time interval over the duration of an effect and not the signal

Hi,

I write a little script in command window of Matlab:

between:

t = (0: 0.001: 10)';

y = sin (2 * pi * 50 * t)

sound (y, Fs);

and

t = (0: 0.001: 100)';

y = sin (2 * pi * 50 * t)

sound (y, Fs);

the 2nd sound is lengthened in duration

OK!

Now I add an effect to this signal with effect.m:

(E)

t = (0: 0.001: 100)';

y = sin (2 * pi * 50 * t)

z = effect (y);

sound (z, Fs);

How to correct (E) for this effect to act in a time interval t1 = (a: 0.001: b) with a> 0 and b <100?

thanks

[jérôme TAÏEB]

sorry,i found the answer with a=30 and b=70:

t= (0: 0.001:30)';

x = sin (2 * pi * 50 * t);

sound(x,20000);

t = (30: 0.001:70)';

y_2= sin (2 * pi * 50 * t);

y=tremolo(y_2,20000);

sound(y,20000);

t = (70: 0.001:100)';

z = sin (2 * pi * 50 * t);

sound(z,20000);

but there lies the following problem:

from an audio file whose variable t does not intervene explicitly:

[y, Fs] = audioread ("audio.wav");

what is the function of Matlab who captures his time t?

thanks