Equivalent circuit of treble attenuation

When the movable arm of the air column moves to the lowest point B, the treble attenuation is the largest. The principle is as follows: due to the large resistance of RPi
, the treble is prevented from passing through the RPi and cl branches, and the RPl and q branches can be regarded as open circuits. However, since B and C4
can be regarded as short circuits to the treble, no matter what position the movable arm of the jigsaw is in, its equivalent circuit can be drawn in
the form of Figure 1-59 (a). At medium and low frequencies , C2 is regarded as an open circuit, V2/Vi≈R2"Ri+ R2)o. As the frequency increases, the capacitive reactance of C2
decreases, and begins to bypass the signal , and the signal input to the lower stage begins to Attenuated. It can be seen that Figure 1-59 (a) has
a treble attenuation effect, and Figure (b) is its attenuation characteristic curve.