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<h1>FFmpeg audio cleanup</h1>
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<div class="header-meta">
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<author>Joshua Seigler</author><date>June 26, 2025</date>
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<span class="tags" style="--totalTags: 11"
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><a class="tag" style="--tagIndex: 5" href="/tags/technical"
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>technical</a
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>
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<a class="tag" style="--tagIndex: 10" href="/tags/ffmpeg">ffmpeg</a>
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<header class="toc"></header>
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<main data-pagefind-body="data-pagefind-body">
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<p>
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I recently needed to process 20+ phone audio recordings. The audio is
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mp3 recordings in stereo, made in an environment with echoes and noise
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from fans/heaters.
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</p>
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<p>
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Although I could do it easily with
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<a href="https://tenacityaudio.org/" target="_blank" rel="noopener"
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>Tenacity</a
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>
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I didn’t want to use a manual process, since it would take days. So I
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tried using FFmpeg filters and Bash scripting.
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</p>
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<p>
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I found an FFmpeg filter called
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<a
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href="https://ffmpeg.org/ffmpeg-filters.html#compand"
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target="_blank"
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rel="noopener"
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>compand</a
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>
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which lets you map an input decibel range to an output decibel range. I
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also used the
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<a
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href="https://ffmpeg.org/ffmpeg-filters.html#anlmdn"
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target="_blank"
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rel="noopener"
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>anlmdn</a
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>
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filter to reduce noise, and the
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<a
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href="https://ffmpeg.org/ffmpeg-filters.html#highpass"
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target="_blank"
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rel="noopener"
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>highpass</a
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>
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filter to help with clarity.
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</p>
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<p>I ran into a couple gotchas.</p>
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<ol>
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<li>
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<code>mpv</code> does something special for audio playback that
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prevents audio from clipping. <code>vlc</code> plays the file as it
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is.
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</li>
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<li>
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Because the compressor has an attack and decay (which is necessary for
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things to sound good) it can cause clipping if the volume rises
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sharply. Applying a <code>delay</code> parameter with half the
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duration of the attack length fixed this.
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</li>
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</ol>
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<p>Here is the script:</p>
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<pre
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class="language-bash"
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><code class="language-bash"><span class="token shebang important">#!/bin/bash</span>
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<span class="token keyword">if</span> <span class="token punctuation">[</span> <span class="token string">"<span class="token variable">$#</span>"</span> <span class="token operator">==</span> <span class="token string">"0"</span> <span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token keyword">then</span>
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<span class="token builtin class-name">echo</span> <span class="token string">"Error: no arguments provided."</span>
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<span class="token builtin class-name">echo</span> <span class="token string">"Usage: <span class="token variable">$0</span> file1 file2 file3 ..."</span>
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<span class="token builtin class-name">echo</span> <span class="token string">" or: <span class="token variable">$0</span> *.ext"</span>
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<span class="token builtin class-name">exit</span> <span class="token number">1</span>
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<span class="token keyword">fi</span>
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<span class="token builtin class-name">trap</span> <span class="token string">"exit"</span> INT
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<span class="token keyword">while</span> <span class="token punctuation">[</span> <span class="token string">"<span class="token variable">$#</span>"</span> <span class="token operator">!=</span> <span class="token string">"0"</span> <span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token keyword">do</span>
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<span class="token assign-left variable">base</span><span class="token operator">=</span><span class="token string">"<span class="token variable">${1<span class="token operator">%%</span>.*}</span>"</span>
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<span class="token assign-left variable">ext</span><span class="token operator">=</span><span class="token string">"<span class="token variable">${1<span class="token operator">##</span>*.}</span>"</span>
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<span class="token assign-left variable">outfile</span><span class="token operator">=</span><span class="token string">"./normalized--<span class="token variable">$base</span>.<span class="token variable">$ext</span>"</span>
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<span class="token keyword">if</span> <span class="token punctuation">[</span> <span class="token operator">!</span> <span class="token parameter variable">-f</span> <span class="token string">"<span class="token variable">$outfile</span>"</span> <span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token keyword">then</span>
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<span class="token builtin class-name">echo</span> <span class="token string">"Processing <span class="token variable">$1</span>"</span>
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ffmpeg <span class="token parameter variable">-i</span> <span class="token string">"<span class="token variable">$1</span>"</span> <span class="token parameter variable">-v</span> warning <span class="token parameter variable">-ac</span> <span class="token number">1</span> <span class="token parameter variable">-af</span> <span class="token string">"compand=attacks=0.3:decays=0.3:delay=0.15:points=-80/-300|-45/-25|-27/-15|0/-12|20/-12,anlmdn=s=10,highpass=f=500"</span> <span class="token parameter variable">-threads</span> <span class="token number">4</span> <span class="token string">"<span class="token variable">$outfile</span>"</span>
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<span class="token keyword">else</span>
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<span class="token builtin class-name">echo</span> <span class="token string">"Skipping <span class="token variable">$1</span>, already processed."</span>
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<span class="token keyword">fi</span>
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<span class="token builtin class-name">shift</span>
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<span class="token keyword">done</span>
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</code></pre>
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<p>
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If this is useful to you please leave a comment or send an email, I
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would love to hear about it.
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</p>
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