This tuba-like device was actually the Japanese military's acoustic locator - a device with which one can "hear" distant engine noise of oncoming airplanes.
Acoustic locators like this (and others) were used from mid-World War I through the early days of World War II, until the invention of radar made them obsolete.
Tokyoflash Treasure Hunt - Win a Tokyoflash Watch of Your Choice!
Going on right now: Tokyoflash Treasure Hunt #26. Win a Tokyoflash watch and neat stuff from the NeatoShop. It's fun and easy to play: Link