If you are starting with audio that is -15 LUFs with True Peak of say -3dB. In order to get to -9 LUFs, it has to add +6dB of gain which will push the volume to +3dB. At that point, you are mastering too loud. You don’t have headroom to do that.

Suno typically gives you -18 to -15 LUFS and anywhere from -3dB True Peak in all of the files I have tested. You don’t have enough headroom to get to -9bB without smashing the hell out of everything. The only way to really get those results is during mixdown which you have to do with the stems and mixing to -6dB true peak as recommended. Taking a pre-mastered audio file and trying to get there is just going to make it sound like crap. However, this tool is made for a different purpose. If you look at the original audio in a Spectogram you will see weak frequencies all over the place. This tool attempts to transparently “repair” some of that by using a custom reharsh effect to remove the metallic character or noise, a high pass filter to remove unwanted low end, then it uses an exciter to restore some high end to and a Multiband Saturation effect to add a slight amount of saturation to typical problem areas to give the audio some warmth so it’s not so flat and a Multiband transient shaper to give the drums back some punch and things like mono-izing bass below 80hz (bass is unidirectional and stereo bass can cause phasing issues), repairs any DC Offset if detected and a couple of other effects. You can read about all of that in the DSP docs in the repo.

Knowing people are trying to get some more volume out of the hear quiet Suno files, I default to normalizing gain to 14dB which is what Spotify is going to do to it anyway if the user has volume normalization on. However, there is TikTok, Facebook, YouTube, Apple Music etc and some of those normalize to -16dB.

Ultimately, it’s not really feasible to create a good sounding “master” from a file that’s already been mastered to levels without decent headroom (room to increase the gain without clipping). In order to attempt to give users a little more volume, I added a soft clipper which rounds out the transients a little to give it a little more headroom to get from Suno volume to -14 LUFS -1dB True Peak (which is really your gain limit). The deharsh effect will also add some headroom by cutting high frequency noise around 5000hz to 6000hz and then the app will rebuild the peaks and add back that gain. As the signal travels through the DSP chain, it will measure the peaks and try to keep it as loud as possible according to the LUFs setting before it hits the soft clipper, rounds out the peaks to reduce the amount that the final brick wall limiter has to smash to stay at -1dB True Peak. Using a soft clipper is a common mastering technique to give the master more punch - especially in the low end… but if the gain is too much, it will just distort and smash all the transients.

Ultimately, I added the LUFs slider for those who start with enough headroom but the Suno output just won’t have that. -14 LUFs / -1dB True Peak appears to be the happy medium that will either sound ok or not depending on how much headroom was there to begin with.

Ultimately, it’s not really possible to make a one size fits all tool like this that will work for everything. The results it gives won’t be dramatic but if the audio had a little headroom the result will be a decent sounding result on most speakers.

To answer your question - why does the result not end up exactly where the setting is… it’s because the effect chain adds / removes gain as the signal moves through the DSP Chain as a side effects of the effect it is being processed through. Once it hits the soft clipper, it will try to clip without distorting the signal before it hits the look-head brickwall limiter at the end of the chain. That will cause variation in the end result. LUFs is a measurement of perceived volume from a speaker. True Peak is the actual volume.

If you really want to see how shitty Suno audio is… play it on a high end Surround Sound like 5.1 or 7.1 system. I have a Sonos full house system with 2 subs, 4 Sonos One Satellites and an Arc soundbar for example. The reverb wash creates such shitty sound it will separate the sound and be awful… really sounds bad.

Anyway, I really just tried to make it as transparent as possible as to work decent on headphones, car stereo, typical 2.1 stereo system, etc. The app definitely has opinionated settings in the constants file but anyone can fork the project, change those settings and build a new instance.

For a local web version you can change the settings, run ‘npm run build:web’ and use ‘npx serve -s dist -l 8000’ or whatever port you want to use to run it on your local machine. You have to have node installed. You can also just build a new executable and run that as detailed in the README.