Change Tempo is different from Change Speed and Change Pitch

Change Tempo alters duration while attempting to preserve pitch. Change Speed links speed and pitch like playing tape or vinyl faster: a shorter result also becomes higher. Change Pitch targets pitch while attempting to preserve duration. Choose by the audible goal, not by whichever effect appears first in a menu.

Pitch preservation is an algorithmic objective, not a claim that the waveform remains otherwise identical. Time stretching divides and reconstructs audio, so transients can soften, sustained sounds can develop modulation, and room ambience can change. Keeping the adjustment modest and starting from a high-quality source improves the odds of a usable result.

Measure the source BPM before processing

Do not enter a target based solely on an unverified catalog value. Analyze or tap a stable section, test half-time and double-time alternatives, and place a grid or markers over multiple bars. If the track changes tempo, one global effect will scale every moment by the same ratio but will not make the source constant-tempo.

Write down the source BPM, target BPM, selected region, sample rate, and file format. If a detector says 75 while the production grid is 150, both may describe related pulses; use values from the same convention when calculating a ratio. Mixing 75 as the source with 140 as the target would request an unintended near-doubling.

Calculate the percentage change accurately

The playback tempo ratio is target BPM divided by source BPM. Percentage change is (target ÷ source − 1) × 100. Moving from 120 to 126 BPM is therefore a five percent increase. Moving from 120 to 114 is a five percent decrease. Negative percentage values slow and lengthen the selection; positive values speed and shorten it.

Audacity's Change Tempo dialog may offer source and target BPM fields, percentage, or duration controls depending on version. Verify that its resulting length matches the same ratio. A 120-second selection sped from 120 to 126 should become about 114.29 seconds because duration changes inversely with tempo.

  • Tempo ratio = target BPM ÷ source BPM.
  • Percentage = (ratio − 1) × 100.
  • New duration = old duration ÷ ratio.
  • Use the same half/double-time convention for both BPM values.

Apply Change Tempo to a duplicate

Import the source, save the project with a new name, and duplicate the track or create a second project for comparison. Select the exact audio to process; no selection may mean the full active track, but verify the highlight rather than assuming. Open Effect > Pitch and Tempo > Change Tempo in current Audacity releases, enter the measured source and intended target or calculated percentage, and preview when available.

Apply the effect and wait for processing to complete. Menu organization can differ across versions, so use the official manual for the installed release. Never overwrite the only source file. Audacity projects are working documents, not automatic backups of every external recording and export decision.

Check timing, pitch, and audible artifacts

Confirm the new duration and tap or analyze the output. That verifies the requested ratio, but it does not prove quality. Compare original and processed tracks at matched loudness, focusing on kick and snare attacks, cymbals, sibilance, sustained vocals, bass notes, stereo ambience, and reverb tails. Listen on headphones and speakers.

Pitch should remain musically stable, yet warble or transient smearing may appear even when note names do not shift. If artifacts are distracting, reduce the change, process shorter musically coherent regions, try quality options documented for your version, or use a specialist time-stretch tool. A technically successful export can still be unsuitable for release.

Treat tempo-changing songs and loops differently

For a constant-tempo loop, scaling the full region may be enough, but trim boundaries to exact musical positions and check for clicks. For a whole live song, a single percentage preserves its relative drift; it does not align every beat to a constant grid. Creating that result requires a tempo map and variable warping, usually better handled in a DAW.

If only one section needs a new pace, process boundaries with care. Time changes alter where later audio lands, so tracks can lose synchronization. In a multitrack project, decide which tracks and label positions must move together. Keep tails inside the selection or they may be cut off or overlap the next event.

Export a new file with clear provenance

Use File > Export Audio and choose a format appropriate to the next task. WAV or FLAC avoids an additional lossy generation for further editing. MP3 is compact for delivery but re-encoding an MP3 source can add artifacts beyond the time stretch. Export once from the highest-quality available source whenever practical.

Use a new filename that includes the target or change, such as mix-126bpm-v2.wav. Retain the untouched original, Audacity project, source and target BPM, processing date, and export settings. Reopen the exported file and check its duration, beginning, end, channels, and playback before deleting any working copy.

Know when Audacity is and is not the right tool

Audacity is effective for a modest offline transformation of a single file. A DAW is generally better for tempo maps, synced multitracks, MIDI, beat-grid warping, and automation. Real-time DJ software is better when the purpose is a temporary live transition rather than a new rendered master.

Copyright and permission also matter. Processing a file does not grant rights to distribute a derivative recording. Use material you created, licensed, or are otherwise authorized to edit, and follow platform requirements. Technical capability is separate from permission and from claims about professional mastering quality.