MSP430FR6047: Acoustic length parameter for clamped on non-intrusive device

Part Number: MSP430FR6047

Hello

I saw that the calculation for the acoustic length is:

Acoustic Length = Distance between transducers / speed of sound in medium(typically water)

My question: if the device is clamped on a pipe, the path will consist different materials (for example: water, brass metal for the pipe and a couplant material)

How do i calculate the acoustic length in such cases?  

Thanks for the assistance