The circuits pictured above are examples of high-pass and low-pass filters with adjustable cutoffs. Due to the reactance of a capacitor (Xc = 1/2pifC), when the frequency in a circuit is high, the reactance is low and inversely when frequency is low, reactance is reactance is high. Reactance is how much the current is impeded. When you have a capacitor going to ground and high frequencies are sent through the circuit, the capacitor is not stopping the signal from being sent out to ground so at high frequencies the output is very low because most of it is going to ground. When low frequencies are sent through the circuit, the reactance in the capacitor is high so it prevents the signal going to ground and therefore more can be sent to the output. Similarly, in a high pass filter, when the capacitor is in series in the signal chain, at low frequencies it simply prevents the signal from going through and at high frequencies provides little impedance to the flow of the current out to the output. The variable resistor simply acts as a voltage divider so you can change the voltage across the circuit and therefore change the frequency at which the circuit cuts off (the cutoff frequency).
1. When you have your RC circuit set as a highpass filter, there is a point at which the corner frequency goes above he range of human hearing. To improve this, you can adjust the value of the potentiometer that you're using so that at it's peak, it does not allow the corner frequency to rise above 20,000Hz.
2. When you have an RC circuit set as a lowpass filter, there is a point where the corner frequency goes below 20Hz and there is no change in sound beyond that point. To fix this, you can first change your potentiometer so that it is unable to reduce the corner frequency to less than 20Hz.
3. In order to make a filter with an even shaper cuttoff, you can put multiple filters in series. This way, you will essentially filter the filter or cutoff the cutoff. At the cutoff frequency you will sharpen that curve even further.
The project I have been researching is entirely analog. As it is essentially only a controller (an extension of the potentiometers on a stompbox), all the parts are entirely analog. If I were to include filters in the design, they would also be analog (distortion, envelopes, etc.). A typical proximity sensor is $50 plus so I will have to stick with the cheaper light sensors. If I can find a way to use them in reverse so that a high amount of light turns the pot off and as the level of light sensed goes down, the value of resistance goes down. Here is a preliminary sketch of my design: