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Why do Circadas Chirp? The Sound of Summer


Summer is watermelons, rainstorms, mosquitoes, burning sunshine and air conditioning. But a summer is not a summer without the chirping of cicadas.

By: Liangwei Shen 
Technical Training Manager Brüel & Kjær China

Cicadas are born underground, and after 2 to 17 years, the nymphs emerge, climb to the nearest tree, shed their old skins and become adults. Regardless of species, adult cicadas perish within a season or two and do not live multiple years in their adult form.

Cicadas live in groups, so it’s not easy to find a single individual. However, I was lucky enough to record an interview with a Mr. Cicada in Haidian, Beijing, using a Type 2250 Sound Level Meter in high resolution setting with a sampling rate of 24 bits.

Vocalism principle

The male cicada chirp is generated by the vibration of the tympanic membrane, which is pulled and compressed by the tensor tympani muscle. The cicada chirp lasts for tens of seconds at a time, reaching deafening levels before fading out again.

 

We carried out an FFT vs Time analysis to obtain an RMS spectral density spectrogram
We carried out an FFT vs Time analysis to obtain an RMS spectral density spectrogram

Period and peak view

Our recording covers the final part of a chirp. By dragging the file into a display in PULSE Reflex™ Core, we can clearly see several 1 s periods in the chirp signal. The peak value is about 0.7 Pa, around 90 dB.

FFT VS Time Analysis

Zooming in on the 1 s period shows that the period signal is composed of pulse trains. We carried out an FFT vs Time analysis to obtain an RMS spectral density spectrogram. The spectrum’s frequency range – from 20 Hz to 12.8 kHz – dominates between 2.5 kHz and 6.1 kHz, with the highest peak occurring at around 5.7 kHz.

Final part of the cicada chirp signal
Final part of the cicada chirp signal

Soon, leaves will fall from the autumn wind and the air will cool. It will become hard for cicadas to chirp as their lives come to an end; their song turns sad, as winter is upon us.

Hear the sound of Cicada Chirp