SOUND WAVES
SOUND travels in waves, much like a water wave. When water waves move, it is not the same piece of water that is traveling but rather the motion being transferred along. Sound waves act the same.
Steve-o "Pops" Kuresko described it in the following way:
Imagine a room filled with ping-pong balls from wall to wall and floor to ceiling. If you touch one of the ping pong balls, it will move the next one a little bit...which will move the next one a little bit...so that the ping pong ball all the way at the other end of the room will be moved without the first ping pong ball moving away from it's location. (Thanks, Steve-o!)
As sound waves move (or more accurately, when they travel by transferring their energy) they interact with physical objects. Soft surfaces will absorb sound while hard surfaces will reflect it. DIFFERENT types of hard surfaces will reflect different aspect of sound more strongly than others. That is why a large wood room does not sound like a large tile room, and why closets are so quiet.
Sound waves love to bounce. If there are parallel reflective (hard) surfaces, the sound may bounce back and forth between them (much like looking into two mirrors that are facing each other and seeing many reflections). This can create ugly ringing echoes. Hard surfaces that are NOT parallel allow sound waves to bounce around in ways that break them up into smaller and softer diffuse sounds. The distinct echoes you hear from parallel surfaces can become a smooth long sound if the sound waves are allowed to spread around while bouncing.
Sound waves vibrate up and down in repeating cycles while they bounce around. Besides having positive and negative PHASES (when the sound is going up or going down) the cycles have physical length needed for a complete cycle back to the starting point ("wavelength"). High frequency sounds are made from rapidly moving sound waves that can complete a cycle in a short distance while low frequency sounds are made from slowly moving sound waves that need more distance to complete one single cycle.
Sound waves do NOT physically move air molecules up and down. That's just how we picture it. What actually happens is that they compress and spread apart. You can find good graphics of this at http://www.physicsclassroom.com/Class/sound/u11l1c.html (thanks Ron).
When sound waves meet other sound waves they combine. The two sound waves may be from the same instrument (such as the direct sound from a drum meeting the echoed sound of the same drum) or different instruments (such as the kick drum sound meeting the bass sound). If the sound waves are both in the same parts of their cycle (both are in the positive or negative part) they are "IN PHASE" and will ADD to each other, creating a stronger sound. If the sound waves are both in DIFFERENT parts of their cycle (one is in the positive and the other is in the negative) then they are "OUT OF PHASE" and will actually SUBTRACT from each other, creating a thinner sound. Instead of reinforcing each other, they carve pieces out of each other.
Many mic pre-amps have a "PHASE" switch that simply flips the phase (reversing polarity). You should always "flip the phase" on individual mics in a multiple-mic setup to make sure all of the sound waves being picked up by all of the mics are in the same part of the cycle (and "IN PHASE" with each other). You do not want your kick mic to be out of phase with your overhead mics...it will sound very thin rather than full.
A "TRANSDUCER" converts sound energy from one form to another. The most basic transducer is a toy telephone made from 2 paper cups and a tight string. The sound waves are converted to string vibrations that are passed to the other cup, where the cup's bottom vibrates and forms new sound waves (much like a speaker). Any microphone uses a transducer to convert physical sound energy in the air to electrical energy. Speakers convert electrical sound energy to physical sound energy in the air. Vinyl records are made when a transducer cuts a physical path representation of the sound into a spinning vinyl disk. When the disk is played back, a needle (transducer) rides along the physical path and the resulting vibrations are used to convert the physical path representing sound into electrical energy. Magnets are used as transducers to convert electrical sound energy to magnetic energy, which can be stored on plastic magnetic analog tape.
A vocal will go through the following transducers from being performed to being heard from a vinyl record:
PHYSICAL SOUND ENERGY IN THE AIR >transducer> SOUND AS ELECTRICAL ENERGY >transducer> SOUND AS MAGNETISM (storage) >transducer> SOUND AS ELECTRICAL ENERGY >transducer> PHYSICAL REPRESENTATION OF SOUND ON VINYL >transducer> SOUND AS ELECTRICAL ENERGY >tranducer> PHYSICAL SOUND ENERGY IN THE AIR
"POLARITY" has to do with what direction a speaker moves (out or in) when a sound wave is in either the positive or negative PHASE. Polarity standards are different in many countries. For example, European audio equipment is wired differently from American and Japanese audio equipment, and has the opposite polarity. That means that if you take a sound wave that pushes a speaker OUT on a balanced system in America and play it on a balanced system in London, the speaker will actually suck IN rather than push out...sounding COMPLETELY different. After mixing an album in Finland, I listened back in New York and thought it was wrong...until I reversed the phase on both the left and right side (so both sides pushed OUT together rather than sucking IN together) and the mixes were right. That is why my "MIX mp3" page has a comment about the polarity...so people can hear the songs as they were intended.
Polarity concerns at a CONSUMER level may be less now that everybody is using home stereo components made by the same companies. Of course, people can still plug the red into the black and black into the red (reversing speaker polarity). But at a PROFESSIONAL level polarity is always important. Your will find that even if your microphones are placed so they are all in phase with each other your phase can still be flipped by gear from another country, a bad wiring job, a missed button or other factors.
Let's say your drums are the same phase and are all pushing the speaker cone together but your bass is out of phase from the drums. When the bass plays it will hold the speaker back rather than pushing it in the same direction together with the drums. This greatly reduces the natural warmth of each instrument by itself and the overall sound of the combined tracks. I have seen many people work hard to put in the warmth they subtracted themselves by not checking phase...but rather than having natural warmth they make the instruments sound like the EQs.
PHASE and POLARITY are important to keep in mind. ALWAYS try flipping phase on individual tracks when you are bouncing them together and use whatever sounds warmer.
Some Producers (such as the amazing Rob Fraboni) will painstakingly check phase at every stage (even through the monitors)...and their records have a realistic natural warmth that is rare. Don't believe me? Check out Wendy Wall "Dig That Crazy Beat" (I will try to get permission from Rob to post a little here).
I will provide future links to demonstrate the importance of consistent polarity and how much phase and polarity can affect overall sound.