Scholars at Tel Aviv University have recorded and analyzed click-like sounds distinctly emitted by plants.

The sounds are similar to the popping of popcorn and emitted at a volume similar to human speech, but frequencies human ears don't detect. The researchers believe plants usually emit sounds when they are under stress, and that each plant and each type of stress is associated with a specific identifiable sound.

Though the frequency is too high for human ears, it is in the range detectable by bats, mice, and insects.

It was already known that vibrometers attached to plants record vibrations but not if they became airborne soundwaves. To find out, the researchers placed  tomato and tobacco plants, and later wheat, corn, cactus and henbit in an acoustic box in a quiet, isolated basement with no background noise. Ultrasonic microphones recording sounds at frequencies of 20-250 kilohertz (the maximum frequency detected by a human adult is about 16 kilohertz) were set up at a distance of about 3 inches from each plant. 

Some plants had not been watered for five days, in some the stem had been cut, and some were untouched. The intention was to test not only if the plants emit sounds, but whether these sounds are affected in any way by the plant's condition. They found that the plants emitted sounds at frequencies of 40-80 kilohertz. Unstressed plants emitted less than one sound per hour, on average, while the stressed plants – both dehydrated and injured – emitted dozens of sounds every hour.

The researchers monitored plants subjected to a process of dehydration over time and found that the quantity of sounds they emitted increased up to a certain peak, and then diminished.

There are confounders. The results were analyzed using machine learning/artificial intelligence
so the algorithms distinguished between different plants and different types of sounds and decided the type and level of stress from the recordings.