In the digital world db(SPL) would normally be a negative value with 0dB being the point at which distortion would typically occur after it has been put through a DAC (Digital to Analog Converter) and then amplified for output on a loudspeaker system.
It's cool, but there's a lot of little errors like this. Honestly reminds me of my first semester notes (good times). Plus it concerns me to see so many blocks of text explaining what decibels are without any maths (just because it's the simplest way to describe how and what a decibel measures).
Regardless, it does put things in simpler terms. My advice would be to go back to other texts that might have stumped you in the past after you've read this.