VI to calculate time decay of acoustic signal?

[ritz19]

im using a b&k8104 transducer to transmit a 75KHz signal sampled at 2MHz generating 100000 samples and receiving it using a neptune d70 receiver in an underwater tank. i need to calculate the reverbration time (i.e. the time taken by the signal to decay by 60 db) but i'm unable to see the time domain plot clearly in labview. its riddled with noise. maybe my vi is faulty. can anyone help me if they've made VIs that calculate such time decays?

P.S. I'm unable to attach my vi here as this is a diferent computer.. but its a simple vi where i simulate a signal and take its output through DAC. Then receive the input through another DAC and see its time domain plot and FFT.

PLEASE HELP! 

[ritz19]

i tried adding a filter at the receiver end but didnt work. i can see a clear impulse on the received FFT plot indicating the presence of a sinusoid wave (which is what im transmitting) .  however i cant see the sinusoid wave on the time domain plot. this is required fopr me to ascertain the decay slope and calculate reverberation time.

PLEASE HELP!!

[ToeCutter]

An FFT is essentially the output from a bunch of filters. Try narrowing your filter around the transmission frequency and increasing the order. If you want more help, post the code you have.

[Bob_Schor]

I assume (but you didn't say) that you present a brief 75KHz "chirp", rather than playing it continuously while taking the 100000 samples.  If so, then divide the post-chirp time into bins perhaps similar to the chirp length and take the FFT of each set of bins.  For example, if your chirp was 2 msec long, take the FFT of 0-2, 2-4, 4-6, 6-8, msec intervals and look at the power at 75KHz (your signal).  Find the time at which the signal has fallen by 60dB.  You basically don't need the time-domain signal to do this.

Disclaimer -- I've never tried to do this, and have no expertise in acoustics.  This just seems to be a logical thing to try ...

Bob Schor