This month my Scientific American column explored the intersection of “Internet of Things” devices (everyday home appliances connected to the internet) and law enforcement. It was prompted by the investigation of a murder in Arkansas in which police asked Amazon to supply any recordings that may have been made of room sound by the suspect’s Amazon Echo.

Amazon replied that, in fact, no audio is ever transmitted to Amazon until you speak the trigger word “Alexa” (or whatever name you’ve chosen for your Echo) and then speak your command.

I thought it might be instructive to see what, exactly, the privacy policies are for allof today’s voice assistants: Alexa (Amazon), Siri (Apple), Google Now and Cortana (Microsoft). Here’s what I found.

Amazon Echo

When the Echo hears you say its name (the “wake word”), it sends audio to Amazon’s servers. “The audio stream includes a fraction of a second of audio before the wake word, and closes once your question or request has been processed,” says the company’s FAQs page. “You’ll know when data is being sent, because “the light ring around the top of your Amazon Echo turns blue.”

In the app’s settings, you can turn on an option that also makes the Echo beep every time it hears its name, and again after it’s finished transmitting—even more cues to help you understand when the Echo is transmitting audio.

You can also turn off the Echo’s microphones entirely when you need super-duper privacy; just press the microphone on/off button on the top.

Finally, Amazon says, “we keep the voice recordings associated with your account to improve the accuracy of the results provided to you and to improve our services.” But you can, in the Alexa app, delete some or all of these stored recordings.

OK Google and Google Home

In general, Google’s policies are identical to Amazon’s. No audio is transmitted until you say “ okay Google”; recordings are stored on Google’s servers forever unless you delete them, which can be done at any time (at myactivity.google.com); and the lights on the Google Home (Google’s Amazon Echo copycat device) glow when it’s transmitting.

Siri

As with Alexa, no recordings are made or transmitted until you say “Hey Siri” or trigger her using a key or a button on an Apple iOS device. Even then, Apple says it doesn’t associate the command with you; it assigns you a random number that’s associated with your request.

After six months, even that number is deleted. Apple keeps the actual, anonymized recording around for up to two years for testing.

If you turn off Siri at any point, all of your identifies and recordings are permanently deleted.

Cortana

Microsoft’s Cortana voice assistant gives you by far the most control. For example, she can perform many tasks even when you haven’t logged in with your Microsoft account: getting answers from the Web, translating phrases, performing calculations, setting alarms and so on. These queries have no association to you.

If Cortana needs access to any of your personal information, she asks first. At any time, you can open her settings and turn off whichever info-bits you’ve shared with her (for example: your location, your calendar and e-mail info, your sports team or restaurant preferences).

Microsoft doesn’t say how long it stores your recordings on its servers, but once again, you can delete them at any time.