The electrodynamic loudspeaker is the most common type used, invented in 1925 by Rice and Kellogg.
A construction process which is still used today in low-cost loudspeakers is applying steam and pressure to paper. The more common approach is felting. Most paper-coned loudspeakers are felted. A screen mold is made in the shape of the desired cone, and wet pulp is pressed into it. By varrying the pressure, heat, and amount of pulp, many different cones can be made from tweeters to subwoofers. Paper cones can change weight in relation to humidity, pompting cones to be made of different materials. Polypropylene cones are a common type used, which are not affected by humidity and have flatter frequency response, however it is difficult to bond the surround material to the polyproplyene cone. Another disadvantage is the low melting point of polypropylene, which makes it unsuitable for continous high power use. Kevlar and Carbon-fiber cones are popular in that these materials are extremely light and stiff, and they have a flater response than paper or polyproplene.
There are two main surround types used today, half-roll and accordian. Half-roll is a commony used in home speakers, but high excursion can pull the cone to one side. Accordion surrounds are generally made of cloth, and are used in high-powered audio setups. Accordion surrounds can move more than half-roll, but at the surround resonance dips in frequency response can occour. Butyl rubber and foam surrounds are used to aleviate some of the problems of paper surrounds. Foam surrounds do disentagrate after about 30 years, but they are light and allow high excursion. Rubber surrounds are heavy which reduces effencency, but they do not disentagrate. On a sidenote, the Qms is generally a measure of friction losses from the suspension, with values from 1-5 indicating a surround that will control a cone well.
The spider is needed to center the voice coil in the mgnetic gap. Spiders are generally made of cotton and steam-pressed. The spider can intoduce noise into a loudspeaker system from air moving through its holes.
The magnet is the most expensive part of a loudspeaker system. In general the heavier the magnet, the lower the Qes of the loudspeaker. A high Qes will dissipate less energy from resistance of the voice coil. This does not mean that high Q coils are better. A low Qes will have better control of a loudspeaker at low frequencies. Thus one can infer that a low Qes from about .4 and below is best suited to a vented box because of the better control at low frequencies, and .4 and above is suited to a sealed enclosure because the air will act as a restoring force instead of the coil.
Voice coil and impedance
There are different ways to make voice coils, egewound, non-egewound, and multi-layer. Egewound coils can fit more turns of wire on a voice coil since the wire is flattened and wound on its edge. Standard coils are just circular wire with no special properties. Multi-layer are generally used in subwoofers where two channels are to come out of one speaker. The disadvantage of this and the resulting unsuitablility for small speakers is the increased inductance which reduces high frequency output.
ENCLOSURE TYPE.Acoustic suspension enclosures are airtight and utilize a loosely-suspended woofer, using the stiffness of the air in the sealed enclosure as an acoustic "spring" to produce extended low- distortion bass from a compact enclosure. A bass-reflex or vented enclosure channels the rear radiation from the woofer through a vent or tuned port in the cabinet to reinforce the bass frequencies from the front of the woofer. Another variant of the ported enclosure is the passive radiator - instead of a port, an undriven woofer cone is activated by the woofer's rear radiation, augmenting and smoothing out the low bass output. Others variations include horn-loading (very efficient); and transmission line (deep bass, but insensitive). Electrostatic and ribbon speakers use a thin, flat diaphragm of plastic or metal foil, respectively, sometimes combined with a conventional dynamic woofer to augment the low bass. These are often dipole radiators, projecting as much energy (out-of-phase) to the rear as they do to the front. Center- channel speakers are for dialog and music reproduction in surround-sound home theater application. They are shielded so they won't interfere with the TV picture when they are placed above or beneath a TV monitor. Satellites tend to be smallish loudspeakers with a midrange and tweeter, which are often paired with a separate subwoofer, for the low frequencies, thus forming a sat/sub system. Additional satellites may be used for the rear channels of a home theater surround-sound system. The surround speakers may be of dipole or bipole design to encrease the envelopment of the listeners in the surround sound field.
- What should I listen to when evaluating speakers?
Do not spend your valuable listening time switching between a dozen pairs every 3 seconds. If you are shopping at a quality store, the dealer will, from the description of your room, your size requirements, your musical tastes, and your budget, be able to show you a couple of pairs that will be close to what you want. Spend several minutes listening to each. When you think you're close, don't be embarrassed about spending half an hour or more listening to the speakers. You're going to have them in your home for a lot longer, and many speakers will cause "listening fatigue" after a short time. Make sure you really like them before you hand over money.
One thing to try is well recorded "Spoken Word" records; most people have a very good ability to tell when a speaking voice sounds unnatural, even if they've never heard the person speaking live. If you play an acoustic instrument, find something that features that instrument solo, or in a small group; make sure it really sounds like it should. Almost everyone has heard a live piano. Piano can be very revealing.