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Message: <blockquote data-quote="quellish" data-source="post: 124500" data-attributes="member: 750"><blockquote data-quote="Orionblamblam"><blockquote data-quote="Abraham Gubler">Active noise cancellation is used in moving devices but inside the vehicle to cancel the noise before it can be radiated outwards from the outer shell. Particularly in submarines.</blockquote>That would be *relatively* easy compared to an aircraft. The noise-makers on a sub are largely *in* a sub, and have to transmit through the shell. A fast enough system could detect the vibration of the shell and cancel it out. it would require a metric buttload of little "speakers" mounted all over the inside of the shell, but it would be doable.</blockquote>Active vibration control and active vibration cancellation are both active areas of research:http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A9534456AH&q=helicopter+active+vibration&uid=790707678&setcookie=yeshttp://data.mecheng.adelaide.edu.au/avc/publications/public_papers/1999/active99cqh.pdfSikorsky has an active rotor vibration control system in several of its models, designed to increase aircrew comfort. The same system could be adapted to signature control. Sikorsky did a project for the Navy, testing integration on the MH-60S:http://www.defenseindustrydaily.com/68m-to-sikorsky-to-rd-mh60s-active-vibration-control-02143/https://www.fbo.gov/index?s=opportunity&mode=form&id=f42ed977d5828377eb0c9543ba70783e&tab=core&_cview=0</blockquote>

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