very normal :) first, it's YOU singing. Secondly, stereo's are built for human ears. In "live" singing, there's many frequencies we don't hear, but your cat does. So even if stereo and normal singing would sound the same to you, the difference to your cat would be like "live singer at stage in front of you in opera house" and "rusty old grammophone in the attic". ^_^
So probably when singing you emit a frequency that the cat likes or thinks it is your purring, and joins in :)
first, it's YOU singing.
Secondly, stereo's are built for human ears. In "live" singing, there's many frequencies we don't hear, but your cat does. So even if stereo and normal singing would sound the same to you, the difference to your cat would be like "live singer at stage in front of you in opera house" and "rusty old grammophone in the attic". ^_^
So probably when singing you emit a frequency that the cat likes or thinks it is your purring, and joins in :)