How to use equalizer on ffmpeg

I would strongly encourage you to read the documentation. Even if there is no example, there is a specific description of how filters are defined on the command line.

A filter is represented by a string of the form: filter_name=arguments (…)

arguments is a string which contains the parameters used to initialize the filter instance

• A :-separated list of key=value pairs.
• …

So, the equalizer filter takes these (required) arguments:

• f – central frequency in Hz
• width_type – for defining the bandwidth, can be one of h (Hz), q (Q), o (octave) or s (slope).
• w – the value of the chosen bandwidth
• g – the gain

Now let's put that all together. For example, you can use this command to attenuate 10 dB at 1000 Hz with a bandwidth of 200 Hz:

ffmpeg -i input.wav -af "equalizer=f=1000:width_type=h:width=200:g=-10" output.wav

Or, for equalizing 2 octaves from 440 Hz (i.e., 220–880 Hz), with a gain of 5 dB (beware of clipping!):

ffmpeg -i input.wav -af "equalizer=f=440:width_type=o:width=2:g=5" output.wav

And if you want to combine these two, separate them by a ,:

ffmpeg -i input.wav -af "equalizer=f=440:width_type=o:width=2:g=5,equalizer=f=1000:width_type=h:width=200:g=-10" output.wav