Phil or Graham ... EQ 'Q' question
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From: Denny Strauser on 24 Jul 2008 03:31 I need to know how the 'Q' factor relates to bandwidth (by octaves). Pleeeeese! Thanks, -Denny
From: Phil Allison on 24 Jul 2008 06:26 "Denny Strauser" >I need to know how the 'Q' factor relates to bandwidth (by octaves). > > Pleeeeese! ** A couple of old customers of mine used to run a PA and lighting hire business together - John owned the lighting rig and Mick owned all the sound gear plus a 5 ton diesel the whole lot went round the place in. For the first few years, they had to " pay their dues " by providing "production" for a lot of novice bands playing the pub circuit in the Sydney area. Mick used to often comment that even if these bands were to improve their playing dramatically, they would rise only to the level of being merely bad. After the first set at one such gig, John and Mick were sitting at a table taking a break and enjoying a beer when the drummer from the band came up and began whining about his drum sound to the bemused pair. Bemusement turned to * uncontrollable hysterics * when the drummer asked in all seriousness " .... can't youse guys do somethin' about my IQ ?? " ...... Phil
From: gwatts on 24 Jul 2008 08:14 Denny Strauser wrote: > I need to know how the 'Q' factor relates to bandwidth (by octaves). > > Pleeeeese! > > Thanks, > -Denny 'Q,' is short for 'Quality Factor,' at least in filter theory, which may or may not apply to live sound depending on who's yelling in your ear the loudest. It's the center frequency divided by the bandwidth. There's a chart on Pro Sound Web at http://www.prosoundweb.com/ee/images/uploads/bandwidthchart.pdf that shows Q in terms of octave bandwidth. A one octave bandwidth filter has a Q of 1.41
From: Arny Krueger on 24 Jul 2008 09:19 "Arny Krueger" <arnyk(a)hotpop.com> wrote in message news:TJWdnakbwo2RHRXVnZ2dnUVZ_uLinZ2d(a)comcast.com > "Denny Strauser" <dennysound_comcast_net> wrote in message > news:IfKdnbDqydpbshXVnZ2dnUVZ_qrinZ2d(a)comcast.com > >> I need to know how the 'Q' factor relates to bandwidth >> (by octaves). > > > Q is defined as -3 dB bandwidth divided by the center > frequency. > http://en.wikipedia.org/wiki/Q_factor > > The relevant equation from this reference is: Hmm the original post got interpreted as a graphic that might not get through Usenet. in text, the equation was: Delta F = Fc/Q Where Delta F = bandwidth Fc = center frequency Q = Q or quality Expressing this in octaves, you come up with the idea that a Q = 1 filter has 1 octave bandwidth, and a Q=10 filter has 1/10 octave bandwidth. So a good rule of thumb is that: Q = 1 / bandwidth(in octaves). Or, a 1/3 octave filter has a Q of about 3.
From: gwatts on 24 Jul 2008 11:12
Arny Krueger wrote: > "Arny Krueger" <arnyk(a)hotpop.com> wrote in message > news:TJWdnakbwo2RHRXVnZ2dnUVZ_uLinZ2d(a)comcast.com >> "Denny Strauser" <dennysound_comcast_net> wrote in message >> news:IfKdnbDqydpbshXVnZ2dnUVZ_qrinZ2d(a)comcast.com >> >>> I need to know how the 'Q' factor relates to bandwidth >>> (by octaves). >> >> Q is defined as -3 dB bandwidth divided by the center >> frequency. >> http://en.wikipedia.org/wiki/Q_factor >> >> The relevant equation from this reference is: > > Hmm the original post got interpreted as a graphic that might not get > through Usenet. > > in text, the equation was: > > Delta F = Fc/Q > > Where > > Delta F = bandwidth > Fc = center frequency > Q = Q or quality > > > Expressing this in octaves, you come up with the idea that a Q = 1 filter > has 1 octave bandwidth, and a Q=10 filter has 1/10 octave bandwidth. > > So a good rule of thumb is that: > > Q = 1 / bandwidth(in octaves). > > Or, a 1/3 octave filter has a Q of about 3. I think you're miscalculating the center frequency in your example, and the wikipedia page doesn't say how they calculate it. I've checked multiple references* and they all agree that the center frequency is calculated logarithmically and is the square root of the product of the low roll off frequency and the high roll off frequency. This method gets a Q of 1.41 for an octave bandwidth filter. Calculating center frequency by averaging the high and low roll off frequencies only works for very narrow filters, defined by the ratio of the high to low roll off frequencies being less than 1.1, or a Q of greater than 10.5, or less than about 1/7 octave. * - 'Electronic Filter Design Handbook,' Williams; 'Handbook of Electronic Calculations for Engineers and Technicians,' Kaufman and Seidman; 'Active Filter Cookbook,' Lancaster. |