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How to do loudness: the LUFS and LKFS FAQ for podcasters

October 9, 2018 · By James Cridland

What are LKFS or LUFS?

LKFS and LUFS relate to the same thing. Most of the world talks about LUFS, so we will too: but the US broadcast industry, particularly, likes to use LKFS.

LUFS describes a measurement of loudness over a period of time. In podcasting, the purpose of LUFS is to make sure that if you play five podcasts one after another, they all sound the same loudness - and you shouldn't need to touch the volume control. It's also useful to get the right loudness in comparison to the voice on your smart-speaker, too.

This isn't related to dynamic compression, which alters the sound of the audio to be consistently loud. LUFS is simply a measurement: "how loud does this audio sound?"

How loud should podcasts be?

The podcast industry has mostly standardised on -16 LUFS. This is the loudness level that Apple Podcasts ask for.

Broadcast radio and television uses -24 LUFS in the US, -23 LUFS in the rest of the world: so if you were to take an audio file made for broadcast radio and upload it as a podcast, it would be too quiet.

Tip: LUFS is expressed in negative numbers, so -26 LUFS is quieter than -16 LUFS.

I use a PPM or a VU meter to measure my audio level. How do you convert this number to LUFS?

You don't; because LUFS is a measure of something rather different.

LUFS is a measure of average loudness - "how loud is this audio overall".

Other forms of metering are concerned with "how loud is this bit right now," which is not the same thing.

What should I measure then?

If you have a LUFS meter in your audio editor - Hindenburg has one - you'll see a bewildering amount of numbers.

Apple asks for the "overall loudness" to be -16 LUFS, and most meters will call this the "integrated loudness". That's the average loudness from the beginning to the end of your podcast, or after a defined time window.

If you're mixing audio, you want to be looking at the "short-term" number. That's the average loudness over the last three seconds. This number is most useful for setting microphone levels. Again, set it to -16 LUFS.

What's the point?

LUFS measures the overall sensation of loudness over a length of time, not the electrical energy of the peak loudness.

That's important, because it lets you be rather more creative with your audio's dynamic range.

If you want a really loud explosion, you can have one. If you want very quiet whispering, you can have that, too. The overall LUFS value is set on the average of the whole piece.

You can also make things appear louder by dynamically compressing them to be more consistently loud. This is why the adverts on the TV used to seem so much louder than the programmes: they looked the same on the old meters, but they were subjectively much louder to our ears.

LUFS metering is clever, since it recognises when something has been treated with dynamic compression: that audio will appear to be louder on the LUFS meter, since it sounds louder in real life.

So, whether you heavily dynamically compress your audio, or don't treat it at all, LUFS metering will work out how loud it really sounds to your ears. That enables podcast producers to make audio which is a wider dynamic range if they want to.

What else do I do?

When it comes to outputting your file, your audio mixing software may let you specify that you want this file at -16 LUFS. (Hindenberg does, Adobe Audition does too). It will listen to your entire audio and work out the right level to output it at so that it hits -16 LUFS.

Alternatively, you can use a service like Auphonic to post-produce your audio to the right loudness level. You can set -16 LUFS in the Auphonic system: it's the default for new productions.

Apple ask for a +/- 1 dB tolerance, which means that your file can be -15 LUFS or -17 LUFS and it shouldn't matter.

Apple also ask for a true-peak value not exceeding -1 dB FS. Your post-production software should do this job for you automatically.

I've just measured my -16 LUFS file, but it's -19 LUFS! Why?

Mono audio is essentially taking two stereo tracks and adding them together, so mono audio can sound louder than stereo. So, mono files need to be made a bit quieter than stereo files to compensate. To achieve -16 LUFS stereo, your editing program should output -16 LUFS stereo... or -19 LUFS mono.

So a mono file at -19 LUFS should sound just as loud as a stereo file at -16 LUFS.

The production process - your audio mixing software - will probably do this work for you when you produce the final mix.

Now, if you put this resulting file into a file analyser that's built to measure stereo files, it will assume the mono file only sounds as loud as -19 LUFS as a result. This isn't the fault of your audio mixing software, and is probably the fault of the file analyser.

We'd recommend setting -16 LUFS for everything, and letting your audio mixing software deal with it. It knows if you're making a mono file, and will make changes accordingly.

Tech note: If you use ffmpeg to monitor loudness, there's an option that says dualmono, which deals with this disparity.

Are there any tutorials I can watch?

Further reading

Got any feedback?

We'd love to hear it. There's a comments form just underneath. Thank you to John Maizels for his kind assistance.

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