What volcanoes and coffeemaking have in common, the physics of playing the didjeridu

What do volcanic eruptions and grinding coffee have in common? According to a team of coffee chemists and geophysicists in the US and the Republic of Korea, they both produce a fair amount of static electricity, so much so that volcanologists are now examining the espresso-making process.

It is well-known that static electricity is created when grinding coffee beans due to the fracturing and friction that occurs. This causes coffee particles to clump together and stick to the grinder. However, not much was known about how this impacts the resulting brewed coffee.

To investigate the possible impacts, the team ground roasted coffee beans from different countries, and with different roast colours and moisture contents. They found no association between static electricity – as measured by a electrometer – and the coffee’s country of origin. However, the team did find that static was lower when coffee had a higher internal moisture content and when the grinding was set coarser. Light roasts also produced less charge than darker roasts, which tend to be drier.

Splash of water

When they compared espresso made with identical coffee beans ground either with or without a splash of water, they found that grinding with water resulted in a stronger and more consistent brew. “It’s sort of like the start of a joke — a volcanologist and a coffee expert walk into a bar and then come out with a paper,” says volcanologist Joshua Méndez Harper from Portland State University. “These investigations may help resolve parallel issues in geophysics—whether it’s landslides, volcanic eruptions, or how water percolates through soil.”

This week the Australian Acoustical Society and the Acoustical Society of America co-hosted the  Acoustics 2023 meeting in Sydney. So it’s not surprising that the acoustics of Australia’s most iconic instrument – the didjeridu – was up for discussion.

In case you haven’t seen or heard the traditional wind instrument, it is a cylindrical or conical wooden pipe that is normally about 1.2 m long (see figure below). Developed by Aboriginal peoples about 1000 years ago, the didjeridu is played using a special breathing technique that creates a continuous drone from the instrument. A skilful player then uses their own vocal tract to modify the sound that the instrument makes.

Now, the physicists Joe Wolfe and John Smith at the University of New South Wales have done acoustic experiments to gain a better understanding of how the instrument is played.

“We were interested in the effect of the player’s vocal tract on various wind instruments,” explains Smith. “The didjeridu seemed like an obvious start because the effect is so striking.”

He goes on to explain the basics of the playing technique, “Resonances in the mouth tend to remove bands of frequencies in the didjeridu sound and we notice the remaining bands. It’s a bit like a sculptor removing marble to leave the things that we notice.”

Their study involved the development of new experimental techniques, such as injecting sound into a player’s mouth to determine their impedance spectrum. This is a measure of which frequencies will resonate with the vocal tract and which will not.

The team also looked at the acoustic properties of didjeridus and compared advanced playing techniques of other wind instruments such as the clarinet and saxophone. “We continue to research subtle features of expressive playing of wind instruments,” says Smith.