Hi Steve, thanks for that... I'm well aware of the ACX -60 noise floor requirements - this brings up an interesting observation, something I hadn't really noticed before - downsampling seems to bring up the noise floor: Here's a bit of the sample as it was submitted to ACX (192kbps mp3) : https://drive.google.com/file/d/1-UQVZhYXvRMWbczYjrplrxKc5PRtcKLz/view?usp=sharing Measuring the long gap just before the end, the noise floor on this is -59.94 / -60.21 (left and right channels) The "original" sample in my opening post was downsampled to 64kbps (22050khz), which is something I do to "get an idea" how my masters will sound streaming on Audible. The same gap measured on this file comes out at around -55, it's still 16 bit, but presumably the lower sample rate and bitrate are bringing up the noise floor? Then, the same gap in the recorded Audible stream of my file measures -44.3 If there has been no resampling by Audible, then this would mean that some form of compression/signal boost + limiting must have been added at some point. As it happens, I also took a recording from the stream via Audible's mobile app to compare it with their browser-based stream. Interestingly, the noise floor of the same "gap" on that comes out around -50 I have pet theory that Audible are adding more signal boost to their browser-based app because they imagine the user to be listening through computer speakers, whereas a mobile listener is more likely using headphones! Probably not true though - I think actually it's a different process on the mobile app, rather than streaming it actually starts downloading the entire chapter to the device when you tap play, then after a little while says "ready to play", while it continues downloading in the background... this is likely to be an aax format download rather than .aa stream. Rather annoying I imagine for anyone who's concerned about storage space on their device. Anyway, the upshot of all this is (referring to my 192kbps original submission): I am within the correct specificaton as regards noise floor. I could certainly reduce the noise floor to around -70 or so and see if that helps. The trouble is, I won't have a way of checking the results of that until my next production has been published. A very long-winded process of trial and error... At the moment, I'm trying to track down what codecs and/or processes Audible use for their streaming platforms by emailing various people at Audible/ACX. If I knew this for sure, I could possibly adjust my mastering process accordingly. However I'm not holding my breath as I wouldn't be surprised if they regard this as "sensitive" information... plus I doubt if I'll ever get them to admit that they have a heirarchy of streaming quality for different titles!
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