We rarely hear music the way the sound engineers intended, whether the problem is caused by external (bad speakers) or internal (bad ears) factors. An audio equalizer (EQ) can help overcome these problems. EQs work by raising or lowering the level of specific frequencies in the sound mix, making the bass line more boomy or the cymbals more rattly.
But since each of us hears a little differently, the one-size-fits-all preset EQ settings in Apple's iOS Music app may not help much. While the "Rock" preset makes the bass stronger, it doesn't give you the option to customize the sound to your ears.
Onkyo's free HF Player app for iOS brings tremendous flexibility to the way your hear music on your iOS device. The app divides sound into 16,384 discrete bands from 32 hz to 32 kHz. As you play a song, it shows a visualization of the frequencies in the music, along with a line to represent "flat" equalization (meaning the levels are unchanged from the original recording). To adjust the tones, just use the touchscreen: Push the line up in a frequency range to boost the level, or drag it down to lower it.
If you think Sting's vocals sound buried on the Police's "Invisible Sun," then just up the level of midrange frequencies (that's where most vocals reside). If you're sensitive to Stewart Copeland's cymbal crashes on the same song, just drag down the treble tones at the high end of the range. Any changes you make are quickly processed so you can immediately hear the difference.
Once you've found a configuration you like, you can save it as a preset to reuse later. As a head start for your own adjustments, opt for an EQ setting created by musicians like Scott Ian from Anthrax or Leo Nocentelli of the Meters.
The HF Player can handle any file you already have on your iOS device, and if you pony up $10 for the HD Pack in-app upgrade, you can also play files that iOS can't otherwise handle, including FLAC, DSD, and WAV. The HD Pack also means you can play high resolution 96 kHz or 192 kHz files – though you'll need a special external digital-to-audio converter if you want to hear all that resolution; the iPhone and iPad can't output at that quality. [Free; itunes.com]