[填空题]
A person hears a pure tone in the 500–1000-Hz range coming1dyc* w ev1,lc.q ip4s5 wh6e8,r yqqcev v t nhwiqnzj+nj0*g /q:.3:fa7x r+d j4b+sfrom two sources. The sound is loudest at points equidistant from the two sources. To determine exactly what the frequency is, the person moves +z/j3tv::fn+7 w d ji*0.4nvq xg+b esrahqnj about and finds that the sound level is minimal at a point 0.34 m farther from one source than the other. What is the frequency of the sound? Hz