Sound Cancellation and Stealth

You may be asking the wrong question. A stealth technology might not require 'canceling' sounds. It might just need a way to prevent the sounds produced from being heard.

In other words, think of a person shouting in a soundproofed room. The sounds are still there, but the sound absorbing qualities of the walls prevent them from being heard.

Perhaps a combination of sound deadening insulation around the engine combined with some type of low level sonic absorbing field. Does it exist? Who knows? But it could.

There is a lot of material on sound cancellation, silencing, etc. around. Someone has been developing active systems that will cancel soun by matching the sounds produced and change as the sound changes. I believe that this has been pretty thoroughly worked through, but it isn't soething for everyone, because it is pricey, and unnecessary. I don't know whether anyone installs stealth equipment on motor vehicles, but it could be only partlly effective, because a large part of the noise is outside the vehicle. A good online search should give you plenty of information.

Look into submarines. That's their main stealth feature. There's billions of dollars that have been spent researching how to keep them silent.

I heard of one exercise, where an American sub was picked up and tracked because it made one noise for about a half-hour every day. After the exercise they spent two weeks trying to locate the sound. It turned out that one bolt securing one of the mixers in their galley was slightly loose.

I think you need to examine the medium, too. I spent 7 years onboard a US Ballistic Missile sub. Trust me, water transmits sound FAR better than air.

Heh, and be careful. For reasons I'm not sure I'm allowed to discuss, it's actually possible to be *too* quiet. Think "ambient sound" and putting something perfectly silent in the way of it.

Sound cancelling works because sound is a wave. Amplitudes are additive. If you have a counter signal that's opposite in phase to the original, and is otherwise identical you can render the original noiseless. The downside is that if the phase isn't the opposite of the original signal, you can increase the emitted noise. So it's usually used when something is going to produce the same wave pattern continually.

Same principle behind the laser computer they came up with a few years back.

Electric vehicles are proving to be much quieter than gasoline/diesel powered ones because they use very different moving parts and simply don't produce as much noise. However, even if the electric engine/actuators were completely silent, an electric car still has tires that make noise on pavement, etc.

It really depends on how quiet you need your vehicle to be, where it has to be used, and for what purpose.

Total silence isn't possible because every solution to one kind of noise will raise cost and will force limitations on the operation of the vehicle. By the time you cancel out every source of noise, you're left with a rock (and rocks are very quiet as long as they're not moving).

If you want to give more specifics about what kind of vehicle you need and how quiet it needs to be, we might be better able to help brainstorm some believable solutions.

Aerial

Ditto what Aerial said. Even a car that has zero engine noise will still have tire noise and noise associated with the disruption of movement through air.

I live by a busy street, and only 1/10th of the noise is engine-related (Harleys & Hot Rods). Walk over a freeway and listen to the noise. Take a drive in a convertible. Listen to bikes ride by. At decent speed, the engine noise is drowned out by tire and wind noise.

Even a slow moving vehicle that's coasting downhill makes some tire noise. The bigger and nobbier the tires, the more noise they make. Especially moving over twigs and leaves and debris, or metal grates.

I can imagine that a quick-reacting computer could engineer sound waves from speakers by the wheels that would obscure the noise from the wheels in perfect interference waves, but I think this would probably still be detectable. As Rob hinted, there'd at least be a subtle shift to ambient noise. The computer would have to detect sounds at one side of the vehicle and "pass them on" at the other side; but there would be reflection. Echo.

My 2c