Thank you for your feedback.
You will only be charged if you start the free trial and do not cancel it before it expires. If you closed the subscription pop-up that appeared on the screen after downloading the app, you are using the free version of Decibel and you won't be charged.
To check if you have an active subscription and cancel it if you do, please go to the app's Settings and then select Manage Subscriptions. From there, you can cancel the subscription you want. Alternatively, you can use Apple's official guide for canceling subscriptions available here: https://support.apple.com/en-us/HT202039.
Wish you all the best!