Lockdown Challenge: Breath of fresh air

Lockdown Challenge #44: Breath of life

Ever since animals – and humans – evolved, they had to breathe oxygen. Most of the time we don’t really notice that we are breathing. But every five seconds or so, six million times a year, we inhale 3-500cm³ of air, absorb some oxygen, desorb some CO₂, and then exhale. Anything going wrong with the process (like viruses) and in a minute we are in trouble, which is why measuring breathing is so important.

Here we look at how we can monitor breathing in two simple ways using a mobile phone. For the first method, we rely on measuring the noise of breathing. (Phone microphones are sensitive, so heavy breathing is not needed!)

First download a sound meter app onto your phone, one that will plot a graph. Alternatively, use one of the multi-sensor apps like PhyPhox or Physics Toolbox (Apple version and Google Play version linked). Now you can either take the flow noise to the microphone or take the microphone to the flow noise. A piece of clean hose will take the noise to the phone. It should ideally be 20mm internal diameter, although a 12mm ID garden hose is OK for quiet breathing. Use 30cm to go from your mouth to the phone on the table. Fix the phone end of the hose so it is a constant 5 or 10mm from the phone microphone – normally on the bottom edge. To take the microphone to the flow noise tape the headset microphone onto a short tube that you can hold in your mouth easily – for example, 20mm diameter, 50mm long.

Now put the hose end, or the tube, in your mouth and breathe normally for a few breaths. Now hold your nose (which increases the signal) for 10 breaths while you get the phone drawing the sound versus time graph. Now press ‘pause’ to freeze the graph and use the snapshot function for storing results.

With luck, you will find the noise level goes up 15dBs as you breathe, each peak sloping down to the right. In a quiet room, you’ll see the noise level go down between inhaling and exhaling. Try repeating the measurement after some exercise, using a big hose or the headset microphone. You should see on the graph faster frequency and increased noise of breathing. Try a spectrogram function and you can see the frequencies of noise changing as you breathe.

This works because of air turbulence. The faster a flow of air goes, the more turbulent eddies are created, and these generate the noise. A kettle whistle emits sound power proportional to flow speed to the fourth power: double flow speed and you get 16x the noise. Most air turbulence follows a similar steep curve. Note that sound is measured on the logarithmic dB scale: each doubling gives a 3dB increase, so that’s 16x = 12dB, for example.

Monitoring breathing via the movement of the abdomen lying down is even easier than monitoring noise. You need an app that reads out angle to horizontal, or the y-axis accelerometer. Again, you can use specialist or multi-sensor apps. Simply place a mobile phone on the chest, with one end between the lowest ribs and the other end further towards your legs, and set the app graph running.

The science here is all about the lungs filling and expanding the volume of the abdomen. Breathing steadily, ‘tidal’ breathing, involves 300cm³ or so going in and out. This is like an area of 30x30cm moving up 5mm – and 5mm is roughly what is seen on an MRI or X-ray of someone breathing. But if the soft part of the abdomen goes up 5mm and 30cm away the part with ribs doesn’t, then you have an angle of 1.6°. Gravity will then give a y-axis acceleration of 25milli-g, which is easily measured on the phone. Try it and see what you get.

Monitor this on a phone using the app Physics Toolbox Sensor Suite. The first trace with the hosepipe shows exhale peaks. 

Meanwhile, the second trace (see below) with a headset mic shows a larger exhale, then smaller inhale peaks separated by lulls.

This trace (below) shows the accelerometer y-axis component showing gentle heaving of the chest with regular breathing as the abdomen changes angle relative to horizontal.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here (Exovent.org). 

There is the back catalogue of Lockdown Challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.