A notch filter (that's what "notchf" stands for) is a band reject filter 
with an extremely small bandwidth.  It can remove noise at a known single 
frequency (e. g. mains hum at 50 or 60 Hz) from a sound recording.

"notchf" can apply several filters in one pass (useful for oscillations 
with upper harmonics, e. g. at 50, 100, 150 Hz etc.) and provides control 
over the sharpness of the notches (to make hitting the right frequencies 
a little easier).  Try this on the included example IFF sound:

  notchf -f0.015 -f0.030 -f0.045 sspa.iff test.iff

New since V1.0: Better information on the bandwidth of the filters, in 
the docs *and* at run-time.

"notchf" is Freeware, C source is included.

 PERMSSN    UID  GID    PACKED    SIZE  RATIO     CRC       STAMP          NAME
---------- ----------- ------- ------- ------ ---------- ------------ -------------
[generic]                13880   28580  48.6% -lh5- 3266 Aug 17  1996 notchf/notchf
[generic]                 3990   10956  36.4% -lh5- f41d Aug 17  1996 notchf/notchf.c
[generic]                 2916    6619  44.1% -lh5- 2aa9 Aug 17  1996 notchf/notchf.doc
[generic]                35950   48176  74.6% -lh5- 9eec Aug 17  1996 notchf/sspa.iff
[generic]                  265     628  42.2% -lh5- 4df6 Aug 17  1996 notchf.info
---------- ----------- ------- ------- ------ ---------- ------------ -------------
 Total         5 files   57001   94959  60.0%            Aug 18  1996