When looking at various music selections on the plot spectrum, the dB level gradually tapers off at the high frequencies. Is there a point at which volume has decreased to a level that is no longer heard? If so, is this what the -60 dB represents? Is this similar to 0dB on an audiogram?
There’s nothing “magic” about -60dB but you probably won’t hear it. ACX requires the background noise on audiobooks to be below -60dB.
You can open a file and run the Amplify effect with a negative amplification, maybe in steps of 10dB, and listen to what you get when you get down around -60. (Of course, don’t touch the volume control during the experiment).
In the analog days we would have killed to get the noise on records can tapes down to -60dB. 
These days it’s not a “great” spec. if you’re talking about (unwanted) noise.
There are lots of complicating factors - There is no standard calibration between SPL levels and digital levels and with enough gain you can turn-up -60dB until you can hear it.
It also depends on other sounds that might be masking (drowning-out) whatever is at -60dB. And it depends on frequency. Our ears are most-sensitive around 2kHz.
Yes, but an audiogram is SPL (acoustic loudness in the air) which is measured in positive dB. And the graph is upside-down compared to what we normally see with audio. The high parts of the graph are where you hearing is weak and the sound has to be turned-up to hear it.
The 0dB SPL reference is approximately the quietest sound that humans (with normal hearing) can hear so SPL levels are positive. SPL measurements are typically A-Weighted to (approximately) account for our ear’s “frequency response”.
Digital levels are dBFS (decibles full-scale) and the 0dBFS reference (with integer formats) is the highest you can “count to” with a given number of bits, so digital dB levels are normally negative.
The numbers in a 24-bit file are bigger than the numbers in an 8-bit file, but the 24-bit file isn’t louder because when it’s played everything is automatically scaled to match the bit-depth of the DAC.
Although there is no calibration, there is a direct correlation. If you adjust the digital level from -10 to -20 dB, the SPL level also drops by 10dB.
P.S.
On an amplitude scale (digital levels or voltage), 60dB is a ratio of 1,000. On a power wattage scale it’s a ratio of 1 million. i.e. If 0dB is 1 Watt, -60dB is one microwatt and +60dB is 1 million watts.
Thank you, Doug. This is helpful!
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