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Articles: Live Sound: PA 101- PA Basics

OK. You need a PA system for your band or church group. you're new to it all and see a variety of boxes with more buttons, indicators and knobs than the flight control deck of a jet fighter. You don't want to buy the wrong thing and it's a large amount of money we're talking about here. People around you are using words like 'Impedance' and 'compressor' and acronyms like 'dB SPL'. Scary stuff...

So I'm going to demystify it for you and let you in on some of the basics you have to know.

What is a PA?
pa1.gif (1775 bytes)PA is "Public Address", an amplification system which takes a source signal (speech or music) at the input and makes it louder at the output. In it's most basic form, a PA system is composed of 3 main components - a sound source, an amplifier and a loudspeaker.

The sound source produces an electrical signal that represents the sound. The sound source can be a microphone or a playback device such as a CD player.

The amplifier is used to increase the level of the electrical signal from the sound source so that it can be heard at sufficient volume from the loudspeaker.

Finally, the loudspeaker is the device which converts the electrical signal from the amplifier into mechanical energy (sound).

pa2.gif (2290 bytes)Of course in the real world we often need to amplify more than one source. The mixer is used to mix the sources together and use just the one amp and speaker for all of them.

Let's now take a look at each of these components in more detail.

Sound Sources
Sound sources vary widely. They can be microphones for capturing sounds from voices or instruments. They can be from a recorded media such as CD or cassette. Or, in broadcast situations, they may be from a radio transmission or the telephone. Whatever the source is we need to understand its properties so that we can connect it properly to our PA system.

Microphones are essentially the reverse of a loudspeaker, where the vibrations in the air cause movements in a small diaphram in the microphone. Microphones usually produce a very weak signal which needs an extra stage of amplification before it can be sent to an amplifier. This is called a pre-amplifier. These are found in most mixers, but they can also exist as separate units in some cases. There are many microphones available, but that is a subject for another article.

Line Level Sources refer to the group of devices whose output is at a suitable level to be connected straight into the amplifier. These devices require no pre-amplification, although it is still common to put them through a mixer first. Devices in this category include CD players, cassette decks, keyboards and vamplifier line-outs.

Amplifiers
As we said earlier, amplifiers take a line-level signal and boost it to the levels required to drive a loudspeaker. We refer to all these amplifiers as power amplifiers as they handle the high powers required to drive loudspeakers.  Here are some of the facts you need to understand to ensure that the correct amp is chosen for your needs.

Power
Output power is the main parameter you will pay attention to when selecting an amplifier. Output power is expressed in Watts - e.g. 500 Watts (500W). In the case of a two channel amp (stereo), this is usually per channel. Care must be taken here as the power delivered by an amplifier depends on the Impedance of the loudspeaker being driven by the amplifier.

Impedance
Impedance is a measure of the resistance offered by the loudspeaker and is expressed in Ohms. The higher the impedance of the speaker, the less power will be delivered by the amplifier (think of doing any kind of work - the more resistance to the work, the less work that gets done). When reading the specification for an amplifier the power will be stated for a specific Impedance - e.g. 500 Watts into 4 Ohms, 250 Watts into 8 Ohms. If only a single figure is quoted then assume this is for a 4 ohm load, which is the industry standard. DO NOT DRIVE AN AMP AT LOWER THAN RATED Impedance - there will be much wailing and gnashing of teeth as your amp and speakers burn out. As most loudspeakers have a standard 8 Ohm load rating, this means you need to know a little about...

Loudspeakers
The loudspeaker is the final link in the chain. In essence it is a very simple device, essentially a box or cabinet containing one or more drivers (speakers) that produce the actual sound. Different combinations of drivers and cabinet construction methods produce speakers with different characteristics, some made for volume, others made for quality, some even made for price. Lets now take a look at the components of a loudspeaker.

The Cabinet
Basically the box in which the drivers are mounted. However, it plays an important part of the overall sound of the loudspeaker.

Cabinets can be constructed from wood, fibreglass or plastic. An effort is made to use materials which are as rigid as possible so it doesn't start vibrating when the speaker is in use as this affects the sound. Bracing (struts for structural reinforcement) aids with this goal in better cabinets.

The way the cabinet is put together is also very important. The dimensions, volume and construction techniques all affect the sound. Most cabinets will have a hole, known as a Port, that will allow the air to move freely in and out of the cabinet as the driver cones move. This extends the bass response of the cabinet. True cabinet design can be very scientific these days, often computer assisted. A good cabinet design can make a cheaper set of drivers sound good, but the reverse is also true - a good set of speakers can sound terrible when slapped haphazardly in a chipboard box.

The Drivers
Drivers are the components that produce the sounds. They are generally comprised of a coil of wire sitting within a magnetic field. As electric current flows through the coil of wire it moves within the magnetic field. By attaching a diaphram to the coil, we can make a volume of air move at the same time and produce sound waves.

A single driver can be capable of producing the full range of audible sounds (you do get "full range" drivers in the smaller sizes). In practice, however, multiple drivers are more common in a loudspeaker for efficiency reasons. A two-way system would have a low-to-mid range frequency driver and a high-frequency driver too. One of the reasons for separating these out is that high frequencies are more directional. High frequency drivers are normally mounted along with a horn in order to control the direction. If you take a look at the specification for a loudspeaker, you should find a Dispersion property. This indicates the angle at which the high frequencies leave the loudspeaker. Narrower dispersion angles (30 - 60) are more suited to long-throw speaker applications whereas wider angles (90 - 100) are better for short-throw.

Crossovers
When a loudspeaker has more than one driver we need a way to split the frequencies so that the low frequencies go to the low frequency driver and the high frequencies to the high frequency driver, etc. This is done using a Crossover.

A crossover splits an incoming audio signal into its component frequency bands. Most two-way or three-way loudspeakers have an inbuilt crossover network that does this job. This is known as a "passive" crossover (as it needs no additional power). More advanced systems use an external crossover known as an active crossover. This works slightly differently as it is inserted before the amplifier. In this case, you will require additional amplifier channels to run each individual driver. This technique is referred to as bi-amping or tri-amping (depending on whether the sound is split into two or three frequencies). Such systems tend to sound cleaner, more detailed and controlled with each driver being run by its own dedicated amplifier channel. The down side is the expense as more amplifiers are required as well as the active-crossover itself.

Impedance
Impedance is a measure of the load that the speaker represents to the amplifier, and is measured in Ohms. Most speakers have an Impedance of either 4 Ohm, 8 Ohm or 16 Ohm, with 8 Ohm being the most common by far.

But what happens when you want to connect two or more loudspeakers to the same amplifier channel? Does the impedance change?  Yes, depending on whether they are connected in series or parallel. When this happens the following maths applies:

impedance.gif (1945 bytes)

pa3.gif (1872 bytes)So, with parallel loudspeakers, the impedence decreases and it increases with series loudspeakers.

To deal with more than two loads you just keep on adding terms to the right. So two 8 Ohm speakers in parallel would give a total of 4 Ohm and three 8 Ohm loads would give you a total load of 2.67 Ohm. Two 8 Ohm speakers in series would give a total of 18 Ohm and three would give you a total load of 24 Ohm.

pa4.gif (2237 bytes)While having too high an impedance will do no damage to an amplifier, be careful not to make the load too small. Most amplifiers have a minimum load that they can drive, and going below that minimum causes the amp to try and put out more power than it is rated for, and it may then distort. And distortion kills loudpeakers more often than too much power.

The Mixer (A.K.A: "Mixing Desk", "Console")
Of course in the real world we often need to amplify more than one source. One option is to have more amps and speakers, but this starts getting unwieldy and expensive as the number of sources rises. The other solution is to mix the sources together and use just the one amp and speaker for all of them. Enter the mixer.

In its simplest form a mixer has several inputs and a single output where the combined signal comes out. To give us a little more control, individual level (volume) controls are fitted to the inputs and a master level control to the output. This then allows us to not only control the overall output level from the unit but also the balance between the individual inputs. Most mixers have more functionality than this and we need these extra functions when setting up a PA system.

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