Rockbox features a parametric equalizer. As the name suggests, a parametric equalizer lets you control several different parameters for each band of the equalizer. Rockbox’s parametric EQ is composed of five different EQ bands: 

Band 0: Low shelf filter  A low shelf filter boosts or lowers all frequencies below the designated cutoff point. The “bass”control on most home or car stereos is an example of a low shelf filter. The low shelf filter in Rockbox is more flexible than a simple “bass” control, because a simple bass control only lets you adjust the amount of gain that is applied. Rockbox lets you control the amount of gain that is applied (i.e., the amount that the bass is boosted or cut) too, but Rockbox also allows you to adjust the “cutoff” frequency where the shelving starts to take effect. For example, a cutoff frequency of 50 Hz will adjust only very low frequencies. A cutoff frequency of 200 Hz, on the other hand, will adjust a much wider range of bass frequencies.

Bands 1-3: Peaking filters  Peaking EQ filters boost or low a center frequency that you select, as well as the frequencies within a certain distance of that center. Graphic equalizers in home stereos are usually peaking filters. The peaking EQs on Rockbox’s parametric equalizer let you adjust three different parameters for each EQ band 1 through 3. The “center” parameter controls the center frequency that is adjusted by that EQ band. The “gain” parameter controls how much each band is adjusted. Positive numbers make the EQ band louder, while negative numbers make that EQ band quieter. Finally, the “Q” parameter controls how wide or narrow each EQ band is. Higher Q values will affect a narrow band of frequencies, while lower EQ values will affect a wider band of frequencies. 
 
Band 4: Hi shelf filter  A high shelf filter boosts or lowers all frequencies above a designated cutoff point. The “treble” control on most home or car stereos is an example of a high shelf filter. The high shelf filter is adjusted the same way as the low shelf filter, except that it works on the high end of the frequency spectrum rather than the low end. 


gain for each eq band can be set between +/-24dB, in 0.5dB increments.
frequencies of each eq band can be set between 20Hz and 20kHz, in 10Hz increments.
bandwith q for each eq band can be set between 0.5 and 6.4, in 0.1 increments.


there are also overall tone controls:

The bass setting can be used to increase (but not decrease) frequencies below 300Hz. Bass boost can be set from 0 to 24 dB in increments of 2 dB. A setting of 0 means that low frequencies are unaltered (flat response). 

The Treble setting can be used to increase (but not decrease) frequencies above 1.5kHz. Treble boost can be set from 0 to 6 dB in increments of 2 dB. A setting of 0 means that high frequencies are unaltered (flat response).