3rd octave filtering 1

[COBWARD]

Hi
I'm trying to get to grips with the use of 3rd octaves in acoustics. Can anyone explain this phenomenon to me or point me to a useful source of information?

 

[pheon]

Any good noise control textbook should give you the background. Try Kinsler and Frey or Beranek. This site explains it quite well.

http://www.norsonic.com/web_pages/frequency_analysis.html

These are the relevant standards

Designation: ANSI S12.1-1983 (R1996)
Title: Preparation of Standard Procedures to Determine the Noise Emission from

Designation: IEC 61260 Ed. 1.0 b:1995
Title: Electroacoustics - Octave-band and fractional-octave-band filters

and here are data on the Band number, center frequency and upper and lower 3dB points.

Third Octave Bands
Band No Center Lower Upper
12 16 14.3 17.8
13 20 17.8 22.3
14 25 22.3 28.5
15 32 28.5 35.6
16 40 35.6 44.5
17 50 44.5 56.1
18 63 56.1 71.3
19 80 71.3 89.1
20 100 89.1 111.4
21 125 111.4 142.5
22 160 143 178.2
23 200 178 222.7
24 250 223 280.6
25 315 281 356.4
26 400 356 445.4
27 500 445 561.3
28 630 561 712.7
29 800 713 890.9
30 1000 891 1113.6
31 1250 1114 1425.4
32 1600 1425 1781.8
33 2000 1782 2227.2
34 2500 2227 2806.3
35 3150 2806 3563.6
36 4000 3564 4454.5
37 5000 4454 5612.7
38 6300 5613 7127.2
39 8000 7127 8909.0
40 10000 8909 11136.2
41 12500 11136 14254.4
42 16000 14254 17818.0
43 20000 17818 22449

Broad Arrow Acoustics

http://home.attbi.com/~pheon

 

[Karinathan]

I think you mean one-third octave, right? The you can follow Pheon from his one-third octave band table..

I can give you some principle in case you then later hear about one-sixed, one-tenth, etc. octave band..

if one tone has a frequency that is twice another tone then the tones are said to be an OCTAVE apart.

f2/f1 = 2

then, for one-third octave band filter
f2/f1 = 2^(1/3)

also for one-sixth,
f2/f1 = 2^(1/6)

further... one-tenth
f2/f1 = 2^(1/10)

grasp the principle?

Then according to Pheon, you may wonder what the band number is, don't you?

Normally the center frequency of one octave is defined as 1000 Hz and all the other center frequencies are derived from this

1000 = 10^3 = 10^(30/10) and The value 30 is the OCTAVE BAND NUMBER...

one more example... 2000 = 10^3.3 = 10^(33/10) then the band number of 2K is 33..

Then you can start...you can try using some spreadsheet programs like excel to do the calculation for you..

start with 1000 , the next upper center frequency is 1000*2^(1/3) =~ 1260 --> 1250, and the next lower center frequency is 1000/(2^(1/3) =~ 794 -->800

now you will know that to calculate the next adjacent upper frequency you can multiply the precent frequency by 2*(1/3) and to calculate the next adjacent lower frequecy you can devide by that...

Hope this help... Good luck..