DIY Audio Frequency Filters

An audio frequency filter modifies the tone of a sound signal.

An audio frequency filter is an electronic circuit that works on electrical signals from microphones, amplifiers and other sound-related equipment. A filter removes a range of frequencies from the sound signal to reduce noise or to create sound effects: Tone controls, graphic equalizers and guitar "wah-wah" pedals are all audio frequency filters. The simplest filters consist of a resistor and capacitor, where one is in a series circuit with the signal and the other is in parallel. If the capacitor is in parallel, the circuit is a low-pass filter, progressively reducing frequencies past a known cut-off point.

Things You'll Need

• Solderless breadboard
• 1-microfarad, 50-volt capacitor
• 10K-ohm, 1/4-watt resistor
• 4 12-inch pieces of 22-gauge wire
• Labeling tape
• Pen

Instructions

1. Insert the capacitor's leads into adjacent columns of the breadboard. Holes in the breadboard columns are connected internally, but adjacent columns are electrically isolated from each other. To connect to a component already in a column, insert a wire or lead into another hole in that column.

2. Set one of the resistor's leads so it shares a column with one of the capacitor's leads. Insert one end of the first and second wires so they connect to the other capacitor lead. Insert the other resistor lead into a free column on the board. Insert the end of a third wire so it connects to this resistor lead. Set the end of a fourth wire so it connects to the other resistor lead.

3. Wrap a label around the first and third wires and write "Input" on the label. Wrap a label around the second and fourth wires and write "Output" on it. Connect the wires labeled "Input" to the output of an audio source such as a microphone, tape recorder or MP3 player. Connect the wires labeled "Output" to the input of an audio amplifier. The cut-off frequency of this filter is 100hz, so it passes lower frequencies than this and reduces higher ones.

Tips & Warnings

• The formula for the low-pass filter's frequency is f = 1/(R*C), where f is frequency in hertz, R is resistance in ohms and C is capacitance in farads. You can easily substitute other resistance and capacitance values to change the filter frequency. At this frequency, signal strength reduces by 3dB or about one-half. For every additional octave, or doubling of frequency, the signal decreases to one-fourth.

References

• Georgia State University: Filter Circuits with Capacitors