In high fidelity (hi-fi) audio system, the harmonic distortion level is kept as low as possible. To make sure that the amplifier handle the sigal perfectly, the response of many hi-fi audio systems is designed beyond the audio range, include the handling capability to process infrasonic (below 20Hz) and ultrasonic (higher than 20kHz) signals. Although this capability is useful to make sure that the audio  signal (20Hz-20kHz) signal is properly processed, the existence of unwanted high frequency ultrasonic and very low frequency infrasonic signal should be avoided. In the high level, this unwanted signal might consumes electric power without being noticed, burn your speaker (usually the tweeter), or just simply distort the audible signal. To overcome this problem, we use rejection filter circuit for ultrasonic and infrasonic signal. Here is the schematic diagram:

ultrasonic infrasonic rejection filter circuit schematic

The first circuit is the infrasonic rejection filter. The filter circuit is a low-pass filter with third order Butterworth response. The ultrasonic rejection filter is a high pass filter, and the response is designed to match the fourth order Bessel filter. To reject both ultrasonic and infrasonic unwanted signals, the two filter circuits shoud be cascaded, conected in series to give the full frequency response as depicted in the last figure. The full frequency response should be a wide-audio-band-pass response. For accurate response, you should use 1% tolerance for all resistors. [Circuit’s schematic diagram source: National Semiconductor Application Notes]