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Old 20th September 2011, 07:24 AM   #1
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Default Why 3 dB?

Why is the bandwidth specified at 3dB points?

Many a times, I tried to find answer for this, but non r convincing.

Gajanan Phadte
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Old 20th September 2011, 07:28 AM   #2
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Look at here.
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Old 20th September 2011, 09:14 AM   #3
diypwj is offline diypwj  Italy
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It is the half power point if I remember well
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Old 20th September 2011, 09:42 AM   #4
DF96 is offline DF96  England
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That wikipedia entry contains a glaring mistake. It talks about voltage dropping by 1.5dB. Wrong! A voltage drop by 3dB (0.707) results in a power drop by 3db (0.5). This often confuses newbies (and some others!!).
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Old 20th September 2011, 09:46 AM   #5
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And it is only part of why it is chosen as the cutoff frequency as opposed to any randomly selected fraction. 3db is the smallest increase/decrease in volume which can normally be perceived by the ear. So everything within a 3db band, sound equaly loud (provided you had totaly flat hearing :P).
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Old 20th September 2011, 09:58 AM   #6
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Quote:
Originally Posted by digits View Post
And it is only part of why it is chosen as the cutoff frequency as opposed to any randomly selected fraction. 3db is the smallest increase/decrease in volume which can normally be perceived by the ear. So everything within a 3db band, sound equaly loud (provided you had totaly flat hearing :P).
This is the answer I gave in a very early interview, but unfortunately all the other things like half peak power and all the other explanations are known to many, which I think r not correct. If they r correct, why should it be not be specified at any other arbitrary power level.

Gajanan Phadte
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Old 20th September 2011, 10:10 AM   #7
DF96 is offline DF96  England
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It is correct to say that -3dB is half power. The issue then becomes "why choose half power"? It is a nice round number, and in many circumstances is the minimum which will be easily noticeable. It also emerges from the theory of single-pole (or first-order) filters: -3dB is the point where the angular frequency is equal to 1/CR, and the phase is 45 degrees. For most amplifiers the frequency limits are defined primarily by first-order filters.
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