PA System Basics - For Beginners Only, Part I
1/05 - Larry Mundy - http://www.colomar.com/Shavano/basics.html
Note: Larry Mundy is the author of Design and Build Your Own Live-Sound Speakers, now available from Amazon, and an occasional contributor to this site.
There are dozens and dozens of helpful articles here on the Shavano site about PA systems, sound reproduction, and equipment hookup. Most of them require that you know at least some semi-technical terms. But we know that some of you are just getting started in all this and could use a basic dictionary for understanding what were talking about.
If you already know the basic principles of sound reproduction, you wont get much out of this article. But if you dont, hopefully this will make the other articles a little easier to understand. What follows is a sort of glossary of terms you should know to evaluate equipment. In a subsequent article, we'll discuss the various configurations PA equipment comes in.
Of course, you know what air is. Youre breathing it right now, or else youre some type of alien life-form. What you need to know is how air and sound interact. Air coats the planet, and tends to just hang around minding its own business unless its somehow disturbed by a speeding car, an electric fan, or your own vocal chords. Any sound is simply a disturbance in air. If we didnt have air, we wouldnt need ears (or noses or lungs, but thats another story).
Air is sort of like water, but a lot less wet. When there is a disturbance at some source point, like the sound and pressure of a pipe-bomb explosion or the faint sound of a pin dropping, that disturbance spreads in a wave from its starting point in all available directions in a sort of chain reaction. A molecule of air next to the source pushes out on the molecule next to it, which pushes on the next one, and so forth billions of times, transmitting the pressure or sound across space until it is felt or heard some distance away.
Air is good at this, but far from perfect. In the process or transmitting sound energy, for example, each molecule absorbs some small bit of that energy, and jostles its neighbor a little less hard than it was jostled itself. Thus a very low-energy sound like a pin dropping gets absorbed in a very short distance and is inaudible across the room. A nuclear explosion that incinerates half of
Baltimoreprobably wouldnt be heard by a farmer in Nebraska. The louder a sound is, the more energy it has and the further it travels through air, but eventually it is absorbed by the compressing air or hits something solid (a wall, perhaps, or a giant mutant crustacean) and stops traveling. How far it gets can be affected by barometric pressure and the type of sound it is, but technically the loudness of a sound in air decreases with the square of the distance from its source. If you slept through high-school algebra, just remember this: the further a sound travels, the quicker it fades. So if you want your sound to fill the room at a certain level, the output capability of your system must grow a lot faster than the dimensions of the room.
Sound also bounces off of things. When the moving air molecules hit something hard and compress, they tend to spring back and send the sound wave off in another direction. As with a shot on a pool table, the angle of reflection depends on the angle of attack. Hard surfaces reflect sound more efficiently than soft ones because they dont cushion the impact of the sound waves and absorb as much energy from them.
Youve probably heard the phenomenon of an echo bouncing off a distant mountainside. What you may not realize is that a good deal of the sound you hear every day is bounced or reflected sound from walls, floors, ceilings and objects in a room. The time delay between hearing the original sound and the reflected sound is not so pronounced that you identify the latter as an echo, but your ears and brain are very, very good at judging the source, distance and nature of sounds, and the size and type of room you are in, from the miniscule amount of this echo or delay. All this happens without any conscious thought on your part.
The ratio of original, direct sound to reflected sound affects both the tonal quality and the loudness of the overall sound you hear. In a small room with hard surfaces, the sound bounces around a lot before it dies out, sometimes adding a reverberant richness. Outdoors, with nothing to bounce off of, the same sound can seem thin and lifeless. The environment in which your system is used (small room, auditorium, outdoor park) can affect the final result in so many ways that the system may have to be set up or adjusted differently every time its moved to a new place.
Sound moves air in little pulses or waves, because its produced by something vibrating a guitar string, vocal chords, a clarinet reed or an exploding iguana. Those waves have one or more frequencies, meaning the number of back-and-forth vibrations in the sound source in a given time interval. The more vibrations in that time interval, the higher the pitch of the sound, so we call such a sound high frequency. The fewer vibrations, the lower the pitch (low frequency). We think of the deep sound of a bass instrument or a bat hitting a sack of flour as a low-frequency sound, and the screech of nails on a chalkboard or the squeak of a mouse as high-frequency. The most important sounds we hear, including specifically the human voice, fall in the midrange frequencies.
A standard measure of frequency is the number of vibrations a sound source makes in one second, called cycles per second, hertz, or simply abbreviated hz. Human ears can hear sounds as low as 20 to 30 hz (very deep bass) and as high as 15,000 to 20,000 hz (the tinkle of breaking glass, a sizzling cymbal). Sound frequencies keep going past human hearing capability, eventually reaching the point (ultrasound) where they can be used to clean your teeth. Low frequencies, below two or three dozen hz, are felt more than heard since they can move your entire body more effectively than your tiny eardrum.
In most sound sources people can stand to listen to, frequencies change constantly, as with vocal inflection for emphasis or a melody played by an instrument. And very few sounds consist of a single-frequency tone anyway. Sonic reflections from a pianos soundboard, the body of a violin and even the inside of a vocalists mouth introduce harmonics. These are small bits of accompanying sound, usually at some multiple of the basic sound frequency, which add timbre and character to the basic sound or note. Sounds reproduced without these sound flat or lifeless. And most musical material consists of multiple voices or instruments exploring various ranges of the frequency spectrum. So what we think of as sound can be composed of a vast number of different frequencies at once, all blended together.
We know that human ears only hear sounds in a given frequency range. Mechanical or electronic sound reproduction devices may or may not be capable of covering that entire range, and you may or may not want them to. Or they may be capable of reproducing sounds beyond human hearing, driving all the neighborhood dogs crazy while you sit there clueless. The frequency response of a sound-reproduction component simply refers to the range of frequencies it can reproduce. Large-diameter speakers (woofers) do a poor job of reproducing high-frequency sounds. Most microphones designed for vocal use are not good at reproducing deep bass notes, and can in fact be damaged by the attempt. Most purely electronic components in the sound-reproduction chain, however, can be designed and built to sense and amplify frequencies over a much broader range than you can hear. Thats because they simply push electrons around in circuits, and have no mechanical interface with the outside air as microphones and speakers do.
A PA system used only to reproduce the human voice doesnt need broad frequency response, because we can hear frequencies we cant make our vocal cords originate. For music, however, an optimum system frequency response would span the entire range of human hearing, reproducing the lowest audible bass notes as well as the highest harmonic overtones we can hear. Lots of PA systems, stereo and home-theater systems, and even portable music players can do that, at least at modest volumes or through headphones. But there is another piece to this puzzle.
The fidelity in high-fidelity means in essence that the original sound that goes into an amplified system comes out sounding exactly the same, only louder. The most important step toward this admirable goal is having a system that reproduces a wide range of frequencies, and doesnt emphasize one range of frequencies over another. Perfection in this area is sometimes referred to as flat frequency response, because if you imagine a graph with frequencies across the X axis and loudness up the Y axis, perfect fidelity would be a flat line across the entire frequency range.
Weve all heard sound-reproduction devices that fall far short of this, although we rarely describe them in terms of improper frequency emphasis. A cheap radio that emphasizes the high frequencies and cant pump bass notes from its tiny speaker is tinny. A guitar amp that emphasizes midrange frequencies sounds honky when voices or recordings are heard through it. An overdone car-stereo system in the car next to you at a stoplight sounds boomy. These are all just ways of describing uneven frequency response that is noticeable enough to be unpleasant.
While a good PA system should allow you to shape the sound to your liking, it shouldnt be tinny, honky or boomy all by itself. Within whatever frequency range its capable of, it should be able to track the original sound faithfully, and not add undue emphasis to any particular range of frequencies.
Tone and Equalization
Tone is probably the most misused word in the laymans sound vocabulary. Some people call a single musical note a tone. More frequently it will be used to describe the overall sound of a reproduction system, as in that radio has a nice tone. What is really meant by this is that radio has a good distribution of frequencies across the audible spectrum, or more simply, that radio has pleasant frequency response, along with other qualities that make it agreeable to listen to, such as a lack of distortion.
This is further confused by the fact that most reproduction systems have tone controls, which allow users to adjust the relative emphasis of one or more groups of frequencies to obtain an overall sound they like better in the simpler systems, this may consist of knobs for bass and treble. In complex systems tone controls are replaced by equalization controls that allow you to adjust frequency response in narrower, more frequency-specific chunks. Used properly, these can electronically adjust the sound to help make up for deficiencies in your sound, your speakers, or the acoustic environment (that is, the room youre performing in). Used improperly, these controls can make even the best systems sound, well, tinny, honky or boomy. As with most complex systems, the more controls and settings you have the better you can dial in the result you want -- but conversely, the more opportunity there is to misuse those controls with disappointing results.
You cant talk about sound without using words like loudness or volume to describe the amount of energy radiating through the air to your ears. But these are at best vague terms; sounds are louder or softer, we turn the volume control up or down. A decibel or db is a scientific, relative measurement of loudness.
The decibel scale starts at zero, which is total silence. Very soft sounds (a whisper, the rushing of a slight breeze) may be 10 to 20 db. Normal conversation between two people is in the 50 to 75 decibel range, depending on how angry they are.
The scale is logarithmic. Each increase of 10 db in the scale represents ten times the sound energy so an increase of 20 db means a sound 100 times more powerful, an increase of 30 db means a sound 1,000 times more powerful, and so on. So at the higher end of the scale, the energy of the sound wave rises very quickly. Up close, a rock concert can reach 120 db, which is sufficient to cause physical pain and eventually, hearing loss. We will use decibels or db in some articles on this site to describe changes in loudness. But its not a bad time to reflect on the fact that powerful amplified sound systems can actually harm your audience, and you.
This simply refers to the ratio between the loudness of the sound entering your system, or one of its parts, and the sound coming out. Thus gain is a measure of overall system power or amplification capability. Sometimes what you think of as volume controls are marked gain, because they affect the increase in decibels between the sound that goes into a source like a microphone, and the sound that comes out of the speaker(s). Since it is primarily the function of electrical amplification circuits to provide gain, their ability to do that is usually described in their specifications, in various ways discussed in articles on this site.
A signal is simply an electronic term for the sound you make, after it is converted into electrical current and proceeds through the sound-reproduction chain. When it comes out the speakers, its sound again.
This is a measurement, for each electronic component in the amplification chain, of how pure the output signal is compared to the input signal, or in other words, whether a particular electronic gizmo adds its own noise to the signal as it passes through. This ratio is expressed in decibels. If that ratio is, for example, 100 db, then the signal coming out is 100 db stronger than any noise or hash added by the gizmo itself, which is excellent. If it is lower than about 85 db, you will hear noise mixed in with the signal at high volume levels, and this is not so good. For an example of what I mean by noise or hash, tune an AM radio in between stations. If the speakers of your PA system sound like that when it is turned up without any input signal, youre listening to noise generated by your electronic components or some sort of external interference.
While noise is an unpleasant element added to a signal by an electronic component, distortion is a change in the signal itself due to shortcomings at various stages in the reproduction chain. At minimal distortion levels, the sound may not sound much different than it did going into the system. At higher distortion levels, the sound may become garbled, buzzy, strident, fuzzy, or whatever word might describe the opposite of clean or pure reproduction. Take that same cheap radio you used to listen to noise, wait until its playing a familiar song, and turn it up all the way. The sound is louder, yes, but it also becomes fuzzy and unpleasant.
All PA components distort the signal or sound to some degree, and the distortion becomes more unpleasant at the highest volume levels, just as with the cheap radio. What you want is a system that keeps unwanted distortion to a minimum, even at very high decibel levels, and carefully chosen components can achieve this. In other articles well refer to harmonic distortion (the addition of harmonic tones to a signal, a phenomenon that occurs inside electronic components), overload distortion (a very unpleasant type that happens when an electronic components input signal is too strong for it to handle), clipping distortion (also very unpleasant, and occurs when your amp is trying to put out too much power), and sometimes just generalized distortion from bad design, weak or damaged components, or pushing the entire system beyond its mechanical or electronic design limits. Sometimes distortion is intentionally introduced early in the signal chain, as when an electric guitarist uses a fuzz pedal to give an intentionally-distorted sound. But distortion is always an enemy in the rest of the sound system, whose job is simply to amplify whatever sound you feed into it.
This term is used in a confusing variety of ways, but generally signifies some sort of reserve capacity in a component or circuit that allows sudden bursts of signal (peaks) to be reproduced without distortion or other ill effects. Sound, and the signals representing sound in an electrical circuit, are very dynamic with loud and soft passages. You set an average sound level for your system, but if your system has adequate headroom, it will have the capacity to reproduce sudden, loud signals cleanly as well. The trick to intelligent setup is to keep a healthy, strong signal passing through your components, which optimizes the signal-to-noise ratio, while retaining sufficient headroom in the system that loud passages arent suddenly distorted. The way to do this is by overdesigning the system, so that with average signal levels it is loafing and will be able to respond to loud peaks without straining.
Like air, you know what this is. Its electricity, little moving electrons, and its the form your signal takes as it passes through the system. There are strong currents that are dangerous, and weak ones that are hard even to measure, and its the job of your system to take weak-current signals from a microphone or instrument and turn them into much stronger ones to drive your speakers.
AC and DC
Youve heard this one before, too. AC current is the stuff that comes from the power company and to the wall plug in a building, while DC current is what a battery makes. They are different in several important ways.
AC is alternating current, meaning it comes in pulses or changes its flow direction (alternates) several times each second. DC or direct current flows in a steady stream in one direction. There are characteristics of AC that make it the only practical choice for high-voltage current that has to travel a long way, like the distance from a power-generating station to the stage where you perform, so thats the stuff that comes out of the wall and provides the source of power to your PA system.
Another important characteristic of alternating current is that it can alternate or change strength and direction a variable number of times per second, which just like the pitch of a musical note, is called its frequency. The power coming from your wall outlet has a frequency of 60 or 50 cycles depending on where you live. A different sort of alternating current is the signal passing through your sound system and being amplified by it. Your system can tell what sound is imbedded in the signal by its frequency or combination of frequencies. This will become important when we talk about moving the signal through the system.
Direct current has no frequency because it only flows in one direction from the power source, through a circuit to do some sort of work, and then to ground. But DC is the preferred diet of many of the little doodads that make up your electronic components. So in any electronic audio component theres likely a signal path that carries the (AC) signal from place to place and a power path that converts wall-outlet AC current into low-voltage DC current and feeds all the transistors and ICs and diodes and similar stuff you dont want to know about, but which do all the work.
A conductor is just something that lets electricity flow through it, AC or DC, it doesnt care. Most metals are good conductors; plastic and wood are poor conductors, which is why youll never see an amplifier made entirely of polished teakwood or recycled 2-liter bottles. But these are good insulators, which keep current from going where its not wanted.
Resistance and Impedance
As current flows through a conductor, it is naturally weakened by its journey, just as sound tends to weaken as it travels through air. This is because both air, and metal wires and circuits, offer some degree of resistance to free movement. Electrical resistance, the resistance of a conductor to the flow of current, is measured in units called ohms, sometimes abbreviated with the Greek symbol Omega that looks like a little horseshoe.
There are instruments commonly available (ohmmeters) that can measure electrical resistance between one end of a wire and the other, or one part of a circuit and another part. Sometimes resisting the flow of current in a circuit is desirable, so a circuit may contain one or more doodads called resistors to accomplish this. More often, resisting the flow of current reduces the efficiency of the system, so the quality and size of conductors is important to getting the performance you paid for.
Impedance is a very close cousin to resistance, but related only to AC current and sometimes introduces a mechanical (i.e. non-electronic) element to the formula. Microphones and speakers, for example, convert sound into signal or signal into sound, and they are a combination of mechanical and electrical parts. Mechanical resistance to movement (pushing a big speaker cone around, for example), added to the electrical resistance of the conductors or circuits involved, are the components of impedance, which still has the effect of resisting current flow and can still be measured in ohms and represented by the Omega or horseshoe symbol, but sometimes is also abbreviated Z.
It is the job of every component in your system to either carry a signal from one point to another (wires and connection cables), or change that signal in some way, either to a stronger or different signal (electronic components) or from sound to signal or back again (speakers, microphones). In a perfectly efficient system, all of the electrical power supplied to the system by plugging the components into an electrical power source would be used to amplify the sound. In the real world, this never happens. Because of the resistance (and impedance) of all the components to the free flow of current, a good deal of the power youre using is turned into heat. Thats why parts of your system get warm or hot as you use them. Efficiency has many measures, but as used in various articles on this site it refers generally to the ratio of useful work done (i.e. sound amplification) to useless heat generated. More efficient components run cooler and use less power than inefficient ones.
This is a less generalized and abstract concept than the ones above, but well talk about it a lot, because its a giant problem with PA systems. When youre speaking or performing with a microphone, some of the sound coming out of your speakers finds its way back to the microphone (i.e., its fed back), with a slight time delay since its reflecting off some surface or traveling though air. Then, of course, it gets amplified again. If the sound is loud enough, and the environment is sufficiently bouncy, the cycle can be repeated many, many times in a split second. Each time it happens, the input sound has been amplified and enters the microphone with more energy. The eventual result, through a process called oscillation, is a very loud and piercing howl or squeal that is painful to anyone in range.
Certain types of microphones are more feedback-resistant than others, as discussed in the article on selecting microphones, and there are electronic gadgets than can help identify and control feedback as it occurs. But the only sure cure for feedback is intelligent microphone and speaker placement, to make it harder for the speakers sound to feed back into the microphone.
This is another attribute of microphones and speakers, since they couple your system to the air and the sound in it. Because they both have some sort of moving diaphragm to capture or emit sound, both tend to be most effective when sound comes into or out of that diaphragm head-on. Singing into the wrong end of a microphone, or sitting behind or to one side of a speaker, isnt very effective because these components are directional and work best when they are pointed right at you. When we talk about types of speakers and microphones, we will frequently refer to this directionality.
With these concepts lodged in that amazing brain of yours, I hope you'll be better equipped to understand some of the more technical discussions on this site, and some of the published spoecifications of PA equipment you might consider buying. PA systems and components come in an amazing array of types, capabilities, and configurations; in Part II we'll focus more on the hardware itself and how to better evaluate what you need.
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