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SOUND & TECHNOLOGY
FREQUENCY / PHASE / POLARITY / COMBINING WAVES
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).
Sound waves can be viewed in terms of frequency and frequency response. Frequency is the speed of the wave. The closer together the cycles are, the higher the frequency (and musical pitch) is. The further apart the cycles are, the lower the frequency (and musical pitch).
The further the cycles go up and down from the center the line, the stronger (and louder) the sound wave is. The direction a sound wave goes (up or down) when it starts refers to the sound POLARITY.
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).
Combining waves with the same frequency (speed), loudness and polarity will lead to a new combined sound wave that is doubled up and twice as strong, but with the same frequency. This is because both sound waves start out in the same direction from the zero-line, and are considered IN PHASE.
As shown above 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.
Combining waves with the same frequency (speed) and loudness that are going in the OPPOSITE directions out from the zero-line (opposite polarities in which one starts going in the positive direction and the other starts going in the negative direction) are considered OUT OF PHASE and will result in PHASE CANCELLATION, with the new combined sound wave being silent as the two waves have cancelled each other out.
So, 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 or complete cancellation. 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 single wave cycle is when it goes out from the zero-line in one direction, comes back and crosses the zero-line to the other direction, and then returns to the zero-line again (one complete cycle of a peak and a valley, regardless of in which direction the wave starts out). Wave frequency speed is how many complete cycles up and down happen every second, which is measured in Hertz (Hz) which stands for cycles per second. Thousands of cycles per second are measured in KiloHertz (kHz).
Sound wave vibrations create musical pitches. Faster sound wave vibrations produce high musical pitches, and slow sound wave vibrations produce low musical pitches.
When two sound waves are identical in type (for example both are simple sine waves), but they are slightly different frequencies, although they will start together the peaks and valleys will become out of sync and the two waves will go back and forth between being completely IN phase and completely OUT of phase. This repeated and cycling “vanishing when out of phase” and “doubling up when in phase” sounds like the waves fade in and out, and is called “BEATING”.
MORE ON POLARITY
"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).