Software quality is the degree of belief that the code will actually execute its intended functions, in the intended operating environment, for the intended life-cycle, without significant error or failure.
Notice that quality is subjective. It is a degree of belief, a Bayesian probability. "A measure of a state of knowledge." This is important for IVV since it means there is a big difference between "software quality" and "consensus software quality."
But that's another topic. The topic here is the proper measure of software quality improvement. (Engineers like quantitative measures and making things better.) So here I would like to note that improving software quality requires conditioning a Bayesian prior. But this, in turn, requires new evidence. The more new evidence presented and the stronger the evidence, the more software quality improves.
This means that the sole test of software quality is, not surprisingly, testing. Testing provides an objective foundation for quality that keeps quality's subjectivity from (hopefully) getting too far from reality.
But that's another topic too. Here I am noting that, IMHO, the strength, power, or intensity of new evidence is best measured in decibels. Bayesian probability ratios are often expressed in decibels. For example, see How Loud is the Evidence?
The decibel is a log scale that simplifies overall power gain/loss calculations and is convenient for ratios of numbers that differ by orders of magnitude. Additionally, log scales have been useful in describing the potential of certain physical phenomenon (e.g., earthquakes) and human perceptions (e.g., hearing). Thus, log scales can be useful when performing risk assessments and other related quality assurance activities.
For more information on evidence measured in decibels, see Chapter 4 of Jayne's Probability Theory: The Logic of Science.
Finally, an analogy. If evidence of software quality is measured in decibels, it suggests software quality assurance can be thought of as singing the quality song about the software. Consensus software quality then is where we all sing the same song, or at least sing our part in a symphony. :-)
(One of my complaints about the state of quality of say, climate science, is that every participant feels she/he must carry the melody.)