Why Can Audacity 3 Change Tempo Leave a Gap or Cut Off My Product-Ad Voiceover?

Understand Duration Changes and Their Limits

Version boundary: the current manual, updated 13 October 2026, says Sliding Stretch replaces Audacity 3 Change Tempo and Change Speed. Its initial and final tempo/pitch controls are a different interface. The fast/high-quality algorithm discussion below applies to the cited older Change Tempo workflow, not a tested description of Sliding Stretch.

Change Tempo adjusts the duration of a selected passage while aiming to preserve pitch. It differs from changing speed and pitch together. The effect provides linked controls for Percent Change, beats per minute, and Length.

For commercial narration, the Length fields are particularly useful because the available video slot may already be fixed. You can inspect the original selection length and enter the intended new duration.

However, specifying a duration is not the same as guaranteeing that every word ending will remain intact. Time stretching requires signal processing, and the chosen algorithm matters.

Audacity's manual identifies possible audible distortion at more extreme settings.

The current Sliding Stretch manual identifies it as the replacement for Audacity 3's Change Tempo and Change Speed.

Distinguish the Fast and High-Quality Algorithms

The Development Manual warns that the default fast algorithm may remove audio from the beginning or end of a selection. When slowing audio, it may also fail to extend the audible ending all the way to the selection boundary. A short silent region can result.

The Use high quality stretching (slow) option uses the SBSMS algorithm. According to the manual, it processes more slowly but retains the complete content. For small to moderate tempo adjustments, it will usually deliver better quality, particularly with percussive material.

That does not make it a universal solution. The same documentation cautions that high-quality stretching is suitable only for small to moderate changes and can deteriorate badly at extreme settings.

Decision branch: If the fast algorithm leaves a missing ending or unexplained gap, try high-quality stretching on the original selection. If the requested duration change is extreme, reconsider the script or edit rather than assuming the slower algorithm will rescue it.

The older Change Tempo manual documents the fast algorithm's missing-content risk and the high-quality option's limits.

Illustrative Exercise: Test Two Narration Inputs

Illustrative exercise — hypothetical timing inputs, not processed recordings. A is a 12-second product description; B is a 15-second description ending with an offer condition. Both have an 11-second target.

For a constant tempo change, tempo increase = (original duration / target duration - 1) × 100%. A therefore needs (12 / 11 - 1) × 100% ≈ +9.1%; B needs (15 / 11 - 1) × 100% ≈ +36.4%. These calculated tempo increases differ from the percentage reduction in duration. Neither value is an established audio-quality limit.

Use the older Change Tempo procedure below only if it matches the installed effect. If the ending is incomplete or speech becomes unclear, reconsider the script, recording or available duration rather than treating a matching clip length as acceptance.

1. Preserve the original selection and make separate copies for the two algorithm comparisons.
2. Set the same target Length on both copies. Process one with the default fast algorithm and the other with Use high quality stretching (slow). Start each comparison from the original audio.
3. Inspect and listen to the beginning and ending of each result, then compare the final rendered audio against the picture.
4. Record completeness, intelligibility and final duration. If an amplitude envelope was present, inspect its level behavior too.

Inspect the Ending Before Export

Zoom into the first and last portions of the processed selection. Listen for clipped consonants, incomplete words, unnatural pauses, and a gap between the final audible sound and the intended video cut.

Check the actual final rendered audio against the picture, especially when the closing product claim or call to action occurs near the boundary. Preserve a clean original for comparison.

The Development Manual also documents an envelope-related issue in which applying Change Tempo can remove an existing amplitude envelope. If the original narration uses envelope automation, inspect its level behavior after processing rather than assuming it survives unchanged.

For delivery, verify the entire spoken message and the final exported duration. These instructions describe documented development-manual behavior, not an executed compatibility test across Audacity versions.

Illustrative Exercise — Duration and Ending Verification

These are illustrative exercise rows. Save a copy and record your own observations; the example is not an executed acceptance test.

Download review worksheet (CSV)