bats and echolocation


kleen kanteen, refillable water bottle
What do we hear as we fill a water bottle?

Filling a re-usable water bottle from the tap, the sound starts off as a low hum, then rises in pitch before a sudden change in note as the water spills over the spout because you have over-filled it. Texturing milk in a pitcher, the sounds change as the bubbles form and break, ready for pouring as latte art. How often do we know what is happening by listening to the sound something makes?

And yet these sounds are revealing more than just when the bottle is full or the milk can be poured. They are teaching us, if we listen carefully, about the physics of what is going on within the water bottle, within the milk pitcher and even within coffee grinds as we bloom the coffee. Consider the water bottle. It is a classic resonator, the basis of many musical instruments. As we fill the bottle, the liquid level acts as an end point to the bottle, reducing the volume of air in the bottle as the water fills it. The note that we hear coming out of the bottle corresponds to the frequency of the sound wave that is resonant in the empty volume of space. As the frequency is inversely proportional to the (square root) of this volume, when the volume decreases (ie. the bottle is filled) the frequency increases, so the note that we hear will go up. The bottle is acting as an approximation to a Helmholtz resonator. You can read about how this can be used for experiments with coke bottles here, or more of the physics (and the maths behind it) here.

Similarly with the milk pitcher, the changing musical note is telling us about the changing conditions within the pitcher, though in this case it gets quite complex. Firstly, as the steam wand is introduced to the pitcher, air is introduced to the milk which “stretches” it. This builds the volume of the milk in the pitcher and introduces air bubbles into the liquid. The combination of the volume change and the introduction of air is going to affect the sound that the jug would make, but the sound you hear, the ‘hiss’ is most probably dominated by the sound of the steam leaving the steam wand. After a short while, the barista will lower the steam wand further into the milk in order to heat the milk in the pitcher. Treating the pitcher again as an approximation of a Helmholtz resonator, we know that the frequency that we hear from the pitcher increases as the speed of sound inside the resonator increases. As the speed of sound in the milk increases with temperature (assuming that it is mostly made of water), to a first approximation we expect the note that we hear to increase in pitch with time. So after the hiss, we will hear a note which rises in pitch as we continue to warm the milk. Is this what we hear?

Latte art scutoid tulip
What sounds do we hear as milk is frothed in a cafe?

Together with other species, we use the information that sound gives us to understand much of the world around us. “Listening” famously helps bats to navigate and hunt and also, helps us to understand more about what occurs in the ocean. Indeed, it has even been suggested that we should listen to the sounds recorded as space probes land on different planets or moons in order to gain further information about what could be hidden just out of view of the camera. Of course, the sounds on another planet may not sound exactly as they do on Earth. Prof. Tim Leighton of the University of Southampton has calculated (and then synthesised) what a methane-fall (like a waterfall but of liquid methane) would sound like on the surface of Saturn’s moon Titan. You can hear the recorded waterfall on earth here, and the simulated methane fall on Titan here. Provided we know what we are listening for and to, better listening can improve our understanding of our surroundings.

the broadcasting equipment at the WW cafe Hackney
London Fields Radio at the Wilton Way cafe. Sounds enter our conscious in a variety of ways, but how many of them do we listen to?

An example of where better listening may improve our understanding of our surroundings comes with bread. One common way of knowing when bread is properly cooked is to tap its base and listen to when it sounds hollow. We can assume that this is because the bread crust acts as the walls of a resonator with the large number of air bubbles that form during cooking (and which make the structure of the crumb) being the bit where the sound wave resonates. The hollow sound shows that what is inside is solid, whereas if it were still dough-y, it would damp the resonance (no pun intended) and make it dull sounding. If this assumption is correct, the note that is made by tapping the bread will decrease as the bread cools and the speed of sound in the air in the bread decreases. But can we also get information about the crumb structure of our loaf by listening to the pitch of the loaf as we tap it? Would not the frequency of the resonance (ie. the sound) change depending on how open the bread structure is (a large, open loaf would perhaps have a lower ‘note’ than a loaf with a small crumb which may have a higher note). Is the bread ‘telling’ us more than just that it is cooked? Experimenting bakers, it’s over to you.