How to edit audio without uploading it anywhere
Many free audio tools upload your file to a server, process it there and send it back. Mp3Blocks does the whole job inside your browser tab instead.
Why it matters
The recordings people cut are often personal or confidential: a medical consultation, a job interview, a family voice message, an unreleased song, a meeting. When a tool uploads them, a copy exists on someone else's computer, stored for an unknown time under a policy most people never read. With in-browser processing, that copy never exists.
It is also faster for large files. A 100 MB recording doesn't have to travel to a server and back, so it opens in seconds even on a slow connection.
What happens when you open a file
- Reading. When you drop a file, the browser gives the page access to that one file on your disk. The page cannot see any other file.
- Decoding. The browser's built-in audio engine (the Web Audio API) turns the MP3 or WAV into raw sound samples in your computer's memory. This is the same engine that plays audio on any website.
- Editing. Cutting and joining just copy ranges of those samples. Playback goes through the same audio engine.
- Encoding. To create an MP3, Mp3Blocks runs lamejs, a JavaScript version of the long-established LAME encoder, inside the tab. WAV files are assembled directly.
- Saving. The finished file is handed to your browser's save function. In Chrome and Edge, if you chose a folder, it is written there through the File System Access API, which asks for your permission first.
Check it yourself
You don't have to take our word for it. Two simple tests:
- Go offline. Open Mp3Blocks, wait for the page to finish loading, then turn off Wi-Fi or unplug the network cable. Open a file, cut it and save it. Everything still works, which wouldn't be possible if a server were involved.
- Watch the network. In Chrome or Edge, press F12 (or ⌥ ⌘ I on a Mac) and open the Network tab. Load a large audio file and export a block. No request carrying your audio appears; the only traffic you'll see is the page itself and, where shown, advertising.
What the site does load
To be complete about it: the page loads its own code, fonts from Google Fonts and, when advertising is enabled, scripts from Google AdSense. Those services see that you visited the page, in the same way they would on any other website, but they never receive your audio. The privacy policy describes this in detail.
The trade-off
Working on your own device means the size of what you can edit depends on your device. The whole recording is held in memory as uncompressed samples, roughly 20 MB per minute of stereo audio while you work. A normal laptop handles an hour without trouble; a phone may struggle with very long files. For everyday recordings the limit never comes up.