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Q is an bit of electronics jargon, which is dimensionless (unlike charge, mass velocity etc), and exists as a relative measure only.

Think of Q as being a comparison-based thing (he is really, really tall) and not a measurable thing (he is 186cm tall).

Q is used as a setting on parametric equalisers. These work like a normal graphic equaliser, with the extra feature, that you can control how narrow or broad you want the peak or dip in response to be. That is, if you start with a frequency response that is flat, like this:

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bass midrange treble

...and then you put a high-Q peak in the middle, you would get a narrow spike in response, like this:

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bass midrange treble

...whereas a low-Q peak would be a broad lump, like this:

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bass midrange treble

Loudspeakers have characteristics which are described by Q in a similar way. A high Q means that the speaker will have a narrow spike in output at it's resonant frequency. A Q of .7 will give flat output to the resonant frequency, while a lower Q system has so little resonance that output is rolled off around it's resonant frequency.

See sealed speaker design to see the way Q interacts with other values.

The Q of a speaker is not a scale for determining how good a driver is, it simply shows how resonant the driver (or driver-enclosure combination) is. The rule of thumb is that big magnets and big enclosures give a low Q. The lower the Q, the more tightly does the magnet control the movement of the cone, whereas a very high Q driver would oscillate wildly at resonance.

Q has a mechanical element, Qms set by by the weight of cone, type of suspension etc, and it has an electrical element, Qes, due to magnet strength, type and size of voice coil etc. These combine to give a total Q, called Qts.

A driver with Qts of less than .2 is quite exceptional, and implies a very powerful (expensive) magnet, and is most likely a speaker optimised for esoteric applications (horn driver). A Qts above .6 implies either a cheap little magnet, or a speaker optimised for esoteric applications (open baffle). Most moderately priced drivers will have a Qts between these extremes.

Finally, Qts is also effected by the enclosure size, giving Qtc. Thus, Qts is the value for a driver in free air, or mounted in an open baffle, while Qtc is the value for a driver-enclosure system.

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