Amazon employs thousands of workers who listen to voice recordings captured by the company's Echo "smart" speaker devices in people's homes, according to a report from Bloomberg.

The employees include both full-time Amazon employees and contract workers located across the globe, from Boston to India, the new service notes. Their job -- to transcribe the recordings and annotate them, then feed them back into Amazon's software. The goal is to improve human speech comprehension in Alexa, Amazon's voice assistant.

The issue raises privacy concerns at a time consumers are becoming more sensitive to the issue, especially when technology companies don't fully disclose how their personal data is used. For instance, Amazon doesn't explicitly tell consumers that its workers are listening to their conversations with Alexa, Bloomberg reports. Instead, it says, "We use your requests to Alexa to train our speech recognition and natural language understanding systems," the company says in a list of frequently asked questions.

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Amazon defends its practices, with a spokesperson saying in a statement that the company uses an "extremely small number" of customer interactions with voice-powered devices in order to improve the service.

"This information helps us train our speech recognition and natural language understanding systems, so Alexa can better understand your requests, and ensure the service works well for everyone," the spokesperson said, noting that customers have the right to delete their voice recordings. "We have strict technical and operational safeguards, and have a zero tolerance policy for the abuse of our system. Employees do not have direct access to information that can identify the person or account as part of this workflow."

"You don't necessarily think of another human listening to what you're telling your smart speaker in the intimacy of your home," Florian Schaub, a professor at the University of Michigan who has researched privacy issues with smart speakers, told Bloomberg. "I think we've been conditioned to the [assumption] that these machines are just doing magic machine learning. But the fact is there is still manual processing involved."

Alexa users have the option to tell Amazon they don't want their voice recordings used to develop new products, but the company told Bloomberg that their voice recordings may still be analyzed as part of its review process.