It’s one of the most iconic sounds heard in the mountains of Switzerland: the deafening tri-toned horn of the PostBus negotiating its way down an alpine pass. Swiss Post has announced that to celebrate 100 years since the iconic horn was first installed, they are planning plenty of Dü-Da-Dos with events and competitions.

PostBus tri-toned horn celebrates 100th anniversary

The story of the horn starts in 1919, when the first-ever motorised Alpenpost buses started to run as public transport services in the Alps. Unfortunately, a combination of small, rudimentary roads and a sudden influx of other drivers on the mountain passes meant that road accidents occurred frequently.

After determining that a standard hand-operated horn was too quiet to be heard by other motorists, “A group of Swiss Post experts, therefore, commissioned the construction of a structure with an electric compressor,” a statement from Swiss Post noted. What they came up with came to be known as the tri-toned horn, which has been used on Swiss Post buses since 1924.

Video: PostAuto / CarPostal / AutoPostale / PostBus / YouTube

PostBus horn in Switzerland based on William Tell Overture

The company noted that the iconic Dü-Da-Do (or Tü-Ta-Ta) sound is a registered trademark and is based on a triad of notes from the William Tell Overture by Gioachino Rossini. The horns themselves are made of brass, aluminium and nickel and have to be regularly serviced and tuned so that they still produce the same notes. The horns themselves produce sound at an ear-deafening 120 decibels - slightly louder than a chainsaw, and slightly quieter than a jackhammer.

Because the device is so noisy, drivers are only able to use it on “mountain post roads”, marked out by a blue sign with a yellow post horn. Today, 700 of the 2.300 buses in PostBus’ fleet are equipped with the horn. 

Over the years, the noise of the PostBus has cemented itself in the public imagination. The noise of the horn even forms the backbone a nursery rhyme sung in Swiss schools.

Big party planned to celebrate 100 years of Dü-Da-Do

To celebrate “100 years of Dü-Da-Do”, PostBus confirmed that it was going to put on a special birthday celebration for its horn, which is as iconic in Switzerland as the whistling theme tune of Migros and the bong-bing-bong of Swiss Federal Railways. While not yet confirmed, the birthday will consist of several celebratory events, including a public competition.