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

gmphadte 20th September 2011 08:24 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.

Gajanan Phadte

Lazy Cat 20th September 2011 08:28 AM

Look at here. ;)

diypwj 20th September 2011 10:14 AM

It is the half power point if I remember well

DF96 20th September 2011 10:42 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 10:46 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 10:58 AM

Quote:

Originally Posted by digits (Post 2717380)
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

DF96 20th September 2011 11:10 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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