How to build a giant speaker from a camper van

In January 2014 myself and my colleague Eric Flament travelled to Nelspruit, South Africa to meet with a BBC film crew. They were interested to find out whether elephants would react to very low frequency (infrasonic) sounds from a distant thunderstorm.

Our challenge was to make a thunderstorm sound track and design a special low frequency loudspeaker capable of generating the sound of a thunderstorm (lightning and thunder below 20Hz).

Creating the sound track was relatively straightforward. We removed the high frequency content from a good quality thunderstorm recording as if the sound had travelled long distances through the air so that we only reproduced infrasound.

Creating a sound source was a little more difficult. For the special speaker that can broadcast infrasound (sounds below 20Hz) we wanted a giant speaker box, around 10.2 cubic meters (360 cubic feet).

Sam Hodgson, the director, suggested using a VW Bus as the speaker box. The VW Bus is known as a Kombi in South Africa – and has the correct internal volume needed to produce very low frequency sounds. The BBC contacted a supplier of classic Kombi's and decided to use "Jackson" a 1970 Westfallia camper.

We found the same type of Kombi locally and using it for a template, designed an insert or speaker baffle for the rear hatch back door that supported a TRW-17 rotary woofer speaker which allowed easy installation and removal for transport.

The pressure from the low frequency sound inside the Kombi caused the roof to flex so we strapped the roof and taped the windows. The sound tests were good, Jackson made an excellent portable infrasound source and we were ready to see how the elephants would react.

When the speaker broadcast the infrasonic parts of the storm, the elephants immediately stopped feeding and rotated around to look in the direction of the van. It was an amazing reaction and demonstration of elephant’s ability to hear infrasound and determine the source location with good precision.

While we did not directly prove that elephants can detect a rainstorm at long distances we did prove that elephants can hear infrasound at the same sound levels and even with very long low frequency wavelengths. Surprisingly they can determine the direction of infrasound source with excellent accuracy.