Lowpass filter

Lowpass filter is used to cut off the higher frequencies, and "pass" the lower frequencies. In Lossy audio it is used to lower the bitrate demand or to spare bits for the lower more important frequencies. Young people can hear a loud sine wave up to 21-22kHz. The same people can't normally distinguish any difference between 19kHz lowpassed music and non-lowpassed music.

Figure 1: FFT analysis (Frequency-Amplitude domain)

The figure on the left side does not have a any lowpass used, while the figure on the right is signal filtered with 15kHz lowpass filter.

Figure 2: Spectral view (time-frequency domain)

The figure on the left side does not have a any lowpass used, while the figure on the right is signal filtered with 15kHz lowpass filter.

A good tool for applying lowpass on audio streams is SoX