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I wonder how it'd sound fretted though. You don't get the sound of the strings actually hitting the neck which is like the iconic upright bass indicator
And at the end of the day if it doesn't change the sound as much as you want it's like $20 - sure, it sucks to lose that, but it's still cheaper than practice mutes/passive amplifiers and stuff for most instruments
They're just little things (people usually homemake them out of cardboard or whatever but they're usually shit) that, well, amplify the sound without needing electricity. You can buy "proper" ones but they get pricey quickly.
this is too good to be true