splMeter Time Weighting Function

Hello,

I need to implement a time weighting for sound preasure. It is implemented for newer Matlab Versions in the splMeter Object, but I only have a 2015 version to run this script.

In general it is a simple filter:

When I implement that, I still get a difference of 0.1-0.2dB if I compare the maximum from my script and the splMeter Object.

Also, the splMeter doens't have that fade-in artefact:

What am I missing to get the complete same result as the splMeter Object?

To compare both, I initialized the SPL Object without any frequency weighting (Z-weighting).

tau = 0.125;
alpha = exp(-1/(fs*tau));
xf = filter( [1/(fs*tau)], [1 -alpha], x.^2 );
spl = 10*log10(xf/(2*10^-5)^2);