MATLAB程序中一段不明白，谁帮我具体解释下。谢啦！

Question

%加噪声
snr=10;
sp=mean(yy.^2);
attn=sp./ 10^(snr/10);
attn = sqrt(attn);
noise=randn(1,length(yy)).*attn;
np=mean(noise.^2);
snr1=10*log10(sp/np);
data=yy+noise;