Sometimes, when the wind blows across a long wire, a low-frequency "moaning" sound is produced. The sound arises because a standing wave is set up on the wire, like a standing wave on a guitar string. Assume that a wire (linear density = 0.0120 kg / m ) sustains a tension of 341 N because the wire is stretched between two poles that are 16.86 m apart. The lowest frequency that an average, healthy human ear can detect is 20.0 Hz. What is the lowest harmonic number n that could be responsible for the "moaning" sound?

Answer is not n= 4.47

The smallest integer is n = 4

Explanation:

Using the equation V= Sqrt(F/Linear density)

V= Sqrt(341/0.0120)

V= Sqrt(28416.7)

V= 168.57m/s

Path distance =[ (n +1)/2]lambda

But V= f(Lambda)

n lambda/2 =L

n = f2L/V

n = (20 × 2 × 16.86) / 168.57

n = 4.0007

The smallest integer is n= 4

in this image, you can see solar radiation going through ocean water.

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electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia