The audio spectrum is the sound frequency range at which humans are able to hear.
This range encompasses 20 Hz to 20,000 Hz and can be effectively divided into seven different frequency areas, with each having a different effect on the total sound.
The seven frequency bands are:
Sub-bass > Bass > Low midrange > Midrange > Upper midrange > Presence and Brilliance
The sub bass gives the first usable low frequencies on most recordings. The deep bass that is emitted in this range is usually felt rather than it is heard, providing a sense of power. Many instruments are unable to enter this band, with the exception of a few bass heavy instruments, like the bass guitar which has a lowest achievable pitch of 41 Hz. It is very hard to hear any audio at low volume level around the sub bass range because of the Fletcher Munson curves (Equal Loudness Curves).
Unless you use very high quality monitor speakers, it's suggested to apply very little equalization boost in this region.
Too much boost in the sub-bass range can make the sound overly powerful, whereas too much cut will weaken and thin out the sound.
Sine wave example at 50 Hz
The bass range determines the thickness of the sound. The fundamental notes of rhythm are situated around this region. Most bass signals in modern music tracks lie around the 90-200 Hz area. The frequencies around 250 Hz can add a feeling of warmth to the bass without losing definition.
If the bass region is too pumped up tends to make the music sound boomy.
Sine wave example at 100 Hz
Generally seen as the bass presence range, the low midrange contains the low order harmonics of most instruments. Pumping the signal at around 300 Hz adds clarity to the bass and lower-stringed instruments. If the sound around 500 Hz is boosted too much, it can make higher-frequency instruments sound muffled.
A lot of songs can unfortunately sound muddy due to excess energy in this region.
Sine wave example at 300 Hz
The midrange dictates how important an instrument is in the mix. If the range around 1000 Hz is too pumped up, it can give instruments a horn like quality. Excess output in this spectrum can sound tinny and may cause ear fatigue. If sound is enhanced too much in this range, especially on vocals, it's most probably a bad idea. Because human ears are particularly sensitive to how the human voice sounds and its frequency coverage.
Sine wave example at 1000 Hz
Human hearing is highly sensitive at the high midrange frequencies, with the slightest enhancement in this range leading to a massive change in the timbre of the audio.
The high midrange is responsible for the attack on percussive and rhythm instruments. If increased, this range can add presence. However, too much emphasis around the 3 kHz range can cause hearing fatigue. Vocals are most emphasized at this range so just like the midrange, it's preferable if this range isn't boosted too much.
Sine wave example at 3000 Hz
Cutting in this range makes sound more distant and transparent.
Sine wave example at 5000 Hz
The brilliance range is comprised entirely of harmonics and is responsible for sparkle and air of a sound. Boosting around 12 kHz make a recording sound more Hi Fi.
Over boosting in this region can accentuate hiss or cause ear fatigue.
Sine wave example at 10,000 Hz