To make an LFO-style pulse repeat consistently in Audacity on Mac, apply Tremolo to a generated tone once, then replay the processed audio. Keep the selection, starting phase and rate fixed when rebuilding it. This workflow creates a reusable sound rather than recalculating modulation during playback.

Why applying the modulation makes playback repeatable

For a simple synthetic pulse, Audacity provides two relevant tools: Tone generates the underlying sound, and Tremolo varies its volume cyclically. The latter supplies the repeating modulation pattern used in this tutorial. Audacity documents its waveform, starting phase, depth and frequency controls in the Tremolo reference.

The key is when processing happens. An effect applied from the Effect menu rewrites the selected audio; a realtime effect calculates processing during playback. Once Tremolo has been applied, the volume movement is part of the audio waveform. Replaying the same passage therefore replays that stored movement. This follows from Audacity’s documented distinction between destructive and non-destructive effects.

There is also a feature boundary: the current Audacity 4 manual lists Compressor, Limiter and Reverb as its bundled realtime effects. Built-in Tremolo uses the selection-based application workflow. Do not look for a Tremolo transport-retrigger switch in that realtime panel; the supported route here is to process the selection. Third-party plugins may provide different controls, which this tutorial does not assume. See Realtime Effects.

The task below is deliberately narrow: create an eight-second synthetic tone with a regular volume pulse, preserve its untreated source, and produce a finished file whose modulation stays in the same places. It uses menu commands rather than platform-dependent keyboard shortcuts. Interface placement can differ between Audacity versions.

Generate a clean synthetic source

Start with an empty Audacity project so that selecting everything later cannot accidentally include unrelated tracks. Open Generate → Tone and enter the following illustrative settings. Audacity’s Tone documentation describes these controls and explains that generation begins at the selection start, or at the cursor when nothing is selected.

Tone control Setting for this example
Waveform Sine
Frequency 440 Hz
Amplitude 0.5
Duration Eight seconds

Generate the tone at the beginning of the project. The 440 Hz value is the audible source frequency; it is separate from the much slower modulation rate you will enter later. A sine source keeps the example straightforward because it contains a single frequency. Amplitude 0.5 is an illustrative working level, below the generator’s full-scale value of 1. Tone — Audacity Manual

Save this untreated project locally before applying Tremolo, and plan to save the processed version under a different project name. Keep the untreated file available for revisions. Audacity distinguishes saving an editable project from exporting a playable audio file; its support guide gives the local project command as File → Save Project → Save Project. Saving and exporting projects

Apply one controlled modulation pass

Select the complete eight-second tone. In this otherwise empty project, Select → Select all selects the audio needed for the example. Audacity also supports dragging a time range directly in the waveform, but selecting the whole source avoids an approximate boundary. Selection determines which audio the next operation changes. Basic audio editing

Open Effect → Distortion and Modulation → Tremolo. Use these illustrative settings, preview them if desired, and apply the effect once. The control names and their functions are documented in Tremolo.

Tremolo control Setting Purpose
Waveform type Sine A smooth repeating volume curve
Starting phase (degrees) 0 A fixed position within the modulation cycle
Wet level (percent) 100 Full-depth modulation, reaching silence at its minimum
Frequency (Hz) 4 Four modulation cycles per second

Keep the starting phase at the same value whenever you rebuild this example. Frequency specifies how quickly the cycle repeats; phase specifies where within that cycle it begins. Matching only the frequency leaves part of the recipe unspecified. Audacity’s starting-phase documentation explicitly identifies this control as the starting position in the waveform cycle.

After applying the effect, the waveform should redraw to show the processed audio. Save the processed project under its separate name. Destructive processing can be undone while the project remains open, but Audacity explains that after closing, the applied change is part of the saved audio. Keeping the untreated project gives you a dependable revision source. Apply your first effect

Calculate a pulse rate for a chosen tempo

The example’s four pulses per second also correspond to eighth-note pulses at 120 beats per minute, assuming each quarter-note beat contains two pulses. This is a calculated relationship, not a tempo-sync feature being claimed for Tremolo. Its documented rate control is in hertz. Tremolo — Audacity Manual

Use the relationship LFO frequency = BPM / (60 × beats per modulation cycle). Here, a beat means a quarter note. For one cycle per beat, use beats per modulation cycle = 1; for two cycles per beat, use 0.5. These illustrative calculations produce:

Intended pulse spacing At 120 BPM Modulation period
One cycle every two beats 1 Hz 1 second
One cycle per beat 2 Hz 0.5 seconds
Two cycles per beat 4 Hz 0.25 seconds

At 4 Hz, eight seconds contains 8 × 4 = 32 modulation cycles. The 440 Hz source contains 8 × 440 = 3520 source cycles over that duration. Both are whole numbers, making this a useful starting example when planning repeated segments. These are arithmetic results from the chosen inputs, not measured playback results.

That calculation does not establish a seamless join. Repeatability means the same stored passage plays again; a seamless loop additionally needs a suitable transition from its end back to its beginning. Inspect and audition that boundary separately. If you change the duration, recalculate duration in seconds × modulation frequency to see whether the segment still contains a whole number of modulation cycles.

Replay exactly the same passage

Keep the full processed clip selected for the first comparison. Play it, stop, return to its beginning, and play it again. This is a procedure for checking your result, not a report of an invented listening test. With no additional processing, the modulation is already stored in the clip.

For repeated playback, align the loop region with the same selection. In the current manual, right-clicking the timeline ruler exposes Set loop region to selection. This makes the loop boundaries match the selected passage, avoiding a separately drawn, slightly different playback range. Set Loop Region to Selection

Selection and loop region are related controls, but they are not interchangeable. The reverse command, Set selection to loop, replaces the time selection with the loop boundaries and selects across every track. In a larger project, check the selected tracks before applying any further effect. Set Selection to Loop

Diagnose a result that seems inconsistent

Use this sequence before changing the modulation settings. Each check isolates a different part of the workflow:

  1. Check the playback boundaries. Compare from the clip’s beginning each time. Starting halfway through the stored sound begins halfway through its existing pattern.
  2. Check the processing selection. A partial selection receives only a partial effect. Select the intended complete passage before rebuilding it. Basic audio editing
  3. Return to the untreated source for revisions. Applying Tremolo again processes already changed audio. Undo the first application or reopen the saved source before trying a different recipe. Apply your first effect
  4. Check clip speed. Audacity’s stretch handles change playback speed. Restore the original speed when comparing against this eight-second example. Basic audio editing
  5. Bypass extra processing. Track and master realtime effects can process the sound beyond its stored Tremolo pattern. Their power controls let you bypass them for a simpler comparison. Realtime Effects

If the task instead requires adjusting a live LFO while playback runs, this rendered workflow does not provide that interaction. Its advantage for the stated task is that the finished pulse pattern becomes reusable audio. Changes to the design require another application from the untreated source.

Export the finished sound

Open File → Export audio. For this single-clip project, export the complete project, or use Export selected audio with the whole processed tone selected. The current export dialog also offers Export audio in loop region; use that only when the loop boundaries are the intended deliverable. Export your audio

Choose WAV as a straightforward working file, or FLAC for lossless compression, and use mono for this single-channel example. Keep the project’s sample rate when no destination requires a different one. Confirm the export range before saving so that a leftover partial selection does not produce a shortened file. Audacity documents format, channel, sample-rate and range choices in Export your audio.

Keep the untreated project, processed project and exported audio as distinct artifacts. Record the source settings, selected duration and four Tremolo values alongside them. You can then replay the finished pattern directly, or rebuild it deliberately when the sound needs a different rate, depth or starting position.

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