Main Page | See live article | Alphabetical index A notch filter (also called a narrow band-pass filter or T-notch filter) is a electronic filter typically used when the high frequency and the low frequency are less than 1 to 2 decades apart (that is, the high frequency is less than 10 to 20 times the low frequency). Example 1: High Freq: 2000 Hz Low Freq: 120 Hz Example 2: High Freq: 500 Hz Low Freq: 30 Hz See also: FM radio. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.