on a dark forest floor, or a dark one on snow, stands out more than a small, colourful object. Movement helps too, because the eye notices moving things faster than still ones.