When a sound wave travels directly toward a hard wall, the incoming and reflected waves can combine to produce a standing wave. there is an antinode right at the wall, just as at the end of a closed tube, so the sound near the wall is loud. you are standing beside a brick wall listening to a 85 hz tone from a distant loudspeaker. how far from the wall must you move to find the first quiet spot?

Explanation:

When standing sound wave is produced in a medium , distance between anti-node and next node is equal to a quarter of a wave length. Node is point of silence where sound intensity is zero or minima.

wave length = velocity / frequency

= 340 / 85

= 4 m

We must  move by a distance of 4/4 = 1 m away from wall

to find the first quiet spot.

compass can be used day and night, cloudy or sunny