Pink noise versus white noise. “And in this corner…”
“Is pink noise the appropriate filtered noise for all sound system setups? If white noise is equal energy at all frequencies shouldn’t that be a better reference signal for testing PA & studio speakers? Even if pink noise is actually white noise filtered to approximate the human ear’s frequency response, shouldn’t the systems be tested with white noise? After all, a human mix engineer will mix to a human ear’s frequency response, right?”
My favorite thing is when people ask the same question three different ways trying to make a point. Well, I guess that’s not really my favorite thing, but I do enjoy it.
A lot of people are confused about the difference between pink and white noise. Pink noise is the right type of noise to use to calibrate audio equipment (at least if you are using it for equalization calibration). Here’s where everyone gets confused:
White noise is equal energy per frequency and pink noise is equal energy per octave. Now, think about how we perceive sound. Think about what an octave is to us. Other than the pitches involved we don’t hear anything more substantial happening when a high note jumps up an octave than we do a low note. It’s the same number of frets on a guitar, or keys on a keyboard either way. But in terms of the actual frequencies being produced the difference is great.
Here’s an example. The difference between 100 Hz and 200 Hz is one octave. The difference between 5 kHz and 10 kHz is also one octave. However, in terms of frequencies the difference between 100 Hz and 200 Hz is only 100 Hz, whereas the difference between 5,000 Hz and 10,000 Hz is 5,000 Hz. The relative relationship is the same, but the actual difference mathematically is quite substantial. With white noise there is a ton more energy in-between 5 kHz and 10 kHz compared to between 100 Hz and 200 Hz because it spans a wider range of frequencies and they all contribute to the overall level per octave. The whole point of pink noise is to distribute the energy according to how we hear. So the pink noise energy between 100 Hz and 200 Hz is the same as between 5,000 Hz and 10,000 Hz. Equal energy per octave.
So it’s not that pink noise is calibrated to the human ear’s frequency response per se. It’s just calibrated to how we hear, which is very well grounded in math. Each time the frequency doubles we hear that as an octave. From one octave to the next we expect to hear an appropriate amount of sound energy (depending upon the program material), which is why we calibrate our audio systems to pink noise.
We’ve done several Tech Tips about how to calibrate your system with noise and an RTA in the past, so feel free to consult the archives for more detail. At a practical level, this means you can run pink noise through your system and set your overall EQ so you get a flat line on a typical RTA display. A flat line would signify equal energy at every octave. Some audio engineers feel that this can produce an unnaturally “bright” sounding system and do an additional rolloff of 3 dB per octave above 4 kHz or so. Or they may just do one 3dB rolloff at some frequency and leave it at that. The specific techniques vary, but the perceived need for it has more to do with the dispersion characteristics of the equipment involved, the measurement accuracy, and personal tastes than anything. Your actual mileage will probably vary.