A small insect is a poor loudspeaker. Wings much shorter than the wavelength of the sound they produce radiate inefficiently, because air simply moves around them rather than being compressed. Larger animals do not face the problem, but a cricket does.
The mole cricket solves it with soil. The male digs a chamber behind a narrow constriction and one or two exponentially flaring exit tunnels. He then sings at the constriction. The bulb acts as a resonator and the flares as horns, matching the impedance of the vibrating wing to that of open air. Males adjust the burrow and their position within it, and tests show that removing or reshaping the burrow reduces the radiated sound considerably. Calls from good burrows carry across hundreds of metres of open ground.
The amplification also advertises to the wrong audience. In North America, introduced mole crickets are located by parasitoid flies of the genus Ormia that home in on the calling song, so the burrow that improves mating success also raises the risk of being found.