The process of filtering instruments out of music is difficult and requires a lot of patience and an attentive ear. While it is impossible to completely remove an instrument from a piece of recorded music unless you have access to studio masters, you can use equalization, spectrum analysis and filtering to remove most of an instrument's presence in a song. Familiarize yourself with frequencies and the corresponding notes that move above or below middle C (C3).
Filtering Using Software
Decide which instrument you wish to filter out. Familiarize yourself with the chart in the references, which shows frequency ranges for instruments.
Import the song into an audio-editing suite that either has filtering, spectrum analysis, and equalization plug-ins, or supports Audio Unit or VST plug-ins with these functions.
Assign a spectrum analyzer to the channel with the music.
Play the music and watch the spectrum analyzer as you listen. Watch for peaks with each instrument. Pay attention to the patterns in the spectrum analyzer corresponding to guitar, bass, drums, piano and any other instruments present in the mix.
Assign a multi-band equalization or a notch-filter to the audio channel.
If you are using a multi-band EQ, assign points to the frequency spectrum that correspond to the instrument you want to filter out. Make a deep notch of at least -9dB to -40dB with a width of no more than 1 to 2 frequency bands in either direction.
If you use a notch-filter, assign the cutoff to the approximate frequency of the instrument you wish to filter.
Using the filter's Q (quality factor), narrow or widen the resonators bell curve to filter the instrument out of the mix as much as possible without altering the other instruments you want to keep.
Filtering With Hardware
The same principles above apply to using external hardware like third octave equalizers.
Select the instrument to filter out and its corresponding frequency range.
Using an EQ, adjust individual frequency sliders down in the vicinity of the instrument you want to filter out. Make sure to keep your equalization curves deep and narrow.
Using a filter, adjust the cutoff frequency to the instrument's frequency mid-range. Adjust the Q to narrow or widen the filter curve.
Instruments have their own characteristic frequency spectrums. Many of these frequencies overlap with other instruments. An acoustic guitar, for example, usually creates a sound wave that makes the human ear respond to frequencies between about 80 hertz (Hz) and 1,000Hz. The guitar shares 80Hz through about 400Hz with the acoustic bass. Attempting to filter out a guitar will also filter out a large portion of the bass's frequency response.
It is good to familiarize yourself with what range of frequencies each instrument creates if you wish to filter them out of music. Remember, multi-band equalization and filtering is not completely isolatory, and will affect neighboring frequencies as the gain envelope of the entire frequency spectrum is shifted up or down.