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-   -   Why 3 dB? (http://www.diyaudio.com/forums/solid-state/196946-3-db.html)

 gmphadte 20th September 2011 07:24 AM

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.

 Lazy Cat 20th September 2011 07:28 AM

Look at here. ;)

 diypwj 20th September 2011 09:14 AM

It is the half power point if I remember well

 DF96 20th September 2011 09:42 AM

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!!).

 digits 20th September 2011 09:46 AM

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).

 gmphadte 20th September 2011 09:58 AM

Quote:
 Originally Posted by digits (http://www.diyaudio.com/forums/solid-state/196946-3-db-post2717380.html#post2717380) 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.

 DF96 20th September 2011 10:10 AM

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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