From the Q&A

With TJ Byers

Pink Noise Filter

Question:

I have a white noise generator and would like to add a filter on its output (zero to one volt, 600 ohms, unbalanced) to provide a pink noise signal. I believe I need a network to give a three dB per octave roll-off. Can you help me to design such a network?

Bill Woods
via Internet

Answer:

White noise — the sound you hear when a TV is tuned to a non-existent station — has a frequency characteristic which raises the power level by 3dB with each increasing octave. By contrast, pink noise is characterized by a uniform power level across all frequencies. For example, the power level in the 40 Hz to 80 Hz octave is exactly the same as in the octave 10 kHz to 20 kHz. For audio testing, a pink noise source will quickly show any anomalies in speaker systems, room acoustics, and crossover networks.

By filtering a white noise source with a 3dB/octave filter, you can create a very good approximation of pink noise. The circuit below is such a filter. The gain of the input op-amp is unity (Vin = Vout). You can adjust the gain as needed by increasing the value of R1 using the formula gain = R1/10k.