AlexaGate is a smart addon which stops Alexa from listening to all of your private conversations, its a cool looking addon that sits nicely on top of your echo devices

to activate clap three times, or tap the alexagate three times, and it toggles on and off so that you can talk to your echo as you would normally.  Turn on with three claps, and you’re once more assured of privacy. The device works by using pulsed ultrasound to jam the echo’s microphone. 

Here is the makers website Alexagate