Guides

How to Batch Process Slow and Reverb Audio for Free (Full Walkthrough)

2026-01-10 · Updated 2026-10-11

Step by step through the free bulk slowed + reverb workflow: add files, Select All, apply settings, treat subsets differently, Download All as a ZIP, and how long each track takes.

Processing one song at a time is fine until you have an album, a playlist or a week of content to get through. Then it becomes the slowest part of the job, and most free tools make it slower by limiting you to one file per session. The bulk slowed reverb tool on this site has no file count limit, no length limit and no account, and every track is processed in your browser without being uploaded anywhere. This is a complete walkthrough of that workflow, from adding files to the ZIP at the end.

Everything here applies to the same editor you would use for a single track. The multi-track features are part of it, not a separate product. If you are new to the settings themselves, the slowed reverb maker page and the complete how-to guide explain what each control does. Here we focus on the mechanics of doing it to many files at once.

Step 1: Gather your source files

Put the files you want to process in one folder before you open the editor. Supported inputs are MP3, WAV, AAC, OGG and FLAC, or anything else your browser can decode. Mixing formats in a batch is fine. The output is always MP3, whatever the input.

A few things to check before you start:

• Source quality. Slowing exposes compression artifacts. If some files are low-bitrate rips and others are lossless, the batch will sound uneven even with identical settings. Use the best sources you have. The FLAC page explains why lossless input helps even though the output is MP3.
• Consistent loudness. The editor does not normalise. If one track is much quieter than the rest, it will stay quieter. Level-match in a DAW first if that matters for your use.
• Filenames. The editor names each output after the input, so meaningful input names save you a renaming pass later.

Step 2: Add the files

Drag the whole selection onto the editor, or use the file picker and select multiple files. Each one appears as a track in the list. There is no limit, though a very large batch on a phone will take a while to decode. On a laptop, forty tracks load without complaint.

Each track decodes in the browser as it is added. Large FLAC or WAV files take a few seconds each. You can start adjusting settings while later files are still loading.

Step 3: Select All and set once

Click Select All Tracks. Any settings change you make now applies to every selected track. Pick a preset or dial in values manually:

Press play on any track to preview. The preview chain is identical to the export chain, so you are hearing the real result. Check at least two tracks from the batch, ideally the busiest mix and the sparsest, because a setting that suits one may not suit the other.

Step 4: Treat subsets differently

Here is where batch processing earns its name. Suppose you have twelve tracks and three of them are much faster than the rest. At 0.85x those three will still feel quick. You want them at 0.80x.

1. Click Clear Selection.
2. Select only the three fast tracks.
3. Set Speed to 0.80x. Only those three change.
4. Clear Selection again, select the other nine, and confirm they are still at 0.85x.

The same approach works for any control. A track with a heavy sub might want Bass at +2 while the rest sit at +5. A track with a very dense vocal might want Reverb at 30% while the rest take 40%. Select, adjust, clear, move on.

If you find yourself making the same subset adjustment often, save it as a custom preset with Create preset. Presets are stored in your browser's local storage on that device, so next week you can select the fast tracks and click the preset instead of redialling. The batch audio editor page has more on selection behaviour.

Step 5: Download All or Download Selected

Two buttons:

• Download All processes every track in the list and gives you one ZIP.
• Download Selected processes only the currently selected tracks and gives you a ZIP of those.

Download Selected is useful when you want to re-export only the tracks you changed, or when you want separate ZIPs for different variations. For example, select the Haze Chamber tracks and download, then select the Wave Drift tracks and download, and you have two archives already sorted.

Each track is rendered through the full chain and encoded to MP3 in a web worker, so the page stays responsive. You can keep previewing while it runs.

Step 6: What the output looks like

The output is MP3 at 96 kbps, 32 kHz. That bitrate is chosen for fast encoding in the browser and is fine for YouTube, TikTok and Shorts, all of which re-encode uploads. It is not a distribution master. If you need WAV or a higher bitrate, process the audio in a DAW from a WAV source instead; the editor cannot export those formats.

Naming:

• A single track downloads as name-remix.mp3, where name is the input filename.
• A batch downloads as all-remixes.zip, containing one name-remix.mp3 per track.

Inside the ZIP the tracks are flat, no subfolders. If you processed two variations, they will have identical names and the second download will not overwrite the first because they are separate archives, but you will want to rename them before merging folders. The naming and packaging guide suggests patterns that include speed and preset in the filename.

How long does it take

Processing time scales with track length and with the device. These are rough expectations, not benchmarks, because every machine differs:

Encoding runs in a web worker, so you can keep the tab open and do other things. Do not close the tab or let the phone lock the screen mid-export on some mobile browsers, which may pause background work.

Step 7: Check, rename, file

Open two or three tracks from the ZIP and listen to the first thirty seconds and the loudest section. You are listening for the same things as a single edit: can you hear the words, does the kick still have a point, does the tail ring over the next phrase. If something is off, select those tracks in the editor, adjust, and Download Selected to re-export only them.

Then rename and file. One folder per batch or per album, speed and preset in the filename, and you are done.

When batch is the wrong tool

• Each track needs a different speed and you have many tracks. Subsets handle a few differences. If every one of twenty tracks needs individual attention, you are not really batching, and doing them one at a time may be clearer.
• You need WAV output. The editor cannot do it. Use a DAW.
• You need trimming, chopping or pitch independent of speed. The editor does not trim, stutter or pitch-shift separately from speed (other than preserving the original pitch). A DAW does.
• Loudness must match across the batch. Normalise before you load the files.

Copyright

Batch processing an album you do not own produces twelve derivative works instead of one. Platforms recognise slowed versions, and the usual outcome on upload is a claim that routes any revenue to the rights holder, sometimes a block. Nothing about doing it in bulk changes that. See the copyright guide for the details.

Open the free editor: upload MP3, WAV, AAC, OGG or FLAC, adjust speed, reverb, bass, tone, width and space, then export MP3 or a ZIP of many tracks. Everything runs in your browser.