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Suppose the no of values in ur wav file after taking it into malab (using wavread) is 5000( call that variable A), initialize another variable with around 7500 with all zeros(call it B). Copy the first 5000 values of A into B starting from 2501 to 7500. Now you have to add A and B, but size of A is lesser than B so pad 2500 zeros towards the end of A to make it 7500. Now add A + .5*B. The multiplication with .5 will reduce the amplitude giving an echo like effect. You have to experiment with adding the number of zeros to get a good output.