Thank you for your feedback.
The Decibel app will only charge you if you started the free trial and did not cancel it before it expired. If you closed the subscription screen that appeared after downloading the app, you were using the free version of Decibel and you won't be charged.
Please note that deleting an app does not also cancel any active subscription you may have to that app. I recommend you check if you have an active subscription and cancel it by using Apple's official guide available here: https://support.apple.com/en-us/HT202039.
Wish you all the best!