You have probably heard the terms pink noise and white noise. You probably cringe when you hear fingernails being drawn across a chalkboard but think nothing of lawn mower noise. These sounds may all have the same sound level, but the frequency content is vastly different.
White noise is a sound that has a uniform spectral level over the range of human hearing. This means that every frequency between 20 Hz and 20,000 Hz has an equal energy level. White noise sounds like it is rich in high frequencies, but this is not the case. This perception occurs because each successive octave is twice as wide as the one preceding it. For example, the 500 Hz octave band has width of 354 Hz. The next octave, the 1000 octave band, has a width of 707 Hz. The net result is that there is more energy in the high frequency components. The sound of water and wind resemble white noise.
Pink noise is a variant of white noise. Pink noise is white noise that has been filtered so that each octave has equal energy. This is done to compensate for the increase in the number of frequencies per octave.
You can demonstrate the difference between pink and white noise using the Sound Level Meter. Set the Sound Level Meter to measure “C” weighted sounds. Pick several sounds that have the same “C” weighted sound level. Now measure these same sounds with the “A” weighting. Compare the difference between the two weightings for each sound.