Blackness - how quiet does it need to be?

Sure, but I was referring exclusively to amps of considerable quality where differences are relatively small.

The OP's question, regarding how noise that is not audible beyond a very short distance from the speaker when no music is playing might have audible significance, is an excellent one, that I've pondered myself at times.

I think that Kijanki's answers are on the mark.

In the digital domain, the explanation is easy, namely jitter effects, as he indicated.

In the analog domain, it is not that clear, but the one explanation that occurs to me relates to intermodulation effects, as he also indicated. The ear is much more sensitive to some frequencies than to others, as can be seen in the figure in this Wikipedia writeup on the Fletcher-Munson Effect. Non-linearities in the speakers, and perhaps also in the electronic components that are in the analog signal path, will result to some degree in intermodulation effects, producing (at very low but conceivably significant levels) new frequencies corresponding to the sum and difference between all of the frequency components that are present. Hiss typically contains a mix of essentially all frequencies within some broad range, especially in the upper treble region (and beyond, at ultrasonic frequencies that are inaudible in themselves). Perhaps intermodulation of some of the frequency components of the music (and perhaps also frequency components of recorded noise, tape hiss, and/or LP surface noise) with those upper treble and ultrasonic system noise components results in difference frequencies in the lower treble or mid-range regions, where the ear is more sensitive.

That's my speculation, anyway, elaborating on what IMO were excellent answers by Kijanki.

Regards,
-- Al

I'm trying to understand the concept of the CDP contributing to noise while playing vs. when not. How would one hear this noise while the CDP is playing? In my case it's a transport to DAC. What I do know, is that on high quality hi'er res material like 96k/24 bit, on quiet passages, the hiss level hardly goes up, even on more than average listen levels. In other words I can only hear the hiss if Im very near the speakers, but certainly not when sitting. Now this only holds true for well recorded sources. Of couse I have hi res recordings that have hiss in the recording like my Miles Davis, Kind of Blue that I can hear from my couch, but this is not the same thing that Kijanki is speaking of.

I don't quite understand what IMD is, in layman terms. So Im off to look that up.

Last_Lemming I'm hesitating if I should even start since Al can explain it much better and more coherent way. If he does follow his explanation.

In both cases, digital and analog, we're talking about modulation that creates new frequencies.

Two frequencies in presence of nonlinear element (nonlinear transistor, nonlinear motion of speaker's membrane) produce additional signals of frequencies that are sum and the difference of original two frequencies. You can think of it as noise since it wasn't in the music and amplifier or speaker manufactured it. Great amplifiers or speakers modulate very little but we also "learned to listen" and can detect smallest differences in clarity.

Time jitter of your transport digital signal when playing single frequency also creates two additional frequencies (sidebands), but this time they are spaced on both sides of original/root frequency by modulation frequency distance (frequency of time jitter). Al mentioned once, that this modulation frequency can be completely random at the moment (random noise affecting digital signal timing) and we call it uncorrelated or can be cause by particular frequency like noise from switching power supply - then it is correlated. Usually it is mix of both. When complex signal (music) contains a lot of frequencies there is a lot of byproducts - basically a hash proportional to amplitude of the music. When music stops hash stops. It is detectable only as a lack of clarity and as such affects pretty much everything from tonality to imaging. Jitter usually has very small amplitude and byproducts (sidebands) are very, very small but still audible since they are not harmonically related to root frequency (like in harmonic distortion) and our ears are sensitive to it.

We're dealing wit two aspects of jitter: amplitude (modulation index) and the frequency. Small modulation frequency (like 60Hz) creates sidebands near root frequency that are easily masked therefore is less dangerous but high frequencies of the jitter create byproduct far apart from the root frequency and very audible. Amplitude of the jitter affects amplitude of the byproduct up to the point where even more than two sidebands can be created. Bad CDP have often a few nanoseconds jitter amplitude while good transports perhaps one tenth of it. I shouldn't really attribute it to transport but rather to system (transport, cable, DAC, ambient noise, power noise etc.)

Al, did I miss something?

Thank you Kijanki. I do believe I get the gist in what you are saying. Since there can only be jitter when a musical signal is present, then noise can't be heard as a "hiss" per say, but as distortion on some level in the music reproduction.