Which acoustic variable refers to the amount of matter per unit volume?

The correct answer is density. Density is defined as the amount of matter contained within a given volume. In acoustics, density plays a crucial role as it impacts how sound waves propagate through different media. It influences the speed of sound; as density increases, the propagation of sound waves can be affected because denser materials may impede the movement of the sound waves compared to less dense materials.

Understanding the concept of density is essential in sonography, as it helps determine how ultrasound interacts with various tissues, fluids, and structures in the body. For instance, different organs have different densities, which can affect the acoustic impedance and, subsequently, the quality of the ultrasound image produced.

In contrast, pressure refers to the force exerted per unit area and does not directly represent the amount of matter per unit volume. Distance involves measurements of space between points and does not pertain to mass or volume. Frequency relates to how often a sound wave cycles through a specific point in a given timeframe but also does not indicate matter per unit volume.