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Upon reading this microphone array tutorial, I'm stuck in the following piece:
The spatial sampling frequency along the x-axis is given by
$f_{x_s} = \frac{\sin \theta \cos \phi}{\lambda}$
where $\theta$ and $\phi$ are the elevation and azimuth angles of the incoming signal, and $\lambda$ is the wavelength.
I cannot grasp the meaning of the equation. How come the sampling frequency depends on the angles? Also, shouldn't it depend on the inter-element distance?
The spatial sampling frequency of a linear array with equidistant sensor (antenna) spacing on the $x$-axis is simply given by
$$f_x=\frac{1}{d}\tag{1}$$
where $d$ is the distance between the sensors (cf. Eq. $(35)$ in the document you linked to). What is meant by the equation in your question is the spatial frequency of the incoming wave along the $x$-axis, according to the $x$-component of the direction vector $\mathbf{\alpha}$ (cf. Eq. $(13)$ in the document).
To avoid spatial aliasing you need to guarantee
$$\frac{1}{d}>2\mathbf{\alpha}_x=2\frac{\sin\theta\cos\phi}{\lambda}\tag{2}$$
for all possible angles $\theta$ and $\phi$. The maximum value of the numerator of the fraction on the right-hand side of $(2)$ is $1$, and the minimum value of the denominator is the minimum possible wavelength $\lambda_{\text{min}}$:
$$\frac{\sin\theta\cos\phi}{\lambda}\le \frac{1}{\lambda_{\text{min}}}\tag{3}$$
Combining $(2)$ and $(3)$ gives
$$d<\frac{\lambda_{\text{min}}}{2}\tag{4}$$
which corresponds to Eq. $(38)$ in the document.
