Otohime or "Sound Princess" is a device found in women's public toilets (not men's) throughout Japan to cover unladylike and embarrassing sounds emanating from the cubicles.
Previously Japanese women would cover these sounds by flushing the toilet repeatedly thus wasting precious water. Toilet maker Toto came up with the Otohime to produce the sound of the flush without the actual need to flush the cistern and waste about 6 liters of water.
Simply wave your hand over the sensor and the speaker emits the requisite flushing sound with a button to stop the noise when required.
Toto also produced a portable version for when a lady encountered an Otohime lacking public convenience.
Otohime (with different kanji characters) is also the name of a mythical Japanese goddess that appears more recently in the game of Yu-Gi-Oh.
Guide Books on Tokyo & Japan