The frequency of maximum attenuation is called the notch frequency.Commonly, both the low-pass and high-pass filter sections are of the “T” configuration, giving the name “Twin-T” to the band-stop combination. Band-stop filters can be made by placing a low-pass filter in parallel with a high-pass filter.Also known as band-elimination, band-reject, or notch filters. A band-stop filter works to screen out frequencies that are within a certain range, giving easy passage only to frequencies outside of that range.The impressive band-stopping ability of this filter is illustrated by the following SPICE analysis: Given these component ratios, the frequency of maximum rejection (the “notch frequency”) can be calculated as follows: Together, this arrangement is commonly known as a “Twin-T” filter, giving sharp response when the component values are chosen in the following ratios: The high-pass filter section is comprised of C 2, C 3, and R 3 in a “T” configuration as well. The low-pass filter section is comprised of R 1, R 2, and C 1 in a “T” configuration. System level block diagram of a band-stop filter.Ĭonstructed using two capacitive filter sections, it looks something like: Not surprisingly, it can be made out of a low-pass and a high-pass filter, just like the band-pass design, except that this time we connect the two filter sections in parallel with each other instead of in series. As always with Audacity, you can Edit > Undo and experiment with different notch filter settings.Also called band-elimination, band-reject, or notch filters, this kind of filter passes all frequencies above and below a particular range set by the component values. You might want to notch the visible spikes at about 100 and 350 Hz as well. Before applying the effect, the hum displayed a prominent spike at this frequency, extending to the top of the graph.ĭepending on the audio, you might find in this case that the spectrum had been notched too deeply, thus affecting the content, and that a higher Q factor would have been preferred. The below image shows a spectrum plot of some music from which a notch of 60 Hz hum has been cut. However a good method for calculating Frequency and Q factor from Plot Spectrum can be found in this Audacity Forum topic. Choosing appropriate Frequency and Q factor settings from the noise spikes in Plot Spectrum is an approximation, due to inevitable interpolation errors from whichever "windowing" function is chosen. Generally, a Q of between 2 and 10 works well for mains hum removal. It is recommended to increase the Q factor when removing higher harmonics so as to lessen possible audio artifacts.Run Notch Filter again at each of the other noise frequencies, then use Effect > Noise Reduction at mild settings to remove any residual harmonics. You may want to increase the spectrum Size and use the "Log" Axis to see more clearly. to identify other broad noise spikes (for example, at 180 Hz and 300 Hz). A good approach is thus to apply Notch Filter to the fundamental hum frequency (for example, 50 Hz or 60 Hz), then use Analyze > Plot Spectrum. Recorded hum often contains noisy harmonic frequencies above the fundamental frequency of the hum. However, there are different notch filter types, and each has a unique circuit design. A low-pass filter attenuates the high frequencies, a high-pass filter blocks the low frequencies, and a summing amplifier combines the results. A table of mains frequencies by country is here. Notch filter designs have three components. Choose 60 Hz as "Frequency" for North America, or 50 Hz for the United Kingdom and most other countries.
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