Your Beolab 28 is added to the Bang & Olufsen app, where you can find the 8-digit serial number under " Product settings -> About".
The Bang & Olufsen app is available for download from the App Store and Google Play Store.
Alternatively, the serial number can be found on your invoice. If these options are not available, the serial number can be found on the product label placed behind the speaker fabric covers.
