For spy tools, drones are pretty easy to spot. And hear, because they're as loud as a gut-busting rock concert. But now the intelligence community's research division, Iarpa, plans to start designing a silent drone inspired by quiet, creeping, flying owls.

Iarpa has reportedly awarded a $4.8 million contract to Connecticut firm D-Star Engineering to develop the ultra-quiet drone, Aviation Week reports. It's the next step in developing a workable drone as part of the agency's Great Horned Owl Program, which the agency hopes will let the military collect intelligence "without anyone knowing you are there," (.pdf) according to an agency briefing.

Sound, after all, is the number one signature "that gives away the location of low-altitude UAVs and gives away their presence." Which sort of defeats the point of having a secret surveillance eye in the sky. In some cases, you might want people to know you're watching. At other times, you want to sneak up quietly.

But it's hard to do without sacrificing payload. The added weight of sensors, and the ability to operate for longer periods, comes with trading out stealthiness. Drones powered by batteries: They're quiet, but can't stay in the air for long. Then there's the added noise caused by airflow generated from propellers, and noise from gasoline or diesel engines (not counting batteries), with their moving pistons, turbofan and gears.

Iarpa wants to keep these efficent and relatively noisy engines for normal flight. But when the drone needs to be stealthy, its operator would switch to battery power, like a hybrid car. That means – for the duration of battery flight – the noisy gears would shut off. The propellers would also likely be ducted, which would mean less noise from vortices whipped up by the propellers and fewer moving parts. Likely, the drone will take off vertically.

The agency doesn't expect the drone to stay ultra-quiet for more than 30 minutes, though, at which point the gasoline-powered turbine engine would switch back on, recharging the batteries. Not enough for (say) a sustained surveillance operation, but quiet enough to take a peek at an enemy without being noticed.

The first step is keeping the sound levels in battery mode below 100 decibels, about equivalent to a chainsaw when up close. But give perhaps a few thousand feet of distance, and the noise drops. Iarpa also wants to reduce noise by cutting down the drone's "phon curve" – or the level of sound pressure interpreted by the human ear. Iarpa plans to start by testing the sound levels of an uninstalled version of the engine.

Iarpa's owl drone is also not the first talk of an owl-inspired aircraft. NASA has looked into the owl's stealthy feathers for inspiration. But it may take years before owl-based aircraft migrate into service.