Why 3 dB?

gmphadte · 20th September 2011, 07:24 AM

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, 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

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, 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.

20th September 2011, 07:24 AM	#1
gmphadte diyAudio Member Join Date: Jul 2003 Location: Dona paula, Goa	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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20th September 2011, 07:28 AM	#2
Lazy Cat diyAudio Member Join Date: Sep 2007	Look at here.

20th September 2011, 09:14 AM	#3
diypwj diyAudio Member Join Date: Mar 2010	It is the half power point if I remember well

20th September 2011, 09:42 AM	#4
DF96 diyAudio Member Join Date: May 2007	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!!).

20th September 2011, 09:46 AM	#5
digits diyAudio Member Join Date: Apr 2011	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).

20th September 2011, 10:10 AM	#7
DF96 diyAudio Member Join Date: May 2007	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.