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Calculating Octave Values, AURA Mapping, Averaging Positions
PostPosted: Sun Oct 15, 2006 9:50 am Reply with quote
amarok
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Joined: 15 Oct 2006
Posts: 3




Hello, I'm a Spanish student (so first of all, sorry for my english), this is my first topic.

I am working in the model of an Auditorium for my acoustic studies with EASE 4.1 and I have a great trouble with the value of 1/3th octave index like C80. I have value criterions based in octave averages index (like C80mid(500Hz,1kHz,2kHz) ). My question is what is the correct form for evaluate this:

octave index=10*log( 10^(0,1*Level 1/3th)+10^(0,1*Level 1/3th)+10^(0,1*Level 1/3th))

or

octave index=10*log( (10^(0,1*Level 1/3th)+10^(0,1*Level 1/3th)+10^(0,1*Level 1/3th))/3 )

My teacher toll me that he thinks first form is the correct but the values are now too high and I doubt if this is correct.

Sorry for my english, thanks!
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PostPosted: Sun Oct 15, 2006 10:27 am Reply with quote
Stefan
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Joined: 19 Apr 2005
Posts: 167
Location: Berlin, Germany




Hi Amarok,

the first expression (energy sum) you gave is related to signal levels which are often summed in fractional octave bands. This also happens in the Live module of our measuring software EASERA, where you will find that the octave level is always greater or equal to any included 1/3rd octave level.

The second expression (energy average) you gave is related to a linear average of fractional octave bands, which is usually applied when considering the impulse response / transfer function but having a signal like pink noise in mind.

In EASE you can actually calculate any octave value directly by choosing the bandwidth to be "1/1 Octave". But since EASE is based on impulse response / transfer function calculations, the average calculated is a bandwidth-weighted average. So every 1/3rd band enters with the weight of the 1/3rd bandwidth relative to the overall octave bandwidth into the calculation.

We denote for the sake of clarity that the expressions you gave should in any case refer to the levels which C50 and C80 are based on, but not to these relative quantities themselves.

When calculating bandwidth-based quantities you will have to ask yourself what the spectrum of the signal will be, because the frequency components may enter the band with different weights.
Generally in ISO 3382 and in EASE the measures C50 and C80 are defined based on the impulse response which in turn can be considered as related to a signal with a flat (white) frequency response (similar to MLS, linear sweep or white noise).

Stefan
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PostPosted: Sun Oct 15, 2006 12:05 pm Reply with quote
amarok
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Joined: 15 Oct 2006
Posts: 3




Hi, Stefan! thanks for your reply!

So, you say that would be better take the values from the mapping with octave average?? I tried to do a ISO 3382 simulation with AURA by chosing 7 listener seats in the room and some different positions to the sound source, and made a 'spatial average', like a real medition. Then, in mapping window I don't take this values, only the complete table for 1/3th octave for each point.

I had a complete AURA Mapping of the room and I see this differents. If I work with this values the complete index (energy sum) for each octave frecuency band is bigger than the AURA Mapping for 1/1 octave (1,1dB vs. 5,51dB for energy sum in 1kHz, for example), so the different is important.

So, the correct way would be use the values for the Mapping (in the example 1,1dB), isn't it?
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Calculating Octave Values, AURA Mapping, Averaging Positions
PostPosted: Sun Oct 15, 2006 2:13 pm Reply with quote
Stefan
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Joined: 19 Apr 2005
Posts: 167
Location: Berlin, Germany




Hi,

I need to comment on several things here.

1) Averaging several room positions, we get this question quite often, also for the measuring software EASERA.

This should be done generally at the very end. Averaging IRs is mostly not a valid means to obtain a "representative" IR for the room. Sometimes you may average the distribution/decay of energy over time such as the Echogram. However, what you are looking for are actually measures representative for the room, so you should look at the measures finally calculated for the individual positions. When averaging them you should always include the error or standard deviation in any reports as well. Otherwise the average is largely meaningless.

2) AURA Mapping Results

See previous forum posts for this also. In general you should make sure first that the numerical variations between several runs are not too large for example due to insufficient particle numbers. For this to verify it is ALWAYS recommendable to perform at least TWO AURA calculations with the same settings in order to obtain an overview over the errors / variations involved.

3) Multiple Mapping points

Yes, you should use the octave values for each seat for your further use. You can use the text file export to write these values to the disk. (There is no direct tabular display for them, indeed.) Alternatively you can store the 1/3rd octave values and calculate the bandwidth-weighted octave average (not the linear average) manually. The Octave mapping should give you the same results.

4) Deriving C50 or C80 octave values

You should not calculate any sums of these energy ratios. This does not make any sense (at least for the applications I know). The values will be meaningless accordingly.

Stefan
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PostPosted: Sun Oct 15, 2006 3:25 pm Reply with quote
amarok
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Joined: 15 Oct 2006
Posts: 3




Thanks a lot Stefan!! the text file export! it's just the solution of my problem, I don't need to repeat any measurement!!!!!, ufff.... Mr. Green

I will use this octave values for the AURA simulation for any single listener seat, don't worry about the mapping, I found this forum searching information about AURA and all the mappings are made with the FAQ recommendations.

Of course, I put in the measurement all standard deviations, it's an academical enginnering work. Wink

Bye!!
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