RMS clarification needed


When ACX state that each file must measure between -23dB and -18dB RMS, do they mean that the Average RMS must be within that range, or that the Min RMS Power must be -23db and above and the Max RMS Power must be -18db and below?

I’m quite new to the idea of RMS, so I apologize if this question seems daft.


“RMS” is an average. It can be a short-term average, or a long term average.
In the case of the ACX specification, they are referring to the average over the entire file, though one would expect that the average over any period of more than a sentence will lie in the same range. The “loudness” should be reasonably constant throughout the book.

Of course that does not mean the book should be read without expressiveness, but the person listening should not have to adjust the playback volume while listening (as that would be really distracting and annoying).

On the file I’m using, which sounds pleasing and constant in volume to the ear, it says the Min RMS Power is -74.35db, and the Max RMS Power is -13.49db. I’m guessing the Min RMS is probably the silence at the beginning and end of the file. But the reading for the Average across the whole file is -23.94db.

Do these figures seem reasonable? Or extreme? Will they be suitable for mastering?

What is “it”?

We publish ACX Check , a plugin which works very much like the testing Robot at ACX. They both check all the “hardware” specifications of your performance sound file: Peak, RMS and Noise and lists them.


Note it only works well on an actual performance. You can get into trouble by trying to measure a non-sound file and you can freak out the noise measurement by not including at least 3/4 second of pure background sound.

This is how to create a simple sound test.


You don’t have to send it to us. You can measure it right there on your computer.

It’s a very painful New User mistake to read a whole book and only then stop to check if it’s OK or not.

For further reading, this is a description of the three testing values.



Also note through all that you still have to be able to read out loud.

Do you routinely have people gather around you when you tell a story? Then you are a prime candidate for audiobook reading. If, however people run from you holding their ears…this process may be extended.

We can’t turn you into an announcer with application of software. That’s a frequent request.


We also publish ACX Mastering. Three simple tools that guarantee Peak and RMS and if you were reading in a quiet room, Noise.



Here’s a thing I read in a quiet room and mastered using our tools. I think I used a little noise reduction, too. It passes ACX.

This is what the sound file looks like.


Note the blue wave top and bottom sharp tips are more or less even over time and don’t look like they were flattened and whacked off with a sharp axe. That’s one of the reasons I sound more or less normal and not dense, compressed, harsh or gritty. That’s why the tools for ACX Mastering were chosen. The minimum processing needed to pass audiobook requirements and no more.