Acidized WAV and REX Files Explained: Loops That Follow Tempo

What is an acidized WAV file?

An acidized WAV is a standard WAV with an extra block of metadata that tells loop-aware software how many beats the loop contains and what key it's in. The audio itself is untouched, so any player opens it like a normal WAV; the extra data only matters to programs that read it.

The term comes from ACID, the loop-based sequencer that has been around since 1998 and is now sold by Boris FX, which describes acidized loops as ones that "automatically match the key and tempo of your project"^[1]. In Sound Forge, the classic tool for acidizing, the ACID Properties window is where you set the ACID type, the root note for transposing and the number of beats^[2]. Save the file afterward and those properties travel with it.

• ACID type One-Shot clips don't change tempo or pitch and aren't meant to loop. Loop clips, usually one to four measures, change tempo and can pitch shift with the project. ACID Beatmapped adds key and tempo information to long files^[2].
• Number of beats Combined with the file length, this gives the software the loop's tempo. Boris FX's help warns that a wrong value changes playback speed: 8 beats entered for a 4-beat loop plays it at half speed^[2].
• Root note for transposing The key the loop was recorded in, used to conform it to the project key. For drums, choose Don't transpose so they never get pitched.

What is a REX file?

A REX file is a loop saved by ReCycle after it has been sliced at every hit. Instead of stretching audio, the DAW plays each slice at the right moment for the new tempo, which is why REX drum loops keep their punch when you change BPM.

Reason Studios, which dates ReCycle back to 1994, says a REX file contains the original audio of the loop, the slices you applied, and any processing you added in ReCycle, stored with lossless compression^[3]. The ReCycle documentation adds that REX2 files, extension .rx2, also carry the loop's original calculated tempo, so Reason stretches the file to the sequencer tempo automatically when you import it^[4].

Why slices beat stretching for drums

Time-stretching smears transients because it has to invent or remove audio between hits. Slicing avoids that: each kick and snare is played back untouched, just earlier or later. Reason Studios pitches ReCycle as letting you play a loop back at any tempo without the artifacts that come from time stretch^[3]. The trade-off appears when you slow a loop down a lot: slices end before the next one starts, leaving small gaps. ReCycle's stretch feature, which extends the tail of each slice, exists to fill them.

Acidized WAV vs REX vs Apple Loops: what's the difference?

All three are loop formats that carry tempo information, but they follow tempo in different ways and are read by different software. Pick the one your DAW actually understands.

Apple documents that Apple Loops of the Loop type "conform to project tempo, scale, and key" and that One-shot Apple Loops ignore that information^[5], which mirrors the ACID loop and one-shot types. If a pack ships all four versions, grab the one that matches your DAW and skip the rest.

Which DAWs read acidized WAV and REX files?

REX support is widespread; acidized tag support is patchy. Reason Studios lists Reason, Ableton Live, Pro Tools, FL Studio, Cubase, Nuendo, Studio One, Reaper, Cakewalk, Kontakt, Maschine and Battery among the apps that support REX technology^[3]. Logic Pro isn't on that list, so check your version's documentation before relying on REX files there.

How do you use REX loops in FL Studio and Ableton?

Drag the .rx2 file into the DAW like any sample, then choose a slice-based playback mode rather than a stretch mode. The slices are the whole point of the format; stretching a REX file throws away its advantage.

1. FL Studio: load into a slicer
   Drop the file into Slicex or Fruity Slicer to get one slice per hit mapped across the keyboard. For the full workflow, see the Slicex tutorial.
2. FL Studio: or use Slice map in the Playlist
   Image-Line describes Slice map as the mode for drum loops with slice markers aligned to the hits, where slices are moved rather than stretched^[7].
3. Ableton Live: drop it on an audio track
   The clip follows the Set's tempo. For more on how Live treats different material, the Ableton warp modes guide explains each mode.
4. Ableton Live: slice to MIDI for editing
   Right-click the clip and slice it to a new MIDI track, which Ableton confirms works for REX files^[6]. You get a Drum Rack with one slice per pad and a MIDI clip you can rearrange.

How do you convert acidized WAV and REX files?

Convert toward whatever your DAW handles natively: plain WAV for universal use, Apple Loops for Logic, or MIDI plus slices when you want to rearrange a groove. You rarely need to convert into REX unless you work in Reason.

• REX to a single WAV ReCycle's Export as One Sample option writes the loop back as one audio file, and you can set a new tempo or pitch in Preview mode before exporting^[9].
• REX to MIDI groove With Export as One Sample turned off, ReCycle's MIDI file contains short events that trigger each slice, which the documentation recommends for creating a groove map^[9].
• Any loop to an Apple Loop In Logic Pro, drag a region into the Loop Browser or press Control+Shift+O, choose the Loop type, and set the key. Apple notes the region must be a whole number of beats for the Loop option to be available^[5].
• Acidized WAV in Ableton No conversion is possible or needed. Live ignores the tags, so set the clip's original tempo manually or let warping detect it.
• Any WAV to REX Open it in ReCycle, adjust the slice points so each hit gets its own slice, set the loop length, and save as a REX2 document.

Why doesn't my loop follow the project tempo?

Either the metadata is missing, the DAW ignores it, or the beat count is wrong. Work through the table before you start stretching by hand.

If you share loops with other producers, the safest format is still a plain 24-bit WAV with BPM and key written in the file name; the guide to audio delivery formats covers why. And if your stretch quality is the real problem, compare the engines in Logic Flex Time vs Ableton Warp.