Note 1 to entry: For a point source at $x_0$, the inhomogeneous Helmholtz equation is$\text{ρ}\nabla \cdot \left[{\text{ρ}}^{-\text{1}}\nabla \left(f;x\right)\right]+k^{\text{2}}H\left(f;x\right)=-\text{4πδ}\left(x-x_{\text{0}}\right)$, where ρ is the medium's time-averaged mass density, k is the acoustic wavenumber and $H\left(f;x\right)$ is the transfer function, x is the position and f is the acoustic frequency.

Note 2 to entry: For a point source in infinite free space, the transfer function is $R^{-\text{1}}\text{exp}\left(\text{i}kR\right)$, where $R=\left|x-x_{\text{0}}\right|$.

Note 3 to entry: Jensen, F. B., Kuperman, W. A., Porter, M. B. and Schmidt, H. Computational Ocean Acoustics, AIP press Springer-Verlag, ISBN: 978- 1-4419-8677-1, 2011. DOI 10.1007/978-1-4419-8678-8. Page 84-85. of Jensen et al. (2011) refers to propagation loss as "transmission loss". Jensen et al. (2011) refers to the transfer function as the "transmission loss pressure".