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Old 29th April 2006, 10:57 AM   #1
Nora is offline Nora  United States
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Default why 3dB point?

I have been taught to design filters for 3dB point.
Why is this the best starting place for basic design?
A google search tuns up info like half-power....
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Nora
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Old 29th April 2006, 11:27 AM   #2
PMA is offline PMA  Europe
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Let's consider simple RC lowpass filter. There is a cutoff frequency Fc:

Fc = 1/(2*pi*R*C)

At Fc, there is a -3dB at amplitude frequency response. This response can be simplified by 2 straight lines, 1st - horizontal - from 0Hz to Fc, 2nd - with slope -20dB/decade, starting from Fc continuing to infinity. That's why the corner frequency (Fc) is important, and on that frequency there is a -3dB amplitude response point.
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Old 29th April 2006, 12:29 PM   #3
lndm is offline lndm  Australia
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It could be considered the point where the filter becomes the dominant element. This is a generalisation.

As applies to audio, 3dB is considered the minimum significant change in audible volume. Yes, I know some of us talk about noticing 1/2dB steps, 3dB is after all a basic electronics concept.

And yes, it is the half power point.
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Old 29th April 2006, 05:21 PM   #4
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Default Re: why 3dB point?

Quote:
Originally posted by Nora
I have been taught to design filters for 3dB point.
Why is this the best starting place for basic design?
A google search tuns up info like half-power....
Thanks-
Nora

3dB is the point where power is reduced to 1/2 its former level.
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Old 30th April 2006, 12:23 AM   #5
djk is offline djk
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It depends on the kind of response you need.

For a two-pole filter:

Butterworth=-3dB

Bessel=-4.78dB

Linkwitz Riley=-6dB

The transfer function of the crossover is the sum of the acoustical and electrical response. A woofer combined with a two-pole low-pass and a tweeter combined with a three-pole high-pass may yield a 24dB per octave Linkwitz Riley transfer function (the target).
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Old 1st May 2006, 10:15 PM   #6
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Hi Nora, I may have misunderstood your question but the reason that people use the -3db point is probably one of those agreed "conventions" that makes the most sense in the real world and allow people to dialogue without reinventing the wheel every time they speak ie going into excessive detail about theory/slopes/damping etc. I know someone will say that RL filters are -6bd at the frequecy of interest (i.e. cross over point) but as I understand it that is a specific use of a more genreralised filter in a cross over application for loudspeakers. I think that in fact 4th order RL xovers are two ordinary Butterworth filters combined. These filters are each -3db at that frequency. If you are looking for the origin of this convention you may have to go a long way back in the relevant literature.
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Old 1st May 2006, 10:45 PM   #7
lndm is offline lndm  Australia
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That reminds me, in the most basic type of filter, the point at which an inductor (or a capacitor) and a resistor resonate (their reactance is equal), the signal on the resistor will be down 3dB.
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