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Acoustic attenuation is a reduction of a propagating sound signal beyond that attributed to the specified geometric spreading. Absorption converts acoustic energy to other forms, while scattering redirects it away from the receiving path. The inferred attenuation therefore depends on what the detector collects and how the spreading reference is defined.

For a plane wave with pressure amplitude $p(x)=p(0)e^{-\alpha x}$, the corresponding sound intensity decays as $I(x)=I(0)e^{-2\alpha x}$ in a fixed medium. An attenuation coefficient defined from intensity has twice the numerical value of this amplitude coefficient. Measuring the signal at several separations can estimate the slope, provided the source response, reflections and geometric corrections are controlled.

The acoustic energy flux is the rate at which a small-amplitude acoustic wave transports energy through oriented unit area. For pressure perturbation $p^{\prime }$ and particle velocity ${\mathbf{u}}^{\prime }$, its leading instantaneous value is $\mathbf{I}=p^{\prime }{\mathbf{u}}^{\prime }$. Averaging over a cycle gives sound intensity.

With physical fields represented by real parts of $\hat{p}{e}^{-i\omega t}$ and $\hat{\mathbf{u}}{e}^{-i\omega t}$, the cycle average isThe relative wave phase matters: large pressure alone does not imply large transported power. In a progressive plane acoustic wave, pressure and longitudinal velocity are in phase, and $⟨I⟩=|\hat{p}{|}^2/(2\rho c)$. In a perfect standing wave they are in phase quadrature locally, giving zero net cycle-averaged transport despite stored acoustic energy. Simultaneous measurements of pressure and particle velocity can distinguish the two situations.

An acoustic point source idealizes a compact emitter as a localized source of sound. The approximation is useful when its size is small relative to the relevant wavelength and observation distances; the actual emitter can be a vibrating object or a small driven volume of gas.

An isotropic outgoing wave in an unbounded lossless three-dimensional medium has pressure amplitude proportional to $1/r$ and sound intensity proportional to $1/r^2$ in the far field. Multiplying that intensity by a sphere's area gives a constant radiated power. The singular behavior at $r=0$ belongs to the idealization: the source's finite geometry and near field must replace it there.

Acoustics studies sound: its generation, propagation, interaction with matter and detection. In a fluid, a small acoustic wave couples a pressure disturbance to the motion of the medium. The disturbance transports energy without requiring the same parcel of material to travel from the source to the receiver.

A loudspeaker and a microphone make a simple experiment. Changing their separation measures propagation delay, while changing the frequency in a bounded room reveals resonance and standing waves. A strong microphone response does not by itself imply a large net sound intensity: oppositely travelling waves can produce a large local pressure while their mean energy transport cancels. Acoustic impedance connects pressure and motion for a specified travelling wave.

Wave intensity is the time-averaged power carried through unit area perpendicular to the transport direction. It has SI units $\mathrm{W}{\mathrm{m}}^{-2}$. For a uniform beam carrying power $P$ through a perpendicular area $A$, $I=P/A$; a nonuniform beam instead requires integration of its local energy flux.

For a progressive plane acoustic wave in a lossless fluid,where $p_0$ is the peak pressure amplitude, $p_{\mathrm{rms}}$ its root mean square, $\rho$ the mean mass density and $c$ the speed of sound. Doubling the wave amplitude therefore quadruples the wave intensity in the same medium. A standing wave can have large local pressure amplitude with zero net averaged sound intensity, so the progressive-wave formula must not be applied to an arbitrary field.

Sound intensity is the time-averaged acoustic energy flux, a vector measured in $\mathrm{W}{\mathrm{m}}^{-2}$. In linear acoustics, $\mathbf{I}=⟨p^{\prime }{\mathbf{u}}^{\prime }⟩$, where $p^{\prime }$ is acoustic pressure and ${\mathbf{u}}^{\prime }$ particle velocity. Its direction identifies net energy transport, and integrating its normal component over a surface gives the acoustic power crossing that surface.

A progressive plane acoustic wave satisfies $I=p_{\mathrm{rms}}^2/(\rho c)$, with mean mass density $\rho$ and sound speed $c$. This formula uses the relation between pressure and velocity for that particular travelling field. Two opposite equal waves instead form a standing wave with zero net sound intensity, despite nonzero root mean square pressure. A microphone alone measures pressure; inferring transported power from it therefore requires assumptions about the field, especially near reflecting boundaries.