Demystifying the iTunes Sound Enhancer
The sound enhancer, cranked up all the way does the following:
+6dB from about 100Hz to 150Hz
-2dB from 250Hz to 475Hz
+3dB at 7KHz
+4dB at 7.5KHz
Also, frequencies presented to iTunes which are completely out-of-phase may be boosted by as much as 7dB. This has the effect of lowering the center image, and boosting the ‘difference’ signal, or in other words, “widening the stereo”.
When using the equalizer, things function pretty much how you expect. The top-most band, as well as the lower-most band operate in the same manner as the rest of the EQ bands - peaking mode. So in other words, if you cut every band to -12dB, frequencies lower than 32Hz and frequencies above 16KHz will not be as affected (the cutting has some bandwidth, or Q, so there is a specific region to either side of every band). There is signifigant overlap between the 2KHz, 4KHz, and 8KHz bands, which will cause frequencies lying inbetween to be boosted or cut more than you’re thinking they might be.
Furthermore, the EQ is designed to prevent user-caused distortion. Example: If you have boosted, either a frequency, or the ‘preamp’ control by 4dB, and a peak in the song is reached (which, on an analog EQ would result in a signal level of 4dB over 0), the iTunes EQ will reduce the output level by 4dB. Partial gain-makeup can occur in about half a second, if the program level dives low enough, however, complete gain makeup will not occur until you re-select and play the song. Sometimes, it takes more than one try at this.
iTunes will compress as much as 8dB if necessary. Boosting more than 8dB will result in signal level too great for iTunes to compress down and not be noticed, so the signal will be reduced by 8dB and then allowed to distort. The compression is stereo-linked mono-band.
The volume slider in the main iTunes window is post all these effects, so whatever distortion the EQ may have created will simply have lower level, along with the undistorted content, if the level is lowered by means of the main iTunes volume control.
“proper” use of the EQ is to lower the preamp by the amount of the highest boost, thus preventing iTunes from compressing, or even worse, distorting. Material that is quiet enough, and does not reach 0dB fs won’t require lowering the preamp control as much.
Hope this was helpful.
