Hundreds of apps and games are monitoring smartphone users through their microphones.

They feature a technology that can even recognise audio when the user puts their phone inside their pocket, or if the apps are running in the background.

Advertisers can then use this information to learn more about consumers and target ads more effectively.

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These apps, some of which are targeted at children, use software from a startup called Alphonso, which quietly collects data about people’s TV viewing habits and sells it on to advertisers.

Around 1,000 games and social apps reportedly use the software, with more than 250 of them available to download from Google Play and a smaller number also available from Apple’s App Store.

The New York Times reports that you can find more of them by searching “Alphonso automated” and “Alphonso software” in Google Play.

Alphonso’s software uses a phone’s microphone to listen in to the environment around a user, to work out what they’re watching on TV.

The company insists that its technology does not record people’s conversations, and says its capabilities are clearly explained in app descriptions and privacy policies.

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The description for Pool 3D reads, “This app is integrated with Alphonso Automated Content Recognition (“ACR”) software provided by Alphonso, a third-party service. With your permission provided at the time of downloading the app, the ACR software receives short duration audio samples from the microphone on your device.”

It continues: “Access to the microphone is allowed only with your consent, and the audio samples do not leave your device but are instead hashed into digital ‘audio signatures’.

“The audio signatures are compared to commercial content that is playing on your television, including content from set-top-boxes, media players, gaming consoles, broadcast, or another video source (e.g., TV shows, streaming programs, advertisements, etc.).

“If a match is found, Alphonso may use that information to deliver more relevant ads to your mobile device. The ACR software matches only against known, commercial audio content and does not recognize or understand human conversations or other sounds.”

Ashish Chordia, Alphonso’s chief executive, told the New York Times that Alphonso has worked with film studios, which provide snippets to the company ahead of a film’s release, making it easier for the software to identify.

Chordia also says his company has a deal with Shazam, which can help it to identify users by analysing audio provided by Alphonso and selling its findings back to the company.

You can protect yourself by denying microphone access to any apps you use, which don’t require microphone access.