Speakers

Transduce electricity back into sound energy. Speaks have greatest effect on sound quality. Speakers have a difficult task: * must accurately follow excursions from 20 to 20,000 Hz. * Must transmit many wavelengths simultaneously. Even with just one high and one low frequency, a speaker cone may move 1/4" back & forth and vibrate rapidly simultaneously. All this must be handled by a simple paper or plastic cone. Three working parts: Cone- moves the air Voice Coil- moves the cone Magnet: provides a magnetic field for the coil to react to. Driver- coil and magnet together. Full-range speakers also have: Dome: center cone reproduces high freq. better. Whizzer: miniature speaker horn built into cone. Also for high freqs. Narrower pattern than dome. SPEAKER SPECS Speakers are specified by: * Impedance * Frequency Response * Magnet size * Power capacity * and Cone resonance Impedance: DC resistance of Voice coil and AC reactance of the coil taken together. Typically 8 ohms, maybe 4 or 16 ohms. Speakers should be run at rated impedance for best sound and power transfer. Frequency response: determines what frequencies speaker handles best. * Tweeters: high frequency speakers, come in several types. * Mid-range speakers: are designed to handle mid-freqs; usually medium small cones. * Woofers: low frequency speakers, usually cone speakers. * Subwoofers: very low frequency woofers. * Full range speakers: theoretically handle the entire range Don't usually work as well as discrete speakers. Mid-range and Woofers: usually cone speakers. Must move a lot of air. Tweeters come in several types: Small cone speakers: Older type, 3 to 4 in. in diameter. Very narrow dispersal pattern. Horns: Sort of megaphone in front of a radiating diaphram; amplify sound mechanically. Exponential horn: sides spread at exponential rate. Shape of determines how fast sound expands and how far it is "thrown". Short throw and long throw horns possible. Horns can be used at any frequency, but must be sized to waves supported. A bass horn would be about 30 ft. long, so horns usually used for highs only for space. Straight horns work well for theatre work; for home systems horns are often folded on themselves. Dome radiator: More common in home than theatre. Much wider dispersion than horns or cones, usually wider than desired for shows. Full range speakers combine characteristics of all of these: May incorporate a dome, a whizzer, or both into cone to handle different frequencies. A Compromise speaker. Full range speakers are not as good at specific frequencies as are more discrete speakers. Magnets: Determine efficiency and power handling character of speaker. More powerful magnets have: --Better damping effect (how quickly it stops vibrating). --Better heat handling at high power. Magnet size gives a rough indication of quality. Magnets are the most expensive part. Most makers won't waste a big magnet on a poor quality speaker. Power rating: tells you how much power a speaker can absorb without damage. but NOT how much sound will come out. A high efficency speaker makes more sound with less power than a low efficiency speaker with a higher rating. Power rating helps to avoid overheating the voice coil, which will warp the coil and destroy the speaker. Speaker efficiency: measures a speaker's ability to change signal into sound. Ratio of Power In to Power Out Usually given in percent. Speakers are notoriously inefficient. * Small home speakers in closed boxes: less than 1 %. * Larger boxes are more efficient, about 5 %. * Horns are most efficient, about 30%. Efficiency affects how much power is need to reach a given sound pressure level. An Example Given a small room 15 ft. x 20 ft: You need: About .0005 W of accoustic power to reach a level of 80 db, fairly loud music. Given a home type speaker of 0.5%, an amp must deliver about 0.1 w. to the speaker. Easy. But, to reach 110 db, like a rock concert: you need a 30 bd increase, or about a 1000 times more power, or 100 watts from the amp. This is still only 0.5 watts of SPL, but to produce it, a lot more amp power is needed. More efficient speakers can do the same with less power. To produce the same 0.5 watts of SPL: * Large cone speaker at 5% efficiency needs only 10 watts. * A horn at 30% efficiency needs only 1 2/3 watts. How speaker manufacturers increase efficiency: Decrease resistance to speaker movement. Can be done by making the cone lighter. BUT This can cause a rougher response. A better way: use a larger speaker enclosures. OR Use a vented enclosure. Another tactic: Increase speaker compliance, how easily it moves, e.g. accoustic suspension or roll suspension speakers. BUT This increases lateral movement of the voice coil in magnet. Another way: Increase force on the voice coil by increasing the size of magnet UP TO POINT where excessive damping cuts off low frequencies. Efficiency then is complicated: The most efficient speakers will have: * A large enclosure * large magnets * good suspension compliance

Networks and arrays

Speaker networks So far, we have discussed single speakers. Difficult for a speaker to handle all frequencies easily well. Therefore, multiple drivers are used to cover the full spectrum. This IS more expensive and harder to design, but is warranted where: * A bass speaker is in a small box, and thereby loses high frequencies. That is why home systems often build woofers and tweeters into an enclosure * In systems with very large woofers, like theatre and concert systems. * Where very large cones can't reproduce high freqs. Multiple speaker systems are sets of speakers feed by a single signal line. Only lows should be fed to woofers and highs to tweeters. Crossover networks. Crossover network: paired band-pass filters. A major advantage: impedance of the speakers is unchanged. Two 8 ohm speakers fed by a crossover will still be 8 ohms. Network acts as a single speaker with multiple drivers for each freq. range. Amps sees only a single full-spectrum speaker. A crossover for a three way is more complex, but principle is the same.

SPEAKER ARRAYS:

A speaker array is another way to put multiple single speakers together. However: An array is multiple speakers acting as collection of speakers all doing the same thing. No crossover network. Each speaker sees what the others do. Essentially a single speaker multiplied several times. Speakers may be arranged in series, in parellel, or in series/parellel. Total imedance will be governed by Ohm's law as applied to series and parellel circuits. Theory to arrays is that, instead of using a single high power speaker, you can use several low power speakers. Power divided between them. May be cheaper. SO why build large speakers? When two speakers are arranged close together, they load each others cones at low frequencies. Efficiency is better above to cone resonance but Bass freqs. roll off bass faster below cone resonance. A single bass cone has better bass response than a bass array. Arrays also tend to have more limited dispersion than single speakers. Two speakers emitting the same signal placed close together tend to have their patterns interact. * In phase, they add, making the sounds louder. * Out of phase, they cancel, reducing sound. Two types of arrays: Square array: speakers arranged in a square pattern. Lineal or Column array: speakers arranged one above another. Square: Good reinforcement in front, and little side coverage. Column: Top and bottom cancelled but sides reinforced. Choice of array depends on frequencies involved. Woofers are best with square arrays. * Benefit from the increased bass loading of a square array * Bass is less directional than treble. * Narrower range helps focus the sound where desired. For tweeters and mids, vertical arrays work better. * Increasing horizontal spread and reduce vertical spread controls unwanted reverb. BUT * Some bass loading always occurs and bass response is less. Generally speaking, arrays work well for voice reproduction, esp. in very live halls like gymnasiums. Large single speakers or multiple drivers are better for music, having more accurate frequency reproduction. Line Arrays Current "State of the Art" in high efficiency systems. Composed of group of speakers that are arranged in a straight line Speakers are: * closely spaced * equal amplitude * operating in phase Result is a very directional and efficient projection of sound. Sound propagates in a cylindrical manner. Individual speakers become "close coupled" with adjacent speakers, i.e., act like a single speaker with multiple cones. In front: Wave pattern from each element adds directly with adjacent waves with little cancellation. To top and bottom: Waves cancel strongly. Line array projects a "beam" of sound to front. Level drops only 3 dB per doubling of distance rather than typical 6 dB. Things to note: * Directional effect strongest in base frequencies. * The taller the array, the higher the affected frequencies. * Only applies to vertical pattern of array; Horizontal dispersion is unaffected. * Line array effect doesn't work well with a small array. Line arrays are the current favorite for efficiency, but is not perfect. There is still some base roll-off, so separate sub-woffers are usually needed to fill in the bottom. 2000, Mick Alderson revised 2012


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Last modified: Dec. 11, 2002
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