An Amazon patent confirms a theory that commercials for Alexa voice control include a secret message to stop Amazon gadgets being triggered. Amazon also says it can now respond to unexpected high-profile mentions.

Because the Echo range of smart speakers aren’t specifically tied to a user’s voice, there’s a risk that any audio of somebody saying the default wake-up phrase of “Alexa” could make the gadget listen to and even act upon the subsequent comments.

That would be an obvious problem with TV commercials demonstrating the voice control. (Microsoft previously had a problem with a Kinect ad causing Xboxs to load up a game.) However, it seems in most cases this doesn’t happen with Alexa.

Last year a Reddit user did some digging and speculated that Amazon was signalling to Echos to ignore the mention. This worked by removing or quieting the audio in a specific frequency range that’s outside that which is most easily detected by humans. The device can spot that the ‘missing’ audio is a deliberate omission, which acts as the signal.

Now Amazon has confirmed that it does take action to stop the devices triggering, but is currently using a different technique. Instead of removing any audio, it sends the devices a snippet of audio for the commercial in advance – specifically the mention of “Alexa”. The devices then use this as a digital fingerprint and know to ignore it if and when they hear it. Thankfully that seemed to work during a Super Bowl ad, which was likely the most number of Echoes hearing the word at the same time.

The Reddit user wasn’t wrong however. An Amazon patent published last year to little attention covers both the techniques. The former isn’t addressed in Amazon’s blog post, so it may have trialled it and decided to go with the latter.

Amazon also says it has a system in place to deal with unexpected mentions of “Alexa” that get a large audience such as in a talk show. (I’ve also heard reports of triggering from the commentary on matches with the WWE wrestler Alexa Bliss.)

If an unusually large number of Echoes wake up at the same time, Amazon will check through the past few seconds of audio for the mention of “Alexa”. If it spots the same voice – such as from a TV broadcast – heard by multiple devices it will immediately create and transmit a digital fingerprint and an instruction to ignore it. This will then block unwanted triggering for any users who watch the same broadcast on a replay, a delay through a PVR, or on a delayed feed from a local broadcaster.