In one of those threads it was explained that if the audio is averaging around say -10dB, you would have 10dB of "headroom". Simply, that is it! You CREATE headroom on your system. It doesn't make much sense to apply the term headroom and sound to noise ratio the same thing, because while in a sense, you COULD say that your sound to noise ratio, and more importantly, your DYNAMIC range is headroom, you are overly complicating what headroom is referring to. Dynamic Range vs. Sound To Noise Ratio is that more complicated thing. I will attempt to illustrate.
Music usually WANTS to room to get louder. Since ALL recording mediums have a maximum voltage that can be stored (yes, even analog....don't confuse the highest voltage value that tape can handle with tape saturation and PERCIEVED volume) you have a "loudest" volume that can be achieved. So, in a way, we have a fixed ceiling to loudness in electronics. If your audio was at full volume 100% of the time, no dynamics would be possible. That would suck. It is not ear friendly at all, even though it is possible to create clean sounding audio with no dynamics (or barely any), it is just tiring to listen to. So, we NEED dynamics. How much dynamics does you song need? Here is the question, and that has do with "headroom". If you need your music to be able to get 10dB louder in certain parts, then you need 10dB of headroom possible. Period! There is nothing else to it. I have looked at audio recordings that don't even allow that much headroom. On certain types of music, it is not terribly offensive to only have 4-6dB of headroom (although, I only say terribly offensive based upon current average volume standards of today....I like having more headroom...I don't like making tonal sacrifices that are neccesary to have such high average volumes...).
Can music still SOUND like it retains dynamics with very little headroom? Sure can. An understanding of the Fletcher/Munson Relative Loudness Curves and multi-band compression will help you more or less do that if you like that kind of sound. 80% of the voltage in your average volume is comprised of frequencies 200Hz and below. It is possible to just compress the hell out of those frequencies so that they STAY at about 80% of your voltage at all times, and allow midrange frequencies to grow. You could make this ratio even higher, like 90% and let the midrange make up the rest if you want REALLY LOUD.
Anyway, think of headroom as the amount of voltage you set aside for the audio to get louder. That is ALL it means. If you don't require much headroom, then you don't. If you do, you need to CREATE it. You have to keep average volumes LOWER to create more headroom. Get it?
Experience will tell just how much headroom you actually need. Like I said earlier, some genre's of music just don't require that much headroom. Others, like orchestral recordings require more of it.
Sound to noise ratio vs. dynamic range are two different things. Sound to noise ratio has to do with the volume that electronic noise is at. But, audio can heard that is actually at a lower voltage than the equipments self noise is at (dithering uses this concept!
). So, DYNAMIC RANGE in gear has MORE of a bearing on minimum to maximum voltages that can be stored than sound to noise ratios do. But, a really crappy sound to noise ratio DOES have a bearing too, just not as much. Think of the s/n ratio as the level at which things start deteriorated really fast. The more distance between the lowest volume you really need recorded and the self noise level is of concerning. It is possible to overcome s/n ratio problems with compression! (this is what noise reduction like dolby do).
So, we know that our ears are sensitive to the 2-5KHz region. Audio that is possibly up to 20dB LOWER than the equipments self noise level would probably be heard! Cool. But, other frequencies that low might not be heard because the broadband noise the electronics make would "mask" it. Too bad...
So, s/n ratio's are considered "nominal" when listed (usually they use 1kHz as the tone used for the displayed spec for s/n ratio. Other frequencies it could be worse, some better.
But, dynamic range is usually FIXED on any device. The unit can only work with a range of voltages. Good circuit designs allow more range, poor designs usually allow for less range.
So, s/n noise ratio/dynamic range aren't really what are being talked about when "headroom" is being talked about. Headroom is just how much voltage ABOVE average levels is set aside for the audio to "grow". You CREATE it and can have as little as NONE, or as much as the total dynamic range possible in the signal chain.
Man, I haven't talked about this kind of shit directly for a long time. Thanks for the opportunity to write it out. I am not sure yet if I have explained it very well. If you don't understand something, or need a clarification, please ask as this helps me sharpen how I describe stuff.
Eddie