the application of -6dB/octave??

db decade to db octave

as we know, for the transfer fucntion in frequency domain, its magnitude will roll-off with -6dB/octave or -20dB/dec when a pole occurs.
could someone tell me what's the difference between -6dB/octave and -20dB/dec from the viewpoint of application??
thanks a lot!!

6-db/octave rolloff

6dB/octave is equivalent to 20 dB/decade .. they both mean the same ..

Regards,
IanP

6db/octave equivalent to db

Here octave meaning that you have twice the frequency ...like if slope of transfer function is 6dB/octave then from 1kHz to 2kHz it will come down by 6dB..

formule 20db/dec

IanP said:

6dB/octave is equivalent to 20 dB/decade .. they both mean the same ..

Regards,
IanP

Click to expand...

Thanks for your help!
but i don't think so.
First, the formula for these two expression are different.
Second, in my opinion, their applications maybe different.
But, I don't know their difference, could you help me ?

Added after 9 minutes:

raduga_in said:

Here octave meaning that you have twice the frequency ...like if slope of transfer function is 6dB/octave then from 1kHz to 2kHz it will come down by 6dB..

Click to expand...

thank you for your help!
could you explain "octave"? for example, 20dB/dec means that if the frequency increase by ten times, the maginitude of the transfer function will fall down 20dB, what's the meaning about the 6dB/octave??
for 20dB/dec, the formula is 20*log10.
for 6dB/octave, does the formula 6*log8 ??

Added after 6 minutes:

In my opinion, these two expression maybe have different application.
in some field, maybe, the 20dB/dec is convenient to depict some problems.
but in some application, 6dB/octave is convenient.
could some give an example for this problem?
thanks a lot!

6db/octave means

Hi liuyonggen
-6dB/octave and -20dB/dacade is caracteristic of the first order low pass filter (one pole)
The Transfert function of such a filter is
H(ω) = A/(1+jω/ω0)
when you turn into Log scale and draw the well known Bode plot
for ω>>ω0 the curve |H(ω)| = |A/(1+jω/ω0)| as fonction of ω is asymptotic
The gain is divided by 10 each time the frequency is mutiplied by 10
That is why you have -20dB/dacade
The gain is divided by 2 each time the frequency is mutiplied by 2
Tha is why you have -6dB/otave

hope I helped you

octave decade app

V(f) - output voltage at frequency f
The transfer function is considered to decrease approx. linearly with frequency, for f>f1=fpole. Therefore if f2/f1 is 10 (decade), istead of 2 (octave) the output will decrease 5 times the value for octave.
The amount of roll-off in dB:
- for f2/f1=2: 20log(V(f2)/V(f1))=-6dB, therefore
V(f2)/V(f1)=10^(-6/20)=0.5;
- for f2/f1=10: 20log(V(f2)/V(f1))=-20dB, therefore
V(f2)/V(f1)=10^(-20/20)=0.1;

meaning of 20 db/octave

Mate,

1 = 1/1 = 2/2 .. = 100/100.. = 1 000 000/1 000 000 .. and so on ..

Regards,
IanP

trasfer between octave and decade

liuyonggen_1,
Another thing to keep in mind, when doing exact comparisons between the 30dB/decade and 6 dB/octave slopes: The exact value is Log10(4) dB/octave = 6.020599913... dB/octave. Using this exact value, 20dB/decade and Log10(4) dB/octave are exactly the same.
Regards,
Kral

6 octave and 20 db decade

thanks for your help!
my final question is that:
it's well known that if the frequency increases by two times, the magnitude of the transfer function will become an half, which is -6dB. But in my opinion, "octave" means that the frequency increases by eight times rather than two times, how to explain it?
expect your further help! thanks!

6 db octave

liuyonggen_1 said:

thanks for your help!
my final question is that:
it's well known that if the frequency increases by two times, the magnitude of the transfer function will become an half, which is -6dB. But in my opinion, "octave" means that the frequency increases by eight times rather than two times, how to explain it?
expect your further help! thanks!

Click to expand...

I think 'octave' scale coming from musican world (tone scale ...c,d,e,f,g,a,b,c... ie. eight sub tones per double frequency periode) not from engineering world.

wikipedia give lot of leads with search word 'octave'

difference between decade and octave

Normalised gain is 20 log (w1/w2). If the ratio of w1:w2 is 1:2 then the expression can be represented as 20 log (1/2) = -20 log 2 = -20X0.30103 = -6dB per octave

If w1:w2 is 1:10 , then the same expression 20 log (1/10)
= -20dB per decade

-Bharat

liuyonggen_1 said:

as we know, for the transfer fucntion in frequency domain, its magnitude will roll-off with -6dB/octave or -20dB/dec when a pole occurs.
could someone tell me what's the difference between -6dB/octave and -20dB/dec from the viewpoint of application??
thanks a lot!!

Click to expand...

6db two times

Hi
Octave means without any doubt that the ratio between the two frequency (that limits a frequency intervall) is 2
for example :
[22Hz ; 44Hz]
or [121Hz ; 242Hz]
or [1GHz ; 2GHz]


In music this intervall contains 8 notes that is why one talk about octave

definition of 6 db/octave

thanks for your help!
i know more about it , thank you very much!