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Understanding live sound.

Live sound applies to both musician and DJ. The first thing you need to learn about sound is the sound itself.

What is the sound? Sound is nothing more than waves of changing air pressure that move your eardrums. Sound begins with a source of physical resonance which can be your vocal cords, a speaker, or anything else that produces sound.


This vibration makes the air move. And when this moving air hits your eardrum, your eardrum will resonate in proportion to the source. This is the essence of sound. All three ingredients are needed; a source, a medium to pass it on, and something to receive it.


Okay, now let's talk about dispersal. Dispersion is the direction the sound will go, once it has left the source. You will usually hear the term dispersion in reference to live PA speakers. You know, 90X60, 60X40 and so on.


Once the sound comes out of a compression or piezo controller, it needs to be controlled. A horn connected to a conductor will do just that. At the base of the horn, is the throat. The throat is a narrow channel through which sound passes.


Once the sound leaves the throat, it wants to explosively spread in all directions. Now it's up to the horn flare to control the scattering angles. However, some horn designs are better than others. If you have a 90X60 horn, you would expect all frequencies to be scattered throughout this model. It's not always the case.


The best horn designs we deal with are waveguides and constant directive or CD horns. These are both designed to disperse all audible frequencies within the specified angles.


The waveguides look like round dishes and naturally they give a cone-shaped dispersion. CD speakers are what you see in most public address cabinets today. They offer a more horizontal dispersion.


Which brings us to the phase. Most of you know that when you are connecting a system, it is important to connect the positive with the positive and the negative with the negative. But what really happens if you don't follow this rule? One speaker will push while the other will pull.


The result: phase cancellation! You will find that some consoles have phase reversal switches on each channel. This is how you can fix phase issues without rewiring. Consider ringing a snare drum up and down.


Even if your cables are wired correctly, since the mics point directly at each other, they are out of phase from the start. Reverse the phase in the console and your problems solved. A more advanced application. Let's say you have a 60 cycle hum in the system.


Take a 60Hz signal generator, plug it into a channel, and offset it so that it cancels out the interference. Most of us can tell when the sound gets twice as loud. This is a good starting point, but you need to be more precise. So if you broke it down into tenths, you'd have a decibel.

The literal translation of "decibel" is one-tenth of a doubling in volume. Where deci means "tenth of" and bel means "twice as strong". In other words, a 10dB boost in sound will seem twice as loud.

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