As I was walking out of my room, the video playing on my computer had a particularly deep note that sounded like a moment of deep thunder or the strike of a very large drum. It occurred to me that such deep, thunder-like rumblings are inherently frightening (humans and animals alike are frightened by huge claps of thunder, even when distant) and that might be the reason why drums of war are so evocative a means to demoralize an enemy.
I thought about the Owl Clan constructing a huge drum to sound the deepest, most primal notes possible. I remembered the huge speakers the team on Mythbusters built in an attempt to make the loudest car stereo system and wondered how a Rokugani people would make such a device. I imagined a drum 20+ feet across. Where would you get the material to make a skin? Does a drum work the same when the skin is patched together from multiple pieces? I assumed not because every real drum I have seen (made from natural materials) is one continuous piece. Perhaps if I used magic to seamlessly merge multiple skins. But where would I get so many skins?
My first thought turned to goblins from the Shadowlands. Now that would be a hell of a drum, constructed from the flesh of Rokugan's enemies. It would also creep the hell out of Rokugani. In fact, it would likely be considered a tainted object of great evil and I would be seen as an evil, insane artisan in the vein of Asahina Yajinden. So I immediately thought of the exact opposite idea: a bell.
Bells are strongly connected to good and right and religion. Temples sound a heavenly, clear peal of the blessed bell to start the day and mark special occasions. But no one uses bells in an advancing army because of the size and weight issues (and you want to sound a bell from high up to let the sound reach as far as possible). Bells are a defensive signal, raising the alarm to alert the defenders while drums are an offensive signal, keeping time for the march and instilling that primal fear of approaching thunder.
However, the thought of an army that sounds chimes and bells as it camps across from the enemy. Large bells have have an ominous, deep ring. Not quite the fearsome thunder but evoking the death knoll has its advantageous connotations.