Barossa Reservoir was created between 1899 and 1902 in the Barossa Valley in South Australia. Its most defining feature is a 36 metre curved wall, which was built at a cost of around £170,000 and was hailed as radical innovation at the time, gaining international acclaim in the process.
The wall has a party trick though, which was discovered by accident during its construction. Have a look at the video below to see it.
Due to this phenomenon the dam has gained the nickname ‘the whispering wall’. Legend has it that it was discovered when a group of workers were complaining about their boss who was safely out of earshot on the other side of the valley. However due to the unique parabolic properties of the wall the boss heard every word and the workers promptly lost their jobs. A few say the story has been fabricated to add to the aura of the place, but it’s a fun one nonetheless.
Now the reason you are able to communicate with someone on the other side is because the curve of the dam is a section of a perfect circle, if you were to imagine the wall continuing all the way round. The sound is carried by ‘whispering gallery waves’ which cling on to the surface, allowing a voice to be heard at the exact same volume 140 metres away on the other side of the dam. The hard concrete surface also contributes to the sound travelling unobstructed.
Whispering gallery waves were first discovered in St Pauls Cathedral in London by Lord Raleigh around 1878.* The cathedral also has a unique acoustic trick where if you were to clap, it would echo 4 times in response.
Whispering gallery waves can also be detected in vibrations in the Earth and distant stars, as their shapes are almost spherical, plus exist for light waves. They are also analogies for them existing in gravitational waves at the event horizon of black holes.
There are a huge number of whispering galleries around the world, including the Grand Central Terminal in New York City, the Temple of Heaven in Beijing, the Royal BC Museum in Canada, and in the Louvre in Paris.
Much closer to home in Scotland you can find the effect in the library of Dollar Academy, as well as the Hamilton Mausoleum.
*Lord Raleigh’s real name was John William Strutt, who also discovered Raleigh scattering, a phenomenon which can be used to explain why the sky is blue!