In noisy factory environments, it's possible to use a loudspeaker to cancel persistent low-frequency machine noise at the position of one worker. the details of practical systems are complex, but we can present a simple example that gives you the idea. suppose a machine 6.0m away from a worker emits a persistent 90hz hum.

to cancel the sound at the worker's location with a speaker that exactly duplicates the machine's hum, how far from the worker should the speaker be placed? assume a sound speed of 340 m/s.