Understanding The Nature of Equalizer's Response Curve: (2) Q

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The Music Telegraph
Text 2022-10-18


▲ Hi Q and Lo Q curves

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Understanding The Nature of Equalizer's Response Curve: (2) Q


The bandwidth of an equalizer curve is a useful number to have for the reasons already described in the previous article. However, it is not the whole picture when it comes to using the curve to alter the timbre of a sound. The spectral nature of most musical audio revloves around the relationship of a sound's fundamental frequency to all the other harmonics of that sound (the octave is twice the fundamental, etc.). A more useful description of an equalization curve would be the relationship between its bandwidth to the center frequency of the curve. This would give us some idea of how many octaves around the center frequency a curve will affect. This relationship is called 'Q' and is defined as the center frequency of a response curve divided by the bandwidth of that curve, or:




There are no units of measurement for this relationship. It is just a number expressing a ratio. The higher the value of Q, the narrower the bandwidth, and vice versa.


Most music equalizers have a range of Q that varies between 4 (1/4 of an octave wide) to .33 (3 octaves wide). There are special purpose equalizers called notch fiilters that can have extremely high Q's (narrow bandwidths) of up to 100 or more. These are not generally considered musical. They are mostly used for eliminating problems like hum, radio frequency interference, wind, and any other noises that find their way into an audio signal. 






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