Speech signal equalization

I'd like to know how equalizer bands usually cover speech frequances , i.e. What bandwidth and what frequences should I use to cover frequences from 20Hz to 20kHz using 4 bands equalizer?
Does anybody know also a good source for common equalizer presets ?
kornilo_ilyaAsked:
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pauljaymesCommented:
Human speech can have components extending between 1kHz and 8kHz but the more important parts (for intelligibility) are between 2kHz and 5kHz.

Around 4kHz is the most sensitive part of the ear and the most vital area for understanding speech.
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pauljaymesCommented:
see http://jaymes.net/speech.gif for a graph
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pauljaymesCommented:
Does that answer your question?
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kornilo_ilyaAuthor Commented:
Not quite actually . What I ment is that I wanted to know principles on which equalizer divides frequences between 20 and 20000Hz and what is the ratio between sentral frequency of a specified band and it's bandwitch . As far as I understand all bands of graphical equalizer should cover the hearing range , but I don't know how exactly does GOOD equalizer do it.
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kornilo_ilyaAuthor Commented:
Not quite actually . What I ment is that I wanted to know principles on which equalizer divides frequences between 20 and 20000Hz and what is the ratio between sentral frequency of a specified band and it's bandwitch . As far as I understand all bands of graphical equalizer should cover the hearing range , but I don't know how exactly does GOOD equalizer do it.
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pauljaymesCommented:
OK,
Graphic equalizers usually use centre frequencies based on "Octave bands" - This is because human perception of frequency is logarithmic, so on a musical instrument, each octave represnts a doubling in frequency.

In mathematical terms a doubling is represented by the inverse log of 0.3 - Octave bands are based on 10^3 (1kHz) and exact octave band centre frequencies are calculated by doing the inverse log (10^x) of multiple of 0.3 up and down from 3.
So 10^3 is 1000kHz
10^2.7 is ~ 500kHz
10^2.4 is ~ 250kHz
10^3.3 is ~ 2000kHz
....and so on
If you have the full range of octave bands you will have a 10 band EQ starting at 30kHz (10^1.5) and going up to 16kHz (10^4.2).

If you want have less than the full range of OBs then manufacturers often make arbitary descisions on which ones to have. If you're going to do a 4 band then you could, say, choose alternate bands and leave one of the upper or lower limits out.

The key thing is, when you design your filter, that the -3dB points of the filter shape roughly coincide for adjacent bands. You can calulate where these points should be by using the Logs -
On a full 10 band EQ, the lower -3dB point for the 1kHz band (and therefore the upper -3dB for the 500Hz band) should be at 10^2.85 = 708Hz
On the same EQ the upper -3dB for the 1kHz (and therefore the lower for the 2kHz) should be at 10^3.15 = 1413Hz
On a 5 band EQ, a 1KHz band would have an upper -3dB of 2kHz and a lower -3dB of 500Hz

Not all the bands have to have the same bandwidth (or any particular centre frequency), providing the adjacent -3dBs always coincide, and the full range you require is covered.

This is an unusual "computers" question!
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kornilo_ilyaAuthor Commented:
Thanx alot ! I didn't know better place to ask this question :)
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