- Silbo speakers
- Spain (La Gomera)
The normal intelligible outdoor range of the male human voice in still air is 180 m (590 ft 6.6 in). The silbo, the whistled language of the Spanish-speaking inhabitants of the Canary Island of La Gomera, is intelligible under ideal conditions at 8 km (5 miles). There is a recorded case, under optimal acoustic conditions, of the human voice being detectable at a distance of 17 km (10.5 miles) across still water at night.