INTRODUCTION

So far, we have covered input devices and signal processing, getting an acoustic sound wave into electrical energy and subsequently manipulating it. But how does it come out? For the recording professional, the final medium is magnetic tape or magneto-optical disc; for those in sound reinforcement, we need to concentrate on loudspeakers and amplifiers. Amplifiers can technically fall into the "signal processing" category but for the sake of clarity we'll put them in this section.

MAKING IT LOUD

Amplifiers are by definition electronic devices that take an existing electrical signal and make it bigger— hence the name, "amplifiers." Amplifiers exist in all sorts of electronic circuits, from the internal devices within some microphones to the input channel on a mixing desk, but the really big ones, called power amplifiers, take an analog line-level audio signal and convert it to something that can properly cause a loudspeaker to vibrate, and thus, make noise.

Ideally, the amplifier shall not introduce any nonlinearities into the audio waveform; that is to say, a comparison of the input signal and the output signal of an amplifier should be exact except in terms of amplitude— the amplifier makes the hills and valleys of an amplitude-versus-time graph bigger, but the waveform shape should not change. That is the scientific ideal, at least, but in the real world amplifiers can add slight variances to the audio signal.

Slew Rate refers to the time it takes an amplifier to sweep from one state to another. A human analog is that fuzzy time in between waking up and getting that first sip of coffee— from a state of relative inactivity to a state of being fully aware. In an ideal universe, the slew rate of the amplifier should replicate exactly the slope of the input signal, but it doesn't, and amplifier designers take into account the amount of time the amplifier needs to really wake up.

What else should I say?

LOUDSPEAKER BASICS

Loudspeakers are transducers— they convert electrical energy into sound pressure waves. Loudspeakers are the last device in the sound system chain over which the 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.

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. Study your physics textbook, and you'll figure it out.

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.

LOUDSPEAKER CHARACTERISTICS

Frequency response, shape, impedance, powered, unpowered, coverage angle.

LOUDSPEAKER PLACEMENT

Coverage angle, intended coverage, system designs

Speaker selection should depend on the type of production and also the type of program material one is planning to use them for. In the past, the standard for music program reinforcement was the Altec "Voice of the Theatre" A-7 cabinet. One can probably still find them in old movie theatres and even in stage theatres. Old '70's rock concerts used A-7s. These are very nice cabinets. They are also behind the times.

"Processor-Controlled" systems have taken the lead in critical reinforcement situations. Processor-Controlled systems involve speakers and a specially-designed-for-a-specific-speaker processor unit, which has all sorts of neat equalization and phase circuitry inside of it that will make its corresponding speakers perform the best (they also include overload-protection circuitry to make sure you don't damage the drivers). Apogee, Meyer, Bose, and Electro-Voice all manufacture processor-controlled cabinets. Apogee, Meyer, and E/V to an extent, have taken the lead in theatrical reinforcement cabinets. The two industry-standard vocal processor-cabinets are the Apogee AE-5 and the Meyer UPA-1C, and Electro-Voice's Delta-Max 1122, to a smaller degree. They all look very similar. They are wedge-shaped, which allows clusters of them to be flown or otherwise placed. These are very clean-sounding cabinets with excellent high- and mid-frequency response. Their low-frequency response is a bit smaller since they employ twelve- to fifteen-inch woofers; to compensate they are designed to be used in conjunction with specially-tuned proprietary subwoofer systems. The AE-5s and UPAs are excellent for vocal reinforcement. Apogee, Meyer, and Bose all manufacture smaller processor-controlled systems-- these can be used for under-balcony fills, surround, or for reinforcement/playback in smaller venues.

These same speakers can be used, in conjunction with their corresponding subwoofer units, for music reinforcement. Meyer has a complete line of units for music reinforcement; if I knew what they were, I'd list them; Apogee also has a complete line, and if they'd send me stuff like I keep asking them to do, I'd list them, too. Using processor systems is ideal, but not necessarily economically viable for small-budget amateur shows. Usually, full-range non-processor-controlled cabinets will suit music reinforcement fine. Cabinets made by JBL, EAW, or E/V should work very well, although they may sound a little less "clean" than a processor cabinet. Look for 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 underbalcony fills, check out small monitors made by Yamaha, JBL, or Bose. 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 (learn to arc weld; it's fun) 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 subwoofers. For a theatrical effect, place subwoofers under seating platforms or even in the plenum below. For more information, check out THX Systems or Dolby Surround Systems or Sony Digital Surround Systems used in movie theatres. Some dance-clubs install subwoofers 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 two linesets 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 center vocal cluster, if there is one, should be located, as the name implies, in the center. 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.

There are, however, designers who don't necessarily use the directional characteristics of speakers in their reinforcement designs (Martin Levan comes to mind). Use some creativity to achieve the sound you want. All this text is meaningless without trying and seeing if it works or not...ke the high-impedance -10dBV unbalanced signal and convert into a low-impedance microphone level balanced signal.

Continue to Sound Reinforcement. Return to the Sound Index.

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/speaks.html - © 1999 - 20002 Kai Harada. 19.05.2002