1/3 Octave Reverberation time

[Tim Nrc]

I am using Labview 7.0 with the Sound And Vibration toolkit 3.0.

I need to be able to measure the reverbation time of our test chambers.

Background

ASTM

[npai]

Hi Tim.

You need to make your own Vi to calculate the reverberation time. There is no sub vi for directly calculating the reverberation time in any of the toolkit. I have the newer version of Sound and Vibration Toolkit 4.0 and its not in there too, so i think you need to make the Vi on your own.

There is nice tutorial available on the NI website about calculating Absorption Coefficients and Reverberation time. you can go to that and its quite good. I have read that
The link is http://zone.ni.com/devzone/conceptd.nsf/webmain/AF13040EF7C8CE7F8625708300524DB0

Regards,

Nitin

[Tim Nrc]

Opps must of clicked something wrong.  Here is the rest of message.

The ASTM standard requires that the reverberation time be calculate for each third octave band.  To do this I need to take 200 measurements with 30 ms slices.  From this data I can calculate the decay time for each third octave band.

Equipment
Lab view 7.0
Sound & Vibration Toolkit
NI 4472 DAQ
Sample rate 32000 S/sec
Time slice 30 ms
Number of samples for 30 ms 960.
Number of presamples required 48000.

I can turn the sound off using a computer controlled relay board.  I use the relay that turns the sound off as an external trigger for the 4472.  I need the card to presample about 1.5 seconds of data so the the Third octave filters in the sound and vibration toolkit can be in a settled state, for the beginning of the decay.

I have tried to create a program that sets the pretrigger samples to be 1.5 seconds of data before the trigger.  However when I look at the data collected it seems to be starting from trigger point.  I need the read vi to sample 960 per cycle, and I need 250 samples.  The first 50 are used to get the filter in a steady state, the last 200 should be the decay starting from the exact time the sound was turned off.

I have code to calculate the reverberation if I can only get Lab view to give the levels from point 50 to 250.

I have created a loop that N=250 and the Read vi is inside the samples has been set to 960.  How can I get Lab view to keep the 48000 samples before the trigger and then process then and the next 192000 points in 960.

Thanks
Tim

[Tim Nrc]

Thanks for the link, I have read it before.

The problem that I am having is capturing the decay and processing it through the third octave filter.  The example NI gives is for Leq, which is does not met the requirements in the ASTM standard for lab measurements. 

Thanks again

Tim

[npai]

Hi Tim,

Can you post the code here for people to take a look at it. 

you can use a shift register to keep the pretigger samples in it. Once the tigger happens you can write the data from the shift register and  for certiain time using the elapsed time vi and wire the boolean function of this elapsed time to the stop button of the while loop. So the data from the tigger point will be written to a file.

Regards,

Nitin