Sound travels in waves that contain moments of peaks and valleys (pushing and pulling) and the waves can be converted to forms that can be easily stored as alternating energy (the alternating part reflects the peaks and valleys of the sound waves in the air).   The positive and negative parts of a sound wave can be converted through various means into positive and negative parts of different types of energy such as physical, electrical or magnetic. 

TRANSDUCERS change sound from one form of energy to another.  At the center of the human ear is the eardrum, which is a flexible membrane that converts sound waves to the physical energy that eventually signal nerves.

A PAPER CUP AND STRING “TELEPHONE” IS A PERFECT EXAMPLE OF TRANSFERING ENERGY BETWEEN TRANSDUCERS.  The sound waves in the air made from someone’s vocal chords and shaped by their tongue, teeth and cheeks will bounce against the bottom of the paper cup.  The cup bottom will shake along with the give and take of the bounced sound waves.  That shaking will travel from the cup bottom to the attached string.  If the string is tight, the shaking will continue to travel along the string and then the shaking string will make the bottom of the second cup shake.  That shaking cup bottom will make the air shake along with it, re-creating the original sound waves in the new location.

EARLY RECORDING involved sound vibrations in the air shaking a piece of rubber, which was attached to a shaking needle that scratched a shaky line into a piece of paper or deeper surface.

EARLY PLAYBACK involved dragging a needle through a shaky scratch in a surface, then allowing the shaking in the needle to make a piece of rubber shake, which will cause the air around the rubber to shake and create sound waves.

ELECTROMAGNETISM uses magnets that are not permanent but rather made by passing an electric current around a piece of metal.  Permanent magnets will always have the same strength and POLARITY (charge direction) at each end.  An electromagnet will not, because the strength and polarity will be based on the electrical charge.  Increase the electrical charge and the magnet will become stronger.  Reverse the polarity of the electrical charge (switch the plus and minus) and you will reverse the polarity of the electro-magnet, turning what was the plus end into a minus end and turning what was the minus end into a plus end.

Now we all know that if we have 2 magnets the opposite polarities will pull together and the same polarities will push away.  The plus side of one magnet and the minus side of the other magnet will try to move towards each other and touch.  If you try to touch both plus sides or both minus sides together, the magnets will move away from each other and not allow you to do so. The stronger the magnet, the stronger the pulling together or pushing away will be.

This means that an electromagnet can have flexible interaction with another magnet.  If we take the plus side of a permanent magnet and put it next to the plus side of an electromagnet with the current turned off, then when we turn the current on and increase it, the permanent magnet will be pushed away with stronger force and speed.  If we then reverse the current going around the metal, the plus side of the electromagnet will suddenly become minus and the plus side of the permanent magnet will suddenly be pulled closer.  The more we turn up this reversed current the stronger and faster the pulling will occur.

So, if we take an electrical signal that is constantly getting stronger and weaker and changing polarity from plus to minus and back again, and send that electrical signal (changes and all) through a wire going around an electromagnet, we will have an electromagnet that is constantly changing both polarity and strength.  This would make any permanent magnet that is close enough alternate between being pulled and pushed, sometimes very strongly depending on the electrical power.

The moving permanent magnet may be too small to move much air by itself, but if you attach a paper cone to move more air you have just made a sound speaker that will create new sound waves that are accurate copies of the sounds used to originally create the electrical signal.

The changing polarity in the electromagnet makes a close permanent magnet shake or vibrate.  The magnet has a cone attached, which makes air vibrate along with the vibrations caused by the changing electromagnetism…creating sound waves.

Microphones work in the exact opposite direction.  Sound waves in the air will move a paper cone (sort of a sound “sail” to capture the power in the air), which will move an attached permanent magnet towards and away from an electromagnet.  The resulting electrical changes are an accurate representation of the changes in air pressure originally made by the sound waves.