How to Create a Loopable Sound Effect

Loop in the Road courtesy xq311z

Do you have a sound effects collection, or are thinking of building one?

A sound library is important for any sound designer or field recordist. It’s more than just a collection of tracks you’ve gathered during your years of editing: it is a valuable asset. It showcases your skill and creativity, and can augment the projects you contribute to.

Strong, evocative tracks create a powerful sound library. But how can you make your even more valuable?

One simple way is by creating loopable tracks.

In today’s post I’ll explain why loopable sound effects add value to your sound library. I’ll also explain how to create them with a step-by-step guide.

What is a Loopable Track?

In the simplest terms, a loopable track is a sound effect that is meant to be repeated, looping over and over endlessly, and seamlessly.

In many cases this is an ambient sound effect. Imagine a 30 second rainstorm playing repeatedly for an hour.

Loopable tracks can be specific sound effects as well. This is common in the game audio industry. A loop of few seconds of a car engine driving provides minutes or hours of continuous audio for a racing game.

A loop is performed in one of two ways:

It loops by returning to the beginning of a single clip when it finishes, just like a single song repeats on your iPod.
Copies of the sound effect are arranged end to end in an editing app timeline, one after another. This repeats multiple copies, from left to right, to create a continuous timeline as long as an editor wishes.

Why Create Loopable Tracks?

Creating looping tracks takes extra time and precision. It’s tricky to do well: to make the loop sound natural, not overly repetitive, and for the seam between the copies to be completely invisible. If it is so much effort, why bother creating loopable tracks?

While it is indeed quite tricky to make a loopable sound effect natural and fluid, the benefits are strong:

Save time and effort. The rainstorm sound effect is only 30 seconds. What if our movie scene is three minutes? In most cases you’ll need to extend the duration of the sound effect. This means an editor needs to repeat that track, and craft a cross fade to smooth out the areas where they join. That’s not so bad if you need to do it once in a single scene.

However, the process becomes tiring, and time-consuming when you need to extend this sound in multiple scenes, or in additional projects. A loopable track saves you this time and effort. You just drop in a loopable track and repeat. Your work is complete in seconds.
Save space. A six minute birdsong atmosphere recorded at 96 kHz, 24-bit is 198 megabytes. That’s heavy. A 30 second loopable version is only 16.5 megabytes. This is especially important for game audio, where file size has an immense effect on memory demands and total app size. Small, loopable sound effect chucks keep things manageable.
Add flexibility. Let’s say our original rainstorm track evolves over six minutes. It begins softly, grows, then adds thunder. Pull three loopable tracks from that file: a loop of soft rain, heavy rain, and rain with thunder. Now a single track has become four: the original with three loops. That gives editors options. It also saves them from editing those versions themselves.

This is especially helpful to music editors. Offering loopable versions of key melodies or riffs allows music editors to slip segments into 30 or 60 second commercials swiftly.
Brevity. A six minute, monotonous rain track doesn’t help anyone. Does it remain the same after the first minute? If so, it’s best to trim it. Why?

The longest duration you’ll usually need for most projects is a minute or so. Television or films may demand more, perhaps up to 3:30. Trim the excess into a loopable track. Sprawling tracks add no benefit to the sound effect.
Expression. A loopable track lops off redundant parts of the sound. What remains focuses on the best part of a sound effect. It highlights the expression a field recordist sought when capturing it on location.

Does the rain track evolve instead? Keep the entire duration. The way a sound evolves adds valuable character. Pull out loops as described in point 3, “add flexibility.“
Appeal. Do you want to share your collection? Perhaps you want to sell them on the Web. Maybe you want share them amongst the other editors at work, or on a sound sharing website. Loopable tracks have strong appeal: they make work easier and faster. They also imply that a track has been given special attention when it was mastered: they are pre-approved.

The extra work creating loopable tracks is worth the effort. When shouldn’t you create a loopable track, though?

The only reason for saving extra long sound effects is to retain character that develops across the duration of the track. Imagine a jungle dawn chorus that awakens and begins to bicker, or a protest crowd that starts with dissatisfied grumbling and bursts into a riot. Save the full duration of these tracks, even if they are long. Extract loopable sections, but preserve the natural expression from the whole. Authentic expression can’t be recreated once it’s removed.

Drawbacks of Loopable Tracks?

Surely there must be a drawbacks to looping a sound file?

It’s true. Loops, by their very nature, are inherently bland. They’re meant to be supporting players in the scope of your project.

Also, loops won’t feature your most interesting recordings. They have limited sonic expression. Why? It’s hard to convey meaning and character in a sound file less than thirty seconds long.

Neither of these is a major problem, though. Simply keep the original sound file, as well as the additional loopable tracks.

How to Create a Loopable Track

Selecting the audio

Listen to the track. Play the track from end to end. Refresh your experience of the sound effect or song. Get a feel for it. Note when distractions jump out. See which details capture your attention and imagination.

The original file: a thunderstorm atmosphere
Find a rough section. Which portion of the sound file do you choose to loop? There are two ways of looking at it:
- Find a section with consistent tone. Choose this option when you’d like to create a background loop. A background loop creates a bed for the rest of your tracks to lay upon. Because of this, you need to choose a portion of the clip with minimal character. This isn’t the time to highlight tittering laughter, or melodic bird cries. The aim is to create a steady tone that can repeat endlessly without drawing attention to itself.
- OR, Choose a region with character. Select a short section that highlights what makes the track special. Consider a jungle dawn chorus. You wouldn’t want just the insects. That doesn’t emphasize the richness of the location, or the life there. (That may be a good choice if you’d like consistent tone, though). Instead, find a span where the tucans are bright and lively.
Find a rough section. This is the most interesting spot.

How long should this section be? Well, it can be long, but that would defeat the purpose of a loop. Short is usally better. But be careful. Too short, and you’ll notice even the smallest features creating a watch-like rhythm. I aim for 30 seconds. That’s long enough to ensure the loop doesn’t sound repetitive, but short enough for a manageable file size.

Don’t worry about being too finicky about this selection. It may have a natural swell and fade. Just ensure you create an “arc” where the beginning of the phrase carries the same energy as the end.

Editing the loop

Edit the broad selection points. Make an edit based on your selection above. Save that audio, and remove the rest.

Edit the broad selection points.
Pick your central edit. Next, find an edit point somewhere in the centre. This will become the start and end points for your loop.

What’s a good place for your central edit? It must be at a natural dip in the sound. Don’t choose an edit in the middle of a “phrase.” Avoid, for example, cutting in the middle of a car pass, or a conversation. Finding this spot is harder than you’d think, especially in dense traffic tracks, or active crowds.

Pick your central edit. This is a good spot to begin a loop.
Zoom in, and cut. Magnify the precise point of the central edit. Find a “zero point,” a place where the waveform crosses the X-axis. Why?

If you sever a waveform at any other point, you risk introducing a pop into your track. That would create a click when the track begins and ends, or loops. That would ruin all your work.

Finding a zero point is hard to do for stereo files. The waveforms rarely cross the zero point at the same time. In these cases, you can add a small four-sample fade to remove the pop.

Finding a good zero point ensures your loop will have a natural beginning and a seamless ending.
Swap regions. You’ll now have two regions in your loop, one on the left, and the right. Drag the left region behind of the right one. This places your new edit (from step 3) at the beginning of the track, and the end.
Fade the newly joined sections. Apply a cross fade at the middle edit. Ensure it is smooth, fluid, and unnoticeable. An obvious fade will ruin the point of the loop. It will stick out when the clips is repeated, and quickly become annoyting.

This will be easier if you choose beginning and ending edits with similar energy (see Find a rough section, above).

Fade the newly joined sections.

Here’s one tip to help with tricky fades. Arrange your fade around a characteristic, louder sound just before, or after a fade. This distracts from the fade itself, and often can trick the ear into accepting slight differences in background tone.
Check the beginning. Test the start of the track. It’s easy to become so focused on the cross fade, that you can forget your central edit has now become the start of your track.
Bounce or render the loop. Commit your edits to a new, self-contained file.

Check your work

Test the track. Solo a track in Pro Tools and loop the selection (CMD-D). For other apps, such as Reaper, you can merely drag either end.

Repeat the loop for a few minutes. Play it. Close your eyes. Listen closely for anything distinctive that may jump out and become annoying. Listen for a “pumping” sensation, repetitive features, or clicks or pops where the loop repeats. The goal is for the loop to be subtle, and supportive.

Test the track by looping it.

That’s it! If you’ve chosen your edits well and faded carefully, you’ll have created a new, bonus track for your sound library.

Looping Tips

Important things to remember when creating loopable tracks:

Loopable tracks shouldn’t sound too repetitive. Ensure that your loop has just enough character to give the track texture, but not so much that it will distract listeners with an annoying pattern as the track cycles.
Since these tracks are meant to loop seamlessly, their length doesn’t matter. The loopable clip can be five seconds long, or fifty. It’s harder with shorter loops, but it may work. It is vital, however, that it loops end-to-end seamlessly. As long as the loopable track doesn’t sound too repetitive, you can choose any length you like.
The loop point must be inaudible. The point where the two looped clips meet must be seamless. There shouldn’t be any pops, jumps in volume, or changes in tone. Some seemingly tonal tracks may seem consistent, but actually have a rhythm. There may be a pulsing hum, a whine, and faint six-count ticking. A hum, for example, may crest and dip. This pattern must be maintained throughout the loop, just like rhythmic surf on a beach. If a clip ends in the middle of crest, then starts again at the dip, it will loop unevenly. This will give the track a lurching aspect, and is distracting.