How loud is loud? — loudness and LUFS

You’ve cleaned up the recording. Now you have to make it the right loudness to publish — for a podcast, a video, a broadcast, a music stream. This sounds trivial (“just turn it up”) and is secretly one of the most misunderstood topics in audio. Getting it right is normalization, and getting it right the modern way means understanding a unit called LUFS.

“Loud” is not “tall”

The naïve way to set level is to look at the peak — the single tallest sample in the file — and turn everything up until that peak just touches the ceiling. This is peak normalization, and it has a fatal flaw: it tells you nothing about how loud something sounds.

A sudden snare hit and a sustained shout can have the same peak height, yet the shout sounds far louder, because loudness is about how much energy there is over time, not how tall one instant is.

   a brief tick                a sustained tone
   (same PEAK height, but much quieter to the ear)

  +1 ┤   █                    ████████████████████
   0 ┼───█──────────          ████████████████████
  -1 ┤   █                    ████████████████████
       one tall spike          loud the whole time
       PEAK: maxed             PEAK: identical
       LOUDNESS: low      ◄──►  LOUDNESS: high

Peak-normalize a quiet, even podcast and a punchy one to the same peak and the punchy one will sound much louder. That’s why, for decades, some adverts felt like they were screaming at you between TV shows: everyone was peak-normalizing and then squashing their audio to be as dense as possible.

LUFS: measuring perceived loudness

The fix was an international standard (its name is ITU-R BS.1770, adopted for broadcast as EBU R128) that measures loudness the way ears experience it, not the way a ruler does. The unit is the LUFS — “Loudness Units relative to Full Scale.” Bigger negative number = quieter. Three ideas make it match hearing:

1. It averages energy over time, so a sustained sound reads louder than a brief spike of the same height — exactly as you hear it.
2. It weights pitches like your ear does. Your hearing is most sensitive in the upper-mid range and less so at the extremes, so the meter gives those mid-high pitches more say. (This pitch-weighting is called K-weighting.)
3. It ignores the silences. Long gaps shouldn’t drag the average down, so the measurement “gates out” the quiet bits and only averages the parts that are actually playing.

The upshot: two pieces of audio at the same LUFS sound equally loud, even if one is a whisper-and-shout drama and the other a steady narrator. That’s why the whole delivery world now specifies loudness in LUFS. Common targets:

Cathar measures true, gated, K-weighted LUFS and turns the whole file up or down by one amount to hit your target: normalize --target -16.

The true-peak safety net

There’s one last trap. When digital audio is turned back into sound, the player draws a smooth curve through the samples — and that curve can briefly poke higher than any actual sample, between the dots. These hidden overshoots are true peaks (“inter-sample peaks”), and if they cross the ceiling they cause nasty distortion on some devices even though no stored sample looked too loud.

So a proper loudness normalizer doesn’t just hit the LUFS target — it also keeps the true peak under a safe ceiling (commonly −1 dBTP). Cathar holds the gain back if pushing for the loudness target would breach that ceiling (--true-peak -1), trading a hair of loudness for a guarantee it never clips on playback.

How the big tools do it

• Every broadcast and streaming workflow on earth now runs on LUFS — it’s the law for TV in much of the world. Loudness meters are everywhere: the free Youlean Loudness Meter, Waves WLM, Nugen VisLM, and the meters built into Pro Tools, Logic, and Audition.
• iZotope RX and Ozone include a “Loudness” module that does exactly what cathar does — measure integrated LUFS, normalize to a target, respect a true-peak ceiling — with presets for every platform.
• FFmpeg’s loudnorm filter is the command-line workhorse the whole web uses for batch-normalizing video and podcast audio; it implements the same BS.1770 standard.

This is a corner where cathar is doing the exact same standardized maths as the professional tools — there’s no ML and no secret sauce in loudness, just a well-defined international measurement. If cathar says −16 LUFS, it means the same −16 that RX, FFmpeg, and a broadcast meter mean.