Dynamic range describes the difference between soft and loud. What’s not defined is how soft or how loud—only the magnitude between the two—which is a problem if the softest sounds fall below the threshold of audibility. When that happens the range of dynamics is truncated, a fact not apparent when quoting dynamic range figures.
I’ll give you an example. The threshold for hearing is defined as the minimum amplitude the average person can detect sound. This level happens to be around 15dB for a middle-aged male. If we want to be able to listen to the full range of a CD, which is 96dB, we have to adjust the system’s loudest peaks to be 111dB (15dB + 96dB). That’s pretty loud and beyond the capability of most speakers to comfortably hit those levels. Which means we aren’t going to get the full dynamic range possible out of a CD.
But, wait a minute. A CD is 16 bits limited in dynamics (96dB). Higher bit depth, like 24 bits, can theoretically go as high as 144dB, though noise and other factors set the practical limit at about 123dB. How then can this greater dynamic range from greater bit depth matter, if we’re already losing dynamic range to the threshold of audibility?
It can’t, at least not by much.
And vinyl? Heck, we’re lucky to get 70dB from well-pressed vinyl, forcing the mastering engineer to compress the presentation into that smaller space. That said, vinyl’s the only medium that actually scales pretty easily. You get it all at most volume levels, though the price is the compression.
So, the bottom line in this ramble is simple. Even though a CD is limited to 96dB of dynamic range, by making sure to turn the level up so the softest passages can just pass the threshold of audibility, we can get close to the dynamics of live music. Deeper bit depth makes it easier, but not by a lot.
Paul McGowan
