Basics of Sound Systems
A MODEL OF A SOUND SYSTEM
In its simplest form, a sound system operates by converting sound waves (physical energy propogated by air or in some sort of medium) into electrical energy, increasing the power of the electrical energy using electronic circuitry, and then converting this resultant electrical energy back into physical energy in the form of sound waves.
If you have no idea of what we speak, go pick up a physics textbook and read the parts about sound and waves and air pressure and why it is more difficult to hear when you're in an airplane.
Devices that convert energy from one form into another are called transducers. Some examples of common transducers are loudspeakers, microphones, contact pickups, and headphones. Microphones and contact pickups (types of input transducers) convert fluctuating sound waves (physical energy in the air or in some other medium) into an electrical current that is an analog representation of the original sound wave. That is to say, if a large mass of air produced by a kick drum is picked up by a microphone, a large electrical current will represent that mass of air. Loudspeakers and headphones (which are mini-loudspeakers, essentially), converts the electrical signal back into physical energy.
Devices the actually amplify and change the characteristics of the electrical audio signal are called signal processors. In its simplest form, a signal processor increases the power of the electrical signal (coming out of the microphone and going into the loudspeaker). This sort of signal processor is an amplifier. Sound systems often include many more types of signal processors, which are used to effect change on any number of different audio signals.
Here are some examples of input transducers:
- Air pressure or velocity microphones:
convert sound waves traveling in the air into an electrical signal, which travels over, say, a microphone cable. A diaphragm, usually made of some very fine metal, vibrates within a magnetic field which results in an electrical current.
- Contact pickups:
convert sound waves traveling in a medium (wood, metal, skin) into an audio signal. Commonly found on instruments such as acoustic guitars, concert basses, violins, mandolins; sometimes found on pianos; sometimes used on dance floors to pick up tapping sounds...
- Magnetic pickups
convert fluctuating waves of induced magnetism into an audio signal, such as found on an electric guitar.
- Magnetic Tape heads
convert fluctuating magnetic fields recorded along a band of metal-coated plastic into an audio signal.
- Phonograph cartridges
convert physical movement of a needle, tracking analogous grooves in a vinyl, metal, or acetate plate into an audio signal.
- Laser pickups
convert patterns of pits and bumps imprinted on a DVD, CD, LD, or MD into a digital data stream that is then translated into an analog audio signal.
- Optical pickups
convert variations in density or light on a band of photographic film into an audio signal. This is how your local cinema projector works.
Here are some examples of output transducers:
- Loudspeakers: woofers:
designed to reproduce low frequencies. A cylindrical magnetic surrounds a cone made of paper (usually anywhere from four to eighteen inches in diameter); the paper is charged with the audio signal, and due to difference in magnetic fields, the paper moves in and out. A woofer loudspeaker works exactly in the same fashion that a dynamic microphone works, but backwards. Subwoofers are loudspeakers specifically designed to reproduce very low frequencies.
- Loudspeakers: midrange:
designed specifically to reproduce middle frequencies. Probably looks a lot like a small woofer.
- Loudspeakers: tweeters:
designed to reproduce the highest frequencies. There are several types of tweeters, including dome tweeters and compression-drivers.
full-range transducers designed to fit in or on the ears.
While this is by no means a complete list of transducers, it gives you an idea of what we speak. There are advances in loudspeaker technology, and developments such as a planar, or flat, loudspeaker, have come into use; however, the basic way in which the loudspeaker works has not changed.
Here are some examples of signal processors, and what they do. It should be noted, however, that the technical definition of "signal processor" is a device which alters the audio signal in a non-linear fashion; thus a simple level control or amplifier is technically not a signal processor:
- Mixing Console, Mixing Desk, Mixer, Console, whatever:
Whatever nomenclature you use, it's still basically the same thing. In its simplest form, a mixing desk takes more than one input (in its electrical form) and sums the signals together into more than one output (still in electrical form). A more complex example could be a desk used for live performance, with forty-eight inputs (vocal microphones, band microphones, etc.), and sixteen outputs (loudspeaker system on stage, loudspeaker system in the house, more loudspeakers in the rear of the house, a couple of tape-recorders, etc., etc.). The mixing desk is your central control station; this is where you make the vocalists louder than the band, or the violinist softer than the cellist.
- Equalisers (or Equalizers, depending on your preferred side of the pond):
Equalizers are units that affect certain portions of the frequency spectrum with a boost (gain) or cut (attenuation). From your electrical engineering knowledge, you know that a circuit that affects a certain portion of a given frequency spectrum is called a filter. You will see the word "filter" used quite often when describing bands of equalization. We'll go into more detail later.
Reverberation is defined as the phenomenon that occurs when sound waves are reflected and reinforced; this phenomenon occurs naturally in nearly all enclosed spaces. Imagine someone standing in the middle of a square, wood-paneled room- this subject claps his or her hands. You are inside the room, and hear the direct sound waves from the hands, and you hear, at a lower level, the sound waves that have been reflected by the wooden walls and subsequently directed towards your ears. This is reverberation at its most basic level. Every venue will have different reverberation characteristics; size, shape, obstacles, construction, etc., will all influence the way in which sound behaves once it is let loose in the room. Certain types of signal processors are designed to artifically create the effect of reverberation; originally designed to enhance sound recordings, making the recording sound "fuller," or "more live," it is also widely found in live sound systems to enhance the sound of a particular hall, or for creating other-worldly effects.
Delay and Echo should not be confused with Reverberation. While reverberation is the homogeneous sound image caused by multiple reflections, delay refers to one or more distinct sound images, repeated over time. Delay units allow for special, other-worldly effects when used in a recording situation; in live sound delay units are used to correct time differences between different systems of loudspeakers. (More will be discussed later).
- Compressors and Limiters:
Compressors and Limiters are signal processors that reduce the dynamic range of an audio signal. Essentially, a compressor and a limiter are the same thing; the difference in nomenclature merely comes from the actual, practical use of the unit. The compressor is designed to prevent audio signals from exceeding a given level-- in simplest terms, it keeps things from getting too loud. Limiters do the same thing, but more drastically. Television and broadcast systems use a lot of compression in order to fit their program material into a specified bandwidth; in practical live sound compressors can be used on vitriolic keyboards or unpredictable voices; many loudspeaker processors have a form of built-in limiter to protect the loudspeaker itself from damaging volume levels.
- Noise Gates:
A Noise Gate is a signal processor that turns off or significantly attenuates the audio signal when the signal level falls below a certain threshold. In practical application, desired program will pass through the unit unaltered, but low-level hiss and noise audible between sections of desired program will not be allowed passage. Noise Gates are used in recording situations; for instance when micing a drumkit a noise gate can be inserted on the snare-drum microphone and adjusted such that when other drums are played, the noise gates prevents the signal from passing, whilst when the snare-drum is played, the signal, which would be a higher volume level than the other drums as picked up from the snare-drum microphone, would be allowed passage. In live sound systems, noise gates are sometimes used as a sort of "automatic mute." Many corporate installations use noise-gates to automatically mute microphones that are not in use, which contributes to higher gain before feedback and a lower noise floor.
Expanders are used widely in tape noise-reduction systems and play a part in RF microphone technology. When an audio signal is recorded on audio tape, a compressor is used to limit the dynamic range of the signal because a very loud signal may cause distortion on magnetic tape. Expanders are then used upon playback to compensate for the compression used in recording. They are widely used in recording, and very seldomly used in live sound.
- Other Effects:
Phasers, flangers, and exciters are all types of other signal processing units; phasers (or phase shifters) and flangers operate in the same by introducing small amounts of phase-shift to one copy of the audio signal and adding to the original signal; it is a widely used effect in sound recording, but rarely used in live sound, unless one is looking for other-worldly effects. Exciters, originally introduced by the Aphex corporation, is a signal processing device designed to add more "punch" to the program material via the use of filters and equalizer circuitry. It has become popular in broadcasting, where it could be used to add "punch" without adding volume, and in dance-club sound systems, where listeners benefitted from an apparent change in level without distortion.
Amplifiers are a basic building block of electrical engineering. In its theoretical form, an amplifier takes an electrical signal and increases its power in a linear fashion- that is to say, the output waveform matches the input waveform, only it's more powerful. While the linearity of common power amplifiers is not necessarily perfect, that is essentially what a common power amplifier does. Power amplifiers are units designed to take an audio signal and make it powerful enough to drive a loudspeaker; due to the loudspeaker construction and its inherent inefficiency, a lot of power is required to produce sound. Other types of amplifiers used in audio include preamplifiers, which are small amplifiers that take an audio signal from a microphone and increase its power so that it can be better manipulated.
A BASIC SOUND SYSTEM
The illustration above illustrates a simple, practical sound system.
Note the three sections- input transducers, signal processing, and output transducers. The three microphones are connected to separate inputs on the mixing desk. On each input the mixing desk provides preamplification, which amplify the microphone level signals to line level; equalization, which provides the means to contour the tonal balance of each microphone; and level control, which allows the operator to adjust the relative level of each microphone individual. The mixing desk then sums the inputs to a single line-level output. The output of the console is connected to a power amplifier, which boosts the console's line level output to a level suitable to drive the loudspeaker (line levels typically run from 0.1 to 100 mW, whilst loudspeakers require 0.5 to 1kW, approximately). The loudspeaker converts the power amplifier output signal into sound pressure waves. The level of the sound is much higher than that of the three orators speaking unaided.
Every sound system is merely an extension of this basic model. The principles that apply to this simple model also apply to large-scale concert reinforcement systems.
THINGS TO REMEMBER
The environment in which the sound system is used can alter, both positively and detrimentally, the output of the system. In a free-field environment, such as on a field-hockey field, there are very few objects that will reflect the sound- trees, grass, and girls in field-hockey garb will tend to absorb sound, rather than reflect it. We can eke more level out of the sound system because we do not have to worry about reflected sound waves getting back into the microphone, causing a rather unpleasant ringing sound known as acoustic feedback. In a small room with wooden walls, we have to worry about reflections caused by the amplified sound bouncing off the walls and affecting the overall intelligibility of the system; we are concerned with the amount of gain we can get out of the system before the unpleasant feedback sound. In a small room with padded walls (heh heh), we are less concerned about intelligibility as the padding will tend to absorb sound rather than reflect it; rooms that are rather nonreverberant are usually termed "dead."
There are other factors to consider when designing and installing a system, including proper speaker selection and positioning, proper microphone selection and positioning, and proper tuning or equalization of a system in a given room. And of course there are issues on ease-of-operation and the system's intended use-- whether for music, speech, theatre, or playback. With this handbook, we'll try to provide you with enough knowledge to make an educated decision about your sound system.
Return to the Sound Index. Continue with Input Devices.
Comments, Questions, and Additions should be addressed via e-mail to Kai Harada. Not responsible for typographical errors.
http://www.harada-sound.com/sound/handbook/basics.html - © 2002 Kai Harada. 30.09.02.