Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 80

As the piston vibrates, the gas immediately next to the piston must have the same velocity as the piston. A small element of fluid is known as a particle, and its velocity, which can be positive or negative, is known as the particle velocity. For waves traveling away from the piston in the positive x‐direction, it can be shown that the particle velocity, u, is given by

(3.10)

where ρ = fluid density (kg/m³) and c = speed of sound (m/s).

If a wave is reflected by an obstacle, so that it is traveling in the negative x-direction, then

(3.11)

The negative sign results from the fact that the sound pressure is a scalar quantity, while the particle velocity is a vector quantity. These results are true for any type of plane sound waves, not only for sinusoidal waves.