How does your coffee sound? Does an espresso sound different to a latte? Could you deduce how the milk had been frothed, or what milk had been used, by listening to your coffee before you drank it?
To see why there may be an effect, it’s worth thinking about your coffee for a moment. The tiny bubbles in the crema of an espresso are different from the larger bubbles of a milk froth made of semi-skimmed milk in a cappuccino. Bubbles of non-dairy milks may be different too, particularly if the initial small bubbles have combined to form larger bubbles as the froth ages. Indeed, sound is used as a characteristic of coffee: think about the sounds made by a steaming wand in milk. Somehow the environment of a café would not be the same without the constant hiss and whistle of a cappuccino being made. But can we use it to experience our coffee more fully? Not just the aroma, taste, sight and feel but also, can we start to listen to our coffee?
Take the example of the sound of a dripping tap: each drop of water falling into a bowl of water left under the tap ready for washing up later. Each “plink” is telling you something about the size of the drop coming from the tap. Intuitively, or perhaps from experience, we know that small drops produce a higher pitch, a higher frequency, than large drops: small drops ‘plink’, large drops ‘plonk’. But there is something wrong with this example, because, despite what we may think, we are not hearing the drops at all, only a consequence of the drops.
As the drop falls, it creates a hole in the surface of the water, a dent that grows and then closes in on itself, so that the drop of water has formed a bubble of air under the water surface. As this bubble is unstable, it pulsates under the water just before it collapses and it is this pulsation that we hear. As the frequency of the pulsation will depend on the radius of the bubble, air cavities of different sizes will produce different sounds. And because a larger drop will generally produce a larger hole under the water, the larger drop will generate deeper sounds: plonks rather thank plinks.
How does this relate to the sounds made by your coffee? Well, it turns out that the sound of a bubble bursting reveals a lot about the surface tension and the size of the bubble. A recent study published in Physical Review Letters measured the sounds made by bursting soap bubbles through 24 microphones placed around each bubble. Analysing the sounds, the group found that not only could they ‘hear’ how the air escaped the bubble, by analysing the sounds recorded in the microphones they could determine, quantitatively, the movement and forces of the bubble ‘skin’ as it retreated back and the bubble burst. They suggested that listening to bubbles and liquid surfaces could be a complementary tool to high speed photography for understanding the forces on a liquid. This may prove useful for example when thinking about how a pond skater moves on the surface of the water.
To think about what this may mean for coffee, take the Aeropresses I’ve been making recently. First, I wet the grounds and allow a first stage of de-gassing to start. The sound here is of an almost continuous hiss, not entirely dissimilar to the sound you hear when you put an empty seashell to your ear.
At this point it was hard to know whether what I was hearing were the grinds or the ‘sound’ of the Aeropress ‘shell’. Topping up the chamber with water, the bubbles on the surface of the coffee became larger, and of a different form. And they sounded different too! A few pops, and a hiss.
Did I learn anything (apart from that putting one’s ear to the top of an Aeropress does get quite hot and a steamed ear is a strange experience)? I learned that there was much more to my coffee than I had appreciated, that there is always more to discover. It was almost as the author of the 1933 paper about determining the size of bubbles in water by the sound said:
“As a matter of fact we know very little about the murmur of the brook, the roar of the cataract, or the humming of the sea.”