Speakers and Amplifiers
Some Key Terms
Diaphragm: The part of the speaker that moved the air. It’s usually a visible cone, but can also be a metallic dome, a metal ribbon, bar magnets, or an arc comprised of ions.
Driver: What sits behind the diaphragm to move (“drive”) it back and forth.
Woofer: A woofer takes care of the low-end of a system. It can reproduce frequencies up to about 1kHz. A mid-range woofer might go up to 6kHz.
Subwoofer: Subs only handle frequencies up to about 200Hz (often only up to 80 or 100Hz in THX or pro systems). The diaphragm is usually from 3″ to 21″ and usually take more power than other drivers in the system. Even a subwoofer doesn’t usually don’t go all the way down to 20Hz.
Full-Range Driver: These are meant to reproduce the full audio spectrum, but are often supplemented by a subwoofer (or an additional subwoofer).
Tweeters and Horns: These take care of the highest frequencies. Since horns can achieve/handle a greater overall output, they’re more often used in pro sound and outdoor applications.
Crossover: The spot in the frequency spectrum where two drivers begin to reproduce the same frequencies. If a crossover isn’t calibrated properly, drivers interfere too much with each other (often creating a bump or dip in level at the crossover point). A 3-way crossover set up on a BSS BLU-100.
Cabinet: The complete enclosure with a speaker or multiple speakers installed.
Amplifier: What powers a speaker’s driver.
Deciding what speakers to go with has a lot to do with your venue. If you’re playing on a different someone’s back porch each week or reinforcing a public figure in a small room, a JBL EON ONE, or Bose L1 might be enough. If you want a great system for your high school auditorium, you probably want a couple line arrays; if it has to be portable, maybe two large subwoofer cabinets and four full-range tops. For small shows, done by a 4-piece band, maybe a couple full-range cabinets and a single sub is enough.
First, here are a few key specs to consider:
Frequency Response: This is the range of frequencies a speaker can reproduce. For full-range cabinets, this is usually in the range of 70Hz-20kHz. Subwoofers get closer to 30-250Hz. A combination of these two can create a great, cohesive sound.
Maximum SPL: Total sound output is measured in dB of sound pressure level. A regular, budget loudspeaker might be 120dB peak output, while a high-output cabinet might be >140dB SPL.
Power Rating: The number of watts a speaker will pull from its amplifier. Common loudspeakers are from about 250-1000W, while large speakers can be >3,000W. This isn’t as much a concern for self-powered speakers but is critical if you need to buy an amp to power your system.
Dispersion: How high and wide the sound dispersion is the dispersion. In a club where everyone is equal height, a super narrow-height dispersion might be ideal. If you have a single cabinet that has to reach people on the ground and others in the stands high above, a much taller dispersion is needed. In general, the more focused the dispersion, the more efficient the projection. For most large venues, line arrays combine lots of very narrow dispersion patterns into a single, controlled, well-projected sound.
Weight, Dimensions, and Visual Appeal: If you’re traveling, you may regret getting cabinets that throw out your back every gig. If you’re hanging them from a ceiling, the structure might not even support the weight. Some speakers are built to be more discreet, to match different spaces, while others are simply gorgeous.
A quick way to get a good idea of what will match your needs the best is to contact a professional. For small systems, visiting a nearby audio store is usually enough. For larger applications, a professional contractor is usually the best way to go. In Utah, Performance Audio, Poll Sound, Webb, and Marshall Industries are a few of the main players; each of these companies has done significant audio, video, and control system work for school districts, colleges, and businesses. If you would like a quick, informed opinion before going to an integrator, feel free to e-mail a picture of a space and the desired application to firstname.lastname@example.org.
Active vs. Passive Speakers and Selecting an Amp
An active speaker has its own amp, which is usually very well matched to the driver it’s powering. A passive speaker needs an external amp for power. We’ve gone over the advantages of each in another section. Selecting an amp can be tricky. Brands and product lines matter, since you don’t want an amp to fail during or before a show; any of the pro brands I’ve recommended will most likely work.
An amplifier is rated at peak watts and program watts. Peak watts is the maximum it can handle at a burst, while program watts is sustained. A good rule of thumb is to keep the total watts of the speakers below 80-90% of the program watts of an amplifier.
Amp transformers (the main component of an amp) are rated Class A through D, with A being the least efficient and generally the best-sounding. For studio use, Class A is preferred for its premium sound quality. For large sound reinforcement systems, Class D is preferred for its energy efficiency and reliability.
Amplifiers require a number of amps (units of current) to run. Large amps can require a special, 30A connector and some serious electrical infrastructure. Most small amps will plug into a regular outlet and run off a normal home circuit. However, this may cause interference from other devices in the venue. A power conditioner in the rack is usually used to avoid this problem; or if it’s a permanent system, it’ll usually have a dedicated circuit.
Variable Impedance (70V/140V) Systems
Most amps for music venues and home audio are 8-ohm amps (or 2/4/16-ohm) since it provides the greatest sound quality. Other venues, like a restaurant or car dealership, may use a 70V, distributive system. These work by keeping the voltage constant and varying the impedance. These provide the greatest driver protection, longevity, and efficiency.
The issue with an 8-ohm system is that the impedance is cut in half with every added speaker. If an 8-ohm amp only runs down to 4-ohm, then it can only handle two speakers total. This is fine for a home stereo, or an auditorium with an amp for every other speaker, but not for a school gym with 30 speakers. A 70V system could run all 30 speakers off of a single amp. Instead of worrying about how many ohms you’re dealing with, you simply add up the total watts drawn by all the speakers and make sure it’s less than what the amp can handle. As an added bonus, many 70V speakers consume very low watts to begin with (for example, 15-30W is common for in-ceiling applications).
Just a Little Math — Ohm’s Law and Watt’s Law
This is the basic mathematical formula that governs electrical power in audio. The three variables are current (in amps), voltage (volts across a conductor), and resistance (ohms of resistance to the flow of electrons).
Basically: I = V/R, or V = IR, or R = V/I. If you have two variables, you can calculate the third using one of the three variations of Ohm’s Law. Full-time audio engineers use Ohm’s Law on a regular basis.
Watt’s Law is similar. It shows the relationship between watts, volts, and amps. W = VA, V = W/A, A = W/V. For example, a 30A amplifier running at 120V circuit produces 3,600W; your laptop charger might be 4A at 19V, producing 76W.