Why AAC Files Need Normalization
AAC files come from many sources — iTunes Store purchases at one loudness, Spotify downloads at another, iPhone voice memos at yet another. A mastered pop track might sit at -8 LUFS, while a podcast recording could be -20 LUFS. That's a 12 dB difference, making one sound roughly 2.5x louder than the other.
When you convert an AAC collection to MP3, these volume differences carry over. Normalization fixes this by adjusting every file to the same perceived loudness before encoding — no more reaching for the volume control between tracks.
No extra quality loss: Normalization is a simple volume adjustment applied before the MP3 encoding step. It doesn't add any degradation beyond the normal AAC-to-MP3 conversion. The gain change itself is mathematically lossless.
Which LUFS Target to Choose
Each platform has its own loudness standard. Here are the targets for the three presets available in the converter above:
| Preset | Target | True Peak | Best For |
|---|---|---|---|
| Podcast (-16 LUFS) | -16 LUFS | -1.5 dBTP | Apple Podcasts, spoken word, audiobooks |
| Streaming (-14 LUFS) | -14 LUFS | -1 dBTP | Spotify, YouTube, Tidal, Amazon Music |
| Broadcast (-23 LUFS) | -23 LUFS | -1 dBTP | EBU R128, European TV/radio |
Most common choice: If you're converting AAC files for Spotify or YouTube, use Streaming (-14 LUFS). For podcast episodes, use Podcast (-16 LUFS).
How Normalization Works During Conversion
When you upload an AAC file and select a normalization preset, Convertio performs two operations in a single pass:
- AAC → MP3 conversion: The AAC audio is decoded and re-encoded to MP3 using libmp3lame (VBR V2 by default)
- Loudness normalization: FFmpeg's
loudnormfilter analyzes the audio's integrated loudness (LUFS) and applies a constant gain adjustment to reach the target level
The normalization is linear — a single gain value applied uniformly across the entire file. It does not compress dynamics, alter frequency response, or change the stereo image. A true peak limiter at -1 dBTP prevents clipping if the gain boost would push any peaks above the safe ceiling.
Normalizing iTunes and Apple Music AAC Files
iTunes Store purchases are encoded at 256 kbps AAC with Apple's mastering standards. While individual tracks are well-mastered, albums from different eras and genres can still vary by 6–10 dB in loudness.
If you're converting an iTunes library to MP3 for a car stereo or portable player, normalization to -14 LUFS creates a consistent listening experience across your entire collection. The volume adjustment is tiny for most modern pop but significant for classical, jazz, and older recordings.
When Should You Normalize?
- Converting a mixed AAC library: Files from different sources (iTunes, rips, downloads) at different loudness levels. Normalize to -14 LUFS for consistent playback
- Preparing podcast episodes: AAC recordings from different microphones or environments. Normalize to -16 LUFS for Apple Podcasts compliance
- Uploading to Spotify or YouTube: These platforms normalize incoming audio anyway — delivering at the target loudness avoids platform-side adjustment
- Creating compilation playlists: Tracks from different albums and artists, all at different loudness. Normalization evens them out
For a deep dive into LUFS measurement, the EBU R128 standard, and platform-specific targets, see our complete LUFS guide. For choosing the right MP3 bitrate for your AAC files, see the bitrate guide.