White Noise (and Pink Noise) are both comprised of a random signal, with all the audio frequencies present, over time, and at various signal levels. Such Noise is generated by the random thermal motion of electrons, and is therefore called random noise. When this thermal noise is amplified, we perceive it as a hiss. White Noise is unfiltered, unaltered thermal noise.
When the energy content of a white noise signal is averaged (integrated over time), it will be found to contain equal energy per Hz (frequency).
If you were to measure a 100hz wide window between 100Hz and 200Hz and then again between 1500Hz and 1600Hz the amount of energy will be exactly the same. Plotting the power of white noise, with respect to frequency, will result in a 3dB per octave buildup in level as the frequency rises.
There are more frequencies per octave as you go up scale, and since White Noise has equal energy per Hz, there will be more energy in the higher octaves.
White Noise is used to calibrate electronic equipment. It provides a signal with which to measure levels, and it drives the circuitry at all frequencies simultaneously. It is almost never used for testing loudspeakers, Pink Noise is used for that.
- Paraphrased from the Sound Reinforcement Handbook
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