Fooling Alexa With Filtering
Tuesday 6th February 2018
Partly inspired by a conversation with Alex, I thought I’d have a play with how Amazon stop its adverts triggering people’s Alexa devices. It seems to be a three step process:
- Filter out 3-6kHz
- Add background noise
- Mumble the name
Here's a video of me testing it. The word "Alexa" is filtered in the first play through, and not in the second. Everything else stays the same.
The image below shows the spectral frequency display in Audition, before and after I processed the sound of me saying "Alexa". The bright orange is loud audio, and the gap in the "after" image shows what's been removed by the filter.
It looks like the key here is the combination of this filtering and the background noise. Without the background noise, it still triggered the device. Some light shuffling/wind noise made it ignore it half the time, and with background music too, it ignored it 90% of the time.
When I only played the removed audio (just the bit between 3kHz and 6kHz), nothing happens at all. It barely sounds like speech to my ears.
