First, the researchers excised the larynxes of eight newly deceased domestic cats, all of which had contracted terminal diseases, resulting in their euthanization. (The owners gave explicit consent for this removal.) The larynxes were promptly flash-frozen in liquid nitrogen and stored at -20° Celsius. They were slowly thawed at room temperature the night before the experiments. Each larynx was cleaned, photographed, and mounted on a vertical tube, which was used to supply heated air with 100 percent humidity to the larynx.

The larynxes were stabilized using LEGO blocks and 3D-printed plastic mounts, and mini-electrodes were attached to the thyroid cartilage, one on each side, to record the electroglottographic (EGG) signal. Gradually opening and closing a magnetic valve in the air supply chain controlled the subglottal pressure by pumping in air, which drove the oscillation in the mounted larynxes. (One larynx also underwent standard histological analysis, while another was CT scanned.)

The authors successfully produced purring sounds in all eight of the excised larynxes when air was pumped through them, with no need for muscle contractions—given that all the adjacent muscles had been removed when excising the larynxes. So what was driving the purrs?

Fascinating, this took a few unexpected turns.