Find the intensity(I) of sound wave whose frequency is 250 Hz. The displacement amplitude of particles of the medium at this position is 1 × 10$^{–8 }$m. The density of the medium is 1 kg/m$^3$ , bulk modulus (B) of elasticity of the medium is 400 N/m$^2$ .

I know I=2$(\pi f A)^2 \rho v$

And v $=\sqrt {B/\mu}$

Where $\mu = $ mass per unit length .

But I could not understand how to use them .

$\mu$ should be density, not mass per unit length.