I am not sure I believe that :) Singe tones should show up noise artifacts better. IMO, -66dB should be just fine for the alleged application.
Let's presume you're using it in a busy pubic space. Room tone plus the hum of activity should be more than
50-60dB. If you make the beeps 90dB, then the noise is -20 relative to room tone.That is not enough to be completely masked, but it's close.
Use one of the many ADPCM routines. If you use the 4 bit version, you can pack 2 12 bit samples into one 8 bit memory location, obviously there are other routines that leave you with various bit depth return values, but the one I have used for years is a 12 bit result, so I'm a bit partial to it.
Put three 10 bit samples into a 32 bit word with just 2 bits lost for every three samples.
You could even use the two spare bits as a primitive NICAM as a common range multiplier for the three samples :-)
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