This makes for exacting gain in each frequency. You can have a program for quiet environment, noisy, music listening, almost whatever you desire. Many of these aids have advanced circuitry which allows for amplifying quiet sounds to where the user hears, and to keep loud sounds from becoming uncomfortable. Background noise is also reduced with this type of circuit. Speech sounds can be distinguished from environmental sounds better.
Cost varies on type, maker, overall gain.
But they are costly. Multichannel aids are those which have the frequencies divided into two or more channels. These aids also feature the above mentioned circuitry. The advantage of these is you can more adequately fit a hearing loss which varies greatly from frequency to frequency, and for people with reverse slope losses. Those who hear high frequency sounds better than the low.