How Audacity Time Tracks Change Playback Speed

A Time Track in Audacity alters playback speed dynamically by applying an adjustable envelope curve across the entire project timeline. Rather than applying a static tempo change to an isolated clip, a Time Track allows you to continuously speed up or slow down playback in real time using movable control points. This guide explains how Time Tracks function, how they manipulate both speed and pitch, and how to apply them across your projects.

What Is a Time Track?

A Time Track is a global automation track that controls the playback rate of an entire project. Unlike standard audio tracks that contain waveforms, a Time Track displays a horizontal line representing the speed percentage relative to normal playback (100%). You can add one by navigating to Tracks > Add New > Time Track.

How Dynamic Speed Adjustments Work

The core mechanism of a Time Track relies on envelope control points to create smooth or abrupt speed transitions:

• Envelope Control: By clicking on the blue envelope line inside the Time Track, you create control points. Dragging a point upward increases the speed above 100%, while dragging it downward decreases the speed.
• Gradual Transitions (Ramping): Placing two points at different vertical levels creates a linear ramp between them. For example, setting a point at 100% and another further along the timeline at 50% causes the audio to decelerate smoothly over that duration, creating a classic "tape stop" effect.
• Instantaneous Shifts: Placing two points immediately adjacent to each other forces an abrupt jump in speed, switching the pace instantaneously from one tempo to another.

Pitch and Speed Coupling (Varispeed Effect)

Time Tracks operate like a physical tape machine's varispeed control. When the playback speed changes, the pitch changes proportionally:

• Higher Speed: Shortens playback duration and raises the pitch (a "chipmunk" effect).
• Lower Speed: Lengthens playback duration and lowers the pitch.

Because the Time Track alters the rate at which samples are read, it does not separate tempo from pitch as algorithms like "Change Pitch" or "Change Tempo" do.

Adjusting Range and Scale

By default, a Time Track usually has a narrow range (such as 90% to 110%). To achieve dramatic effects like extreme slow motion or rapid fast-forwarding, you must adjust the scale:

1. Click the dropdown arrow on the Time Track panel.
2. Select Set Range.
3. Define the Lower Speed Limit (minimum 10%) and Upper Speed Limit (maximum 1000%).
4. Choose between a Linear or Logarithmic scale depending on whether you need fine control at lower speeds.

Global Project Impact and Rendering

The Time Track governs the entire project globally; it cannot be assigned to modulate one specific track while leaving another untouched. During playback, Audacity reads the Time Track in real time to govern the transport speed. When you export your project (via File > Export) or use Tracks > Mix and Render, Audacity renders the dynamic speed and pitch curves permanently into the final audio file.