Loudspeakers are transducers they convert electrical energy into sound pressure waves. Loudspeakers are the last device in the sound system chain over which the sound  designer has full control. With the exception of room characteristics and acoustic treatment, once the sound wave leaves the loudspeaker  cabinet, it's all up to the audience to actually listen, and it is up to the designer to properly utilize loudspeaker equipment to  provide an even listening pattern to as much of the venue as is humanly  and technologically possible.

Most loudspeaker cabinets and their components share similar design  characteristics. Because of the large range of frequencies that are  needed to properly reproduce the audible range of sound, in most cases, more than one component is used. A loudspeaker component, called a driver, that reproduces the lower frequencies of sound is called a woofer, while a component that is dedicated to reproducing higher frequencies is called a tweeter. Midrange driver is the name given to a component that is designed to reproduce the middle frequencies. Super tweeters are sometimes used to reproduce very high frequencies; there are some designers who use them in theatrical reinforcement but they are  primarily found in old, 1970s-era night club sound systems. At the  opposite end of the frequency spectrum, sub woofers or sometimes sub-bass cabinets reproduce very low frequencies, down to the point where the listener feels the sound rather than hears it.

Because each component is designed to reproduce a different portion  of the frequency spectrum, their construction differs slightly, but all  follow a basic principle: a cone of paper or paper-like material is suspended within a frame, and connected to the loudspeaker's input. A permanent magnet surrounds the center of the cone, and as alternating  current is applied to the loudspeaker cone, changes in the magnetic  field potential causes the cone to move away from or towards the  permanent magnet.

Since low frequency sounds have a larger wavelength, low-frequency drivers tend to be larger more mass is needed to convincingly provide  enough air movement at the given frequency. Common sizes of woofers range from four inches in diameter to eighteen inches in diameter.  Diameters above this size generally become too inefficient for  real-world applications due to the amount of mass in the speaker cone; loudspeaker engineers can provide two drivers side-by-side to move a greater volume of air without sacrificing efficiency.

Loudspeaker characteristics

Most designers use off-the-shelf loudspeaker systems which are usually a wooden box with a woofer or two and a tweeter or two. Off-the-shelf loudspeaker systems come in many shapes, sizes, weights, constructions, and even colours. The more important characteristics of a loudspeaker are frequency response, power handling, and coverage angle, although their physical shape and electrical impedance play significant roles as well. The designer can specify the proper type of cabinet given its application whether it is a loudspeaker dedicated to  reproduce music, play sound effects, reinforce vocals, hang underneath a balcony, line the front of the stage, or some combination of uses.

Frequency response is the range of frequencies a component or system of components can faithfully reproduce with very little deviation. A loudspeaker cabinet, comprised of a woofer and a tweeter in a box, may tout a frequency response of 47 Hz - 15,000 Hz, 3 dB. These numbers mean that the speaker will happily reproduce all frequencies between 47  Hz (very low) and 15 kHz (very high). It doesn't necessarily mean that  the loudspeaker cannot reproduce frequencies above or below the specs,  but the performance of the speaker at higher or lower frequencies may  suffer. Manufacturers often provide a frequency response graph, detailing the response of the speaker from 20 Hz - 20 kHz, showing peaks and valleys as the loudspeaker's response varies. These calculations are usually made with a measurement microphone on-axis with the  loudspeaker in a free field at a distance of approximately one meter and an input signal such as pink noise (all frequencies with equal energy  per octave). These numbers, then, are the best-case scenario of the  speaker's performance.

Another important measurement is the speaker's coverage angle. Although a loudspeaker has a front and a back and generally it will sound better when the listener is in front of it, just how much to the left or the right can we go before the loudspeaker starts to lose its  efficacy? Since low-frequencies have a long wavelength, they are able to penetrate walls and bend around obstacles. Many engineers consider a low-frequency cabinet omnidirectional and this is mostly true; turn up the bass on your home stereo and stand behind the loudspeaker  listening to the low frequencies. High frequencies behave in a different fashion. Because of their shorter wavelength, the sound waves dissipate quicker and are restricted to a given field of focus. Loudspeaker  manufacturers thus provide coverage angle data. A loudspeaker may exhibit a 40V x 90H coverage pattern. This measurement indicates that, using the centre of the high-frequency component as the 0 axis, the loudspeaker can effectively reproduce its touted frequency response  20 above and below the 0 axis, and 45 left and right of the 0 axis.  Rotating the speaker 90 will of course mean that the speaker will be effective 45 above and below 0, and 20 left and right of 0. Coverage angle helps designers choose loudspeakers. If multiple loudspeaker  cabinets, fed the same or essentially the same input signal, both cover a given area, phase cancellation in the form of comb filtering will occur due to differences in arrival times at the listening position. The result? Bad sound. Using time-alignment and level adjustment, it is  possible to alleviate these problems and create a good sound image for  the listener, but that's for another chapter.

Manufacturers often also provide a polar graph of the  loudspeaker's response, which uses both the frequency response and the  coverage angle. The polar graph is circular, with 0° being the axis point (centre of loudspeaker, usually), and response versus physical location can be easily seen; as we travel 45 degrees away from the  axis, we notice the horizontal coverage getting worse and worse; at the  rear of the loudspeaker, high frequency coverage is at a minimum until  we circle back around, approaching 315 (-45).

Loudspeaker placement

Speaker selection should depend on the type of production and also the type of program material one is planning to use them for.

Usually, full-range non-processor-controlled cabinets will suit music reinforcement fine. They should have a very wide frequency response (especially in the low-end if you are doing somewhat of a rock  musical), and also look for high efficiency ratings.

For surround-sound or under balcony fills, small monitors work well. Many small studio monitors will work well as reinforcement speakers - just be careful not to overload them with  excessive program material. Another feature to look for is mounting options or rigging points. Speakers cabinets with no rigging points will not take well to having holes drilled in their cabinets. This is very  very dangerous. As a last resort, build strong frames out of metal to hold them, and attach rigging points to these. Remember the safety ratio of 5:1-- if a speaker weighs ten pounds, the rigging materials should be rated for at least fifty pounds.

For strong low-frequency response, for dance-clubs or loud music reinforcement/playback, check out sub woofers. For a theatrical effect,  place sub woofers under seating platforms or even in the plenum below.  Some dance-clubs  install sub woofers underneath the dance floor. Some even have systems  that vibrate the dance floor in time to the music. Weird, but cool.

Proper location of speakers is also key in the whole design. A vocal cluster should not be located twenty feet upstage and slightly to the  right. Make your decisions quickly on speaker placement and let the lighting and sets people know them early in the game. [You don't want to have to let the sets people move your speakers upstage an  hour before the house opens and find that they have broken the Speakon  connector on one of the main cables. It can happen.] The centre vocal cluster, if there is one, should be located, as the name implies, in the centre. The idea is to cover the audience as equally as possible--  moving the vocal cluster to the left or right without compensating equally on the opposite side will make for some interesting reflections  in the house and will most likely not cover the house equally. The house left and house right stacks should be hung, or stacked,  equally in the vertical plane. Otherwise, holes in frequency response  may occur in certain parts of the house. Check out the dispersion angle characteristics of each speaker and align them according to that... or simply listen to them in different parts of the house and align them that way. If a surround effect is your goal, place the speakers according to that goal. Try to balance the house between left and  right-- don't have lopsided design for reinforcement.

Delay units

Delay units can play a very  significant and important role in sound reinforcement systems - especially for theatrical sound systems. Remember that the goal of the  theatrical reinforcement system is to not be heard. Judicious use of delay will aid in not being heard. Concentrating on vocal clusters, this is the theory behind using a delay unit: In 1947, a man named Helmut Haas performed some tests into delayed sound and how we hear it. Haas found that if a human was listening to two sound sources of the same material, and one was slightly delayed with respect to the other and also slightly louder, the ear would associate the direction of the sound source to the sound it heard first, not the sound it heard loudest. However, this only works within a window of 10dB. If the  second, delayed speaker is 10dB louder than the first, non-delayed speaker, the effect starts to wane. This is known as the Haas Effect. Thus, the goal is to arrange the system so that the  delayed, amplified sound arrives after the direct sound, thereby  focusing the audience's attention on the real source.

By inserting a delay line before the amplifier to a vocal cluster, we can try to achieve what Helmut Haas proved. By delaying the cluster  from 10ms to 30ms, we can shift the audience's perception of the sound  source to the stage itself, not to the vocal cluster. This effect is limited by the sound coming from the stage; often if actors are miced  well and sound natural while facing front and suddenly turn around to  face the back wall (upstage), a sudden shift in perception is noted as  the original sound (their voice) is not being heard clearly.

Another application for the delay unit is in venues with balconies.  Underneath the balcony, intelligibility from the centre vocal cluster will diminish, since there is a distinct shadow caused by the balcony  overhead. Thus, insertion of small speakers with good high- to  mid-frequency response is necessary to improve intelligibility. However, without the use of a delay line, sound will come from these small  under balcony speakers first - which will confuse if not irritate the audience. Insertion of a delay line will fix this problem - by delaying the under balcony fill speakers, sound will seem to "appear" from the  stage first, and the fill speakers will provide for improve  intelligibility.