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3rd octave filtering

3rd octave filtering

3rd octave filtering

(OP)
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?

RE: 3rd octave filtering

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

RE: 3rd octave filtering


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..

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