Such phonons propagate in straight lines with unchanging wavelength, frequency and velocity.Sound in, say, a swimming pool or a smoothly flowing river travels straight from its source to the ear.

• 这类声子以固定波长、频率与速度沿直线传播，像声音在游泳池或是平顺流动的河流中，就是从音源一直线传递到我们耳朵。