Vocals
Roughly speaking, the speech spectrum may be divided into three main frequency bands corresponding to
the speech components known as fundamentals, vowels, and consonants.
Speech fundamentals occur over a fairly limited range between about 125Hz and 250Hz. The fundamental
region is important in that it allows us to tell who is speaking, and its clear transmission is therefore
essential as far as voice quality is concerned.
Vowels essentially contain the maximum energy and power of the voice, occurring over the range of 350Hz
to 2000Hz. Consonants occuring over the range of 1500Hz to 4000Hz contain little energy but are essential
to intelligibility.
For example, the frequency range from 63 to 500Hz carries 60% of the power of the voice and yet
contributes only 5% to the intelligibility. The 500Hz to 1KHz region produces 35% of the intelligibility,
while the range from 1 to 8KHz produces just 5% of the power but 60% of the intelligibilty.
By rolling off the low frequencies and accentuating the range from 1 to 5KHz, the intelligibility and clarity
can be improved.
Here are some of the effect EQ can have in regards to intelligibilty. Boosting the low frequencies from 100
to 250Hz makes a vocal boomy or chesty. A cut in the 150 to 500Hz area will make it boxy, hollow, or
tubelike. Dips around 500 to 1Khz produce hardness, while peaks about 1 and 3Khz produce a hard metallic
nasal quality. Dips around 2 to 5KHz reduce intelligibilty and make vocals woolly and lifeless. Peaks in the
4 to 10KHz produce sibilance and a gritty quality.