Little known fact:
Sound is technically a subgroup of the electrics department, because it requires electricity to amplify sound.
The purpose of the sound department is not only to make it loud enough, but to make it sound good in every corner of the room. This is the trick, and the reason that most audio engineers are so precise with the placement of their speakers.
Sound itself is the vibration of matter (usually air) as acoustic energy passes through it. The vibrations are detected by the eardrums. The number of vibrations per second measured by Hertz. The human ear is capable of detecting from about 25 Hz to 25,000 Hz. The higher the frequency, the higher the perceived pitch. The low end is called Bass and the high end Treble.
Volume is measured in Bells, which is actually a measure of relativity between one volume and another. Bells are typically divided into sets of 10 deciBels, or 10/10 of a Bell (1 Bell). A change of about 3 dB would be required to make a noticeable change in volume. An increase of 10 dB actually doubles the volume. So if you start at 0 dB, which is actually the lowest volume capable of being heard by the human ear, then everytime you add 10 dB, the sound increases exponentially (keeps doubling over and over).
For example, 100 dB is about the loudness of loud classical music, and in actual power is about 10,000,000,000 times as powerful as 0 dB. Rock music runs about 110 dB and the threshold of pain about 120 dB!
A microphone is a device who's purpose is actually to convert acoustic energy to electricity.
Dynamic mics and Ribbon mics make their own electricity, but Condenser mics only regulate power, and so require an external power source, called Phantom Power, which is typically sent from the Console.
Wireless mics could be either Dynamic or Condenser, but its more difficult to make a dynamic element small enough for wireless, so most wireless mics are condensers. A wireless mic is actually a mic and a radio transmitter, usually FM. These often use batteries, AA or 9V, which provide the phantom power.
Microphones can also be divided into Omnidirectional (picks up sound from all directions) or Unidirectional (only picks up sound from one direction).
There are Handheld mics, that the talent walks around holding or puts on a mic stand. They can be wired or wireless. There are also Laveliers/ Lavs which are almost always wireless mics that are taped to the face or forehead so as not to make it obvious in a play or musical that the actor has a microphone.
The God Mic is a microphone that allows the sound engineer (or sometimes the director) to speak to everyone on stage during sound check. It can be pumped into certain monitors, the main speakers, or everything as needed.
Power a system up by moving from small to large. Turn on external AC, then sources, then mixer, then line-level devices/processors, then amps. Power down in reverse order: amps, then Line level devices/processors, then mixer, then sources, then AC. Doing so will help prevent accidental damage to equipment.
From the Mic
The common cable for transmitting sound signal on the source end of things (microphones and instruments) is the XLR cable.
A Direct Input Box (DI Box) allows an instrument cable (1/4 inch cable for guitars and keyboards) to be transfered to XLR.
Preamplifiers (pre-amps) boost the amount of electricity coming from the source in order to make it easier to manipulate and amplify.
From there, the sound moves to the Mixer (the sound board or console), where the engineer can make the appropriate tweeks.
At the Mixer, the engineer has the list of inputs in one place, and the list of outputs in another. He/She can adjust how each input interacts with each output. Gain refers to the INPUT level, where is Volume refers to the OUTPUT level.
From the mixer, sound can be sent to Line Level Devices. Amongst these are Compressors, which keep the sound level within certain limits, Effect Machines, which can make sound effects like Reverb (controlled echo), and Equalizers.
EQ's can help control individual frequencies. This is not only important to improve quality of sound, but also to avoid Feedback.
Feeback is when a frequency of sound recycles in a system over and over creating a higher and higher volume. So, the frequency goes into the microphone, comes out of the speaker, and the microphone picks it up again and puts into the speaker again, etc. When this happens, the sound engineer pulls down the frequency in the EQ a bit to avoid that happening again.
A Buzz/Hum, sometimes called 60 cycle hum because it is usually around the 60 Hz frequency, can be heard coming from the speakers or monitors while sound checking, and is often the result of a nearby power source. AC power can operate around the same level as 60 Hz, which makes that frequency double in the sound system, and that makes it audible. A Grounglift is a small adapter that allows you to plug in a three prong edison cable without the ground. This sometimes helps relieve the hum.
The best way to avoid it though is to avoid putting any non-sound equipment into the same power sources as sound equipment. This is why sound and lights usually have different feeder sources.
At the end of the path is the final output, the speakers. They turn AC electricity back into sound.
Speaker cable connections are called Neutrix (a brand name), or NL-4's and NL-8's. (The NL-8's are larger and usually used for larger speakers.)
The Cones inside of a speaker vary in size, and the larger cones produce lower tones. High-end cones are typically composed of metal and a magnet.
Subwoofers put out a Bass signal, the low end. They are typically very large, due to the larger cones.
Tweeters put out the very high tones of sound, and involve very small cones.
Mids put out the mid-range of sounds.
Line Arrays are a set of speakers (tweeters and mids) that are flown in the air in a downward arc in order to deliver sound to a vertical range.
There is a wide variety of line arrays that are assembled a variety of ways, but always in several stages. Usually the cabinents come out of the truck in straight stacks, and then pins and specially made pucks are used to set the angle between each box. Each one is different, so just follow the sound tech's instructions carefully.
Monitors are speakers placed on stage that playback aspects of the show for the performers.
In a theater show, the background music will be played through all of monitors for the actors to sing along to and to listen for their cues.
In a concert, each musician may want their own Mix of music from the other musicians, so that they can play along.
Monitors in the same mix can be jumped together with NL-4's. Each mix must get its own home run to the monitor rack.