Table 3.3 gives values of a's and b's for a 5th-order low-pass Butterworth filter at a selection of frequencies relevant to speech applications. Refer to the first column, labelled f₈₀₀₀, if the sampling rate of the original signal is 8000 samples/s, and find the desired low-pass cut-off frequency. Reading along the row will give the relevant b's and a's. If the sampling rate is 16000 samples/s, use the second column. For other sampling rates  f_s, divide the desired cut-off frequency f by the sampling rate f_s, and use the third column to find the right row.

For example, to filter out frequencies above 3.3kHz if the sampling rate is 11025 Hz, f/f_s = 0.299. This is very close to 0.3, for which the coefficients are 0.0069, 0.0347, 0.0693 ...

Example (2): to get rid of frequencies below 55 Hz in a signal sampled at 8000 samples/s, refer to the second row of table 3.4.