in context
For products and services with voice recognition capabilities, we collect and store a copy of the voice input you make to the product or service and corrections you make to the returned recognition result. To improve processing of your voice input, we may also continuously record in temporary memory a few seconds of ambient background noise. This recording stays only temporarily on the device and is not sent to Google. If you choose to personalize voice recognition, we will associate your voice data with your Google Account.
i've used the google app on iPhone that uses voice recognition, and it's very good, so I assume that so long as the app is running, the app records a rotating buffer 2-3 seconds of ambient noise to establish a baseline then "subtracts" it from the your query prior to processing it to give more accurate results...