+6dB gain in Audio Units

First, I think Audacity is great software, I am really grateful for it.

What I noticed though is that, as I think, it does not behave correctly with FX processing as Audio Units get +6 gain on their input without obvious reason.
I attached the pic where I am playing a pink noise file with peak value of -3.5 dB, the level meter in Audacity shows correct level, but Audio Unit gets almost 6 dB more!
Then probably level is reduced back so no actual volume change takes place. But the problem here is that many users may be unaware that they are overloading the FX units/plugins. Not all FX in Audacity have built-in level meters to check and adjust their incoming levels.

Roundtrip AU on the picture is behaving correctly (displaying -3.5 dB) when used in other software.

I wanted to know -maybe this is done intentionally, then what is the reason for doing so?
Screen Shot 2016-12-24 at 12.02.38 AM.jpg

Sorry for large image, will reduce size next time.
Forgot to mention that I have latest stable Audacity 2.1.2 version

It seems that AURoundTripAAC is confused if Audacity’s sample rate (the “Project Rate” shown in the lower left corner of the main Audacity window) does not match the sample rate that it expects for the playback device.

On my Mac Mini, AURoundTripAAC shows the correct level when using the Built-in Output device and Audacity’s sample rate is 44100 Hz. For my USB audio device, Audacity’s Project Rate must be 48000 Hz in order to get the correct reading from AURoundTripAAC, and for HDMI output the project rate must be 22050 Hz.

In short, this effect is not fully compatible with Audacity, but it is not a general problem - when testing the AUPeakLimiter effect, the output is correct regardless of the project rate.

Glad you got to the bottom of that problem. I noticed this with some AU plugins in other DAW’s and considered it normal for AU plugins. Never figured I should check sample rate. After all, how could sample rate change the level?

I also believed all AU plugins behaved like that. It’s not a problem for me, but I can imagine it can be a problem in a chain, if the next plugin doesn’t have an input level control to reduce with 6 dB.

It might be related to (or the same as) this Audacity bug in which Audacity does not report the sample rate correctly to the plugin: http://bugzilla.audacityteam.org/show_bug.cgi?id=1502.


Possible, of course, but then why would REAPER show the same +6 dB behaviour with some AU plugins?

A likewise bug?

You would have to ask REAPER about that. Are Apple AU’s the only ones that do it?


Maybe. I’ve never seen it with VST’s, but there are many VST’s out there…

And I figured it happened with all AU units, based on a too limited test, it seems. :confused:

It’s nothing to worry about, as long as the following plugin on a chain has an input level control, or isn’t dependent on the audio level.


Using Audacity 2.2.2, I see the same behaviour with VST on Windows (maybe the topic should be moved or copied ?)
For example, using HOFA 4U Meter/Fader/MS-Pan, or ReaJS w/psycho_dither shows +6dB at input meters…
Obviously the test track has been normalized to -0.1dB