Psycho-acoustics is the threshold of the people who make money from all the folklore that goes around technology circles. It's the output of the hardware that makes your sound, your ears and where that sound is reflecting (or not as the case may be).
 
Anyway.....
 
Most sound we are likely to hear is relative to 20-bit (120dB SPL).

SonicExplorer
Hmmm...  I did a lot of reading on the whole 32 vs. 64 bit mix engine last night and still came away not knowing what to believe.  On paper it seems not realistic to hear a difference in most scenarios, but in practice some people claim to hear it.

 
In some practice people claim to hear the differences in cables, even though we can prove in blind testing they can't (and prove they are the same on a 'scope). People claim all kinds of stuff they can't substantiate. They want to hear a difference, so they do. 
 

If enough things are rounded, eventually the decimals can add to pennies which can add to dollars, it all depends on how you group and round, and how many times that transpires.  Being an old school analog relic I just don't know enough about the inner workings of this digital stuff.

 
That is the reason for having something like a 64-bit engine: Insurance against rounding. With 32-bit, rounding errors will occur. 32-bit FP is actuall 8-bits exponent, 24-bits significand. That means it has 24-bits of precision over a range of 8-bits, more or less. So when working at normal audio levels, assuming 24-bit input and output, you do get minor rounding errors. However they are going to be on the order or 1, maybe 2-bit at most in all likelihood. So you don't worry about it. Even the best DACs don't resolve more than maybe 21-22bits of signal and other kinds of noise will be WAY higher.
 
However in theory, enough operations, particularly ones that involve things like multiplication or division in sequence, could increase the rounding error. It doesn't really happen in practice, but it could in theory. Well we can easily prevent that just by going double precision. Now we have an 11-bit fraction and 53-bit of significand. That means we have so much precision we will never have any rounding error ever reach the 24-bit output signal, even in edge cases. Since modern processors kill at double precision performance, no real reason not to use it.
 
If you want to see a case where it does matter, look at graphics. Most monitors and video formats are 8-bits per channel output. However you need to do processing in 32-bits per channel (floating point) if you want truly smooth colour gradients. Do it in 16-bit half precision and you can see errors sometimes.
 

And not to derail my own thread, but I also read a comment last night that said XP sounds better than W2K.  Which really left me scratching my head as I was unaware one OS version from another by itself could change the quality of sound.  Huh??

 
For sound going through the OS layer? Sure. Remember that unless you use something like ASIO to talk straight to the soundcard, the OS is modifying the sound. It runs it through a mixer and resampling engine to allow for more than one program to play sound at once. Sometimes does other stuff too. How good or bad it does that can vary. In general newer OSes are better than older ones since there is more CPU power these days so they use better algorithms.

Sycraft

SonicExplorer
 
And not to derail my own thread, but I also read a comment last night that said XP sounds better than W2K.  Which really left me scratching my head as I was unaware one OS version from another by itself could change the quality of sound.  Huh??

 
For sound going through the OS layer? Sure. Remember that unless you use something like ASIO to talk straight to the soundcard, the OS is modifying the sound. It runs it through a mixer and resampling engine to allow for more than one program to play sound at once. Sometimes does other stuff too. How good or bad it does that can vary. In general newer OSes are better than older ones since there is more CPU power these days so they use better algorithms.

Thanks to everybody so far for the responses.
 
I'm using WDM drivers that I thought, along with RME FF and Sonar, effectively bypassed the OS layers to go straight to hardware.  If that's not the case then maybe this is why I seem to have some harshness/clarity issues going on (for lack of better description).  Hmmm...

Nothing really goes "straight to the hardware", but it doesn't matter. Extra layers add to overall latency, but shouldn't affect harshness or clarity. Wish I could tell you what your problem really is, but all I can say is that whatever issues you're experiencing they can't be ascribed to the drivers or the O/S. 

Ok, thanks, OS not likely impacting sound quality.
 
I could swear however I recall hearing a subtle but noticeable and important (to me) difference between 32 and 64 bit mix engine.  I also now vaguely recall making two mixes and comparing them.  Will check around and see if I can find those files....will post back if I discover anything definitive....
 
Sonic

The actual sound changed in Windows 2000 to XP which was it became WASAPI which means it operates at a different application level, giving you direct access to the hardware before it passes information through the application layer. Something Technet-ish and geeky anyway.
 

SonicExplorer
Ok, thanks, OS not likely impacting sound quality.
 
I could swear however I recall hearing a subtle but noticeable and important (to me) difference between 32 and 64 bit mix engine.  I also now vaguely recall making two mixes and comparing them.  Will check around and see if I can find those files....will post back if I discover anything definitive....
 
Sonic

Be aware that if you do a test, what you are likely to find is that exporting the exact same mix repeatedly using the same engine will have differences due to the randomness inherent in many synths and plugins. These differences will often be orders of magnitude greater than the difference between the 32 bit and 64 bit engines, making it impossible to know the cause of a difference one is hearing - assuming one actually hears a difference under double blind conditions. 

@drewfx1, wow, thanks. So I will avoid any A/B testing then as I don't need to be chasing my tail with something that won't be a fair/stable test.  
 
@interpolated, about the W2K/XP thing, this is rather important as I plan to soon build one last backup system and am on the fence about which to choose for OS.   Just because something bypasses the app layer doesn't mean the app layer did something to alter the sound, other than possibly more latency.  I'd need to understand more about WASAPI and any other thing that changed to learn if XP tends to produce superior audio quality over W2K.  I'll try searching around on the forum and see if I can learn anything about it.....
 
BTW, will Sonar 5 and 6 be able to utilize both CPU cores on XP (Core 2 Duo) ?  

bitflipper
 
Mathematically, it's actually quite similar to how your property taxes and bank interests are calculated: when real dollar amounts are multiplied by some rate multiplier, the results are stored in imaginary currency (millicents, or thousandths of a cent). These are intermediate values that will never appear on a tax bill or bank statement. That extra precision reduces the overall rounding error, so that when those numbers are ultimately expressed in real currency the cumulative error will be less than a penny.
 

Bit's financial analogy is excellent for what is happening inside the box (ITB) and where calculations may (or may not) benefit from 64-bit over 32-bit. Depending on a given VST used, 32-bit is often sufficient for most real-world applications since the excessive detail of 64-bit is not audible (but adds CPU overhead).
 

Bristol_Jonesey
 
When playing back what you hear is dictated by your interface, the majority of which operate at 24 bit and in the real world, this can mean the effective bit depth is around 20

Coupled with that is also Jonesey's post made above. When the ITB calculations exit through your audio interface (AI) so you can hear it, it must be passed the format that the AI can work with (often 24-bit). Basically, you are not "hearing 32 vs 64 at the interface," but the manifestation of the calculation chain, which is always passed to the AI in what it is expecting to see (under the hood SONAR function).
 
You can get into a lot of discussion on different aspects of the above, but to answer your initial question, it is highly unlikely to hear a difference with the 64-bit double precision engine (DPE) over 32-bit. I have actually had my 64-bit DPE disabled for some time as it has caused issues over time with different builds of SONAR (ones after S5/S6).

Okay, I've spent quite a few hours searching and reading about this W2K/XP/WASAPI/OS sound situation.  Seems to me, when it comes to quality sound cards, their affiliated drivers and reputable DAW software (such as Sonar) there is generally no sound loss in practice due to OS.  Any notion of a kmixer like situation is typically in those cases where the receiving entities accept a common conduit (and therefore potentially combined or altered) signal path.  Otherwise the data mostly passes through (or around if IOCTL or other mechanism if supported) and goes "direct" to sound card.  I say this with understanding there is an I/O OS stack involved.  Slight latency would/should be the only typical impacts.  Yes, in theory there could be buffering etc, but nothing that should alter the actual sound quality per-se.
 
Anybody disagree with this general conclusion?   And BTW, along these lines I found nothing that provided evidence or explanation of XP being superior in sound over W2K (in context of quality cards/drivers/DAW software such as we are discussing - RME FF, WDM Stream and Sonar more specifically)
 
Sonic