shouldnt be... as i said, dithering noise usually tends to be outside of audible *frequency* range... e.g., very high freq. noise and such.
here, read this:
"Improved Dithering Techniques
Where there's a will, there's a way. Although the required amplitude of the dither is about -96 dB, it's possible to shape (equalize) the dither to minimize its audibility. Noise-shaping techniques re-equalize the spectrum of the dither while retaining its average power, moving the noise away from the areas where the ear is most sensitive (circa 3 KHz), and into the high frequency region (10-22 KHz).
Here is a picture of one of the most successful noise-shaping curves (courtesy of Meridian Audio, Ltd).
As you can see, it is a very high-order filter, requiring considerable calculation, with several dips where human hearing is most sensitive. The sonic result is an incredibly silent background, even on a 16-bit CD. The 0 dB line is around -96 dBFS in this diagram.
There are numerous noise-shaping redithering devices on the market. Very high precision (56 to 72 bit) arithmetic is required to calculate these random numbers. One box uses the resources of an entire DSP chip just to calculate dither. The sonic results of these new noise-shaping techniques range from very good to marvelous. The best techniques are virtually inaudible to the ear. With 72-bit arithmetic, all the dither noise has been pushed into the high frequency region, which at -60 or -70 dB is still inaudible. Critical listeners were complaining that the high frequency rise of the early noise-shaping curves changed the tonality of the sound, adding a bit of brightness. But it turns out that it is the shape of the curve in the midband that affects the tonality, due to masking. Two or three of the latest and best of these noise-shaping dithers are tonally neutral, to my ears. It took a long time to get there (about 10 years of development), but now we can say that the best of these processors yield 19-20 bit performance on a 16-bit CD, with virtually no tonal alteration or loss of ambience from the 24-bit source.
Noise-shapers on the market include: db Technologies model 3000 Digital Optimizer, Meridian Model 618, Sony Super Bit Mapping, Waves L1 and L2 Ultramaximizers, Prism, POW-R, and several others. When using dithering plugins, be sure to use them with the right version of workstation software to retain a 24-bit wordlength until the final mastering step.
Apogee Electronics produced the UV-22 system, in response to complaints about the sound of earlier noise-shaping systems, declaring that 16-bit performance is just fine. They do not use the word "dither" (because their noise is periodic, they prefer to call it a "signal"), but it smells like dither to me. Instead of noise-shaping, UV-22 adds a carefully calculated noise at around 22 KHz, without altering the noise in the midband."
(copied from http://www.digido.com/portal/pmodule_id=11/pmdmode=fullscreen/pageadder_page_id=27/)