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The spectrum subdivision, white noise and pink noise
We already know that we are capable of hearing frequencies between 20Hz and 20KHz.

A first rough subdivisions is commonly made between low, low-mid, mid, high mid and high frequencies: the following picture below will give you an idea of the different 'families'. In many occasions however, there is the need to finely adjust a frequency spectrum, for example while tuning the sound system for a concert, ore when we are fine-adjusting the frequency content of an instrument in our mix. It is very common to divide the audio spectrum into 31 bands: each of these 'bands' is named after a central frequency, and the width of a band is always 1/3 of an octave. Now, we haven't really talked about the relationship between frequency and musical intervals such an octave, which will happen in the first class of the next section (music theory). For now, let's say that doubling a frequency means covering a one octave interval.

If you take a look at this chart, you will see the 31 bands across 10 octaves:

click to enlarge

Please note that the 'families' of frequencies are not really standardised, so some people for instance would consider 400Hz to be a 'low frequency' and others a 'low-mid'. Similarly, the boundaries between adjacent families are blurred, so please take this chart as a reference but consider some flexibility. During this curriculum, we will be calling frequencies and families as shown in the chart. Please, listen to the pure tones of the 31 bands now:

 low low mid mid hi mid high 20Hz 400Hz 1KHz 5KHz 10KHz 25Hz 500Hz 1,25KHz 6,3KHz 12,5KHz 31,5Hz 630Hz 1,6KHz 8KHz 16KHz 40Hz 800Hz 2KHz 20KHz 50Hz 2,5KHz 63Hz 3,15KHz 80Hz 4KHz 100Hz 125Hz 160Hz 200Hz 250Hz 315Hz

Then, as you can see all the following ranges of frequencies cover an octave:

20Hz - 40Hz

40Hz - 80Hz

80Hz - 160Hz

160Hz - 320Hz

...

Every interval is twice as large than the one before (and of course half of the following).

 octave ƒ contained 20Hz - 40Hz 20 40Hz - 80Hz 40 80Hz - 160Hz 80 160Hz - 320Hz 160 ... ... 10KHz - 20KHz 10.000

so if we, say, would play all the whole frequencies in an octave together, the intensity of that sound would be twice as much than the octave below and 1/2 than the octave above. That's why, if we hear all the audible frequencies together at the same amplitude, the overall sound will tend to sound crisp and sharp, which is due to the higher intensity of the high end frequencies. This sound is named white sound (similarly to the white colour, which is the sum of all the visible colours).

Now, why would we listen to such a sound, since is not particularly pleasant? It is often used in the fine calibration of audio equipment, where we need all the frequencies at the same level, to verify that a machine response is linear.

However, as we said, white sound does not take into account that high octaves have much more intensity, due to the more frequencies that they comprehend. Being the intensity double in the higher octaves, if we lowered each octave by 3dB, it would sound as the same intensity as the previous one. Such a sound is called pink noise.

This noise is usually employed to calibrate sound systems taking into account the amount of intensity of each octave. It is often used in concerts or for loudspeakers calibration.

Let's compare the two noises in an intensity/frequency graph

click to enlarge

After all we have said, you will certainly understand that, being every complex sound made of pure sounds summed together, if you would be able to identify each of the 31 frequencies straight away, it will be an invaluable skill when it comes to EQ your mixes. Well, as we said, this is one of the objectives of this first set of classes. Start by taking one family at a time (i.e. low frequencies) and practice with the tests in the free tests section. You may find it easier to listen to a reference tone at first, but remember that in order to pass the competency tests you must be able to tell the frequency without a reference. Once you are confident within a family, move on to a different one, and only when you have a lot of practice start the tests that cover the whole spectrum.