I don't think one can hear the difference between 0.1% and 0.02% pre amp output. Human ears/brains and most speakers in room with noise floor at 20 dB or higher will not allow discernment to be that good, especially when watching movies. We all want great measurements for other reasons. Lots of people own 200, 400, 1 kW amps when they only need 50 w, almost the same idea.
0 dB SPL is defined as the threshold of human hearing in the area that we are most sensitive.
It is not difficult to observe that bass sounds travel better room than the high-frequencies.
This is true of ambient noise. To understand the impact of the noise floor you must analyze the spectrum.
There are rooms with 40 dB ambient noise but 10 dB at 2 to 6 kHz where it matters most.
Anything above 0 SPL in the 2 to 6 kHz regions is possibly audible but not likely.
Distortions are also going to have the same issue. They may not be masked and within the most sensitive frequencies.
I would stay below 20 dB distortion is a max for critical users. That is SINAD of 70 dB or 0.03 THD+N from .1 to 1 watt as the max.
It is important to recognize that the distortion measurements are at 1 kHz so not indicative of the entire frequency range and intermodulation distortion is not present.
Next, you have to examine the speakers sensitivity. The higher the sensitivity the great the chance for audibility since many amps distortion numbers are at or near maximum output.
So that's just noise and distortion, next there needs to be an assessment of linearity into load.
We know a great deal about amplifier performance at 1khz signals into 8 and 4 Ohms.
Sometimes, 20 to 20kHz but even then bandwidth of the measurement can matter because some systems are sensitive to distortions above 20 kHz and can modulate that back into the audible range.
Basically, the numbers thrown around are rules of thumb only, but that does not necessarily make them invalid.
- Rich
