You want a speaker that sounds great and that starts with picking the right materials for each part of the speaker. You’ve got to consider what to use for the voice coil, the magnet, the basket, the cone (or diaphragm), the grill, the suspension elements, and the enclosure.
The voice coil, operating inside a permanent magnet field, is what changes the electrical energy fed to a speaker into mechanical energy and makes the cone vibrate back and forth to create soundwaves. A voice coil is typically made of copper wire wound around a bobbin. The bobbin can be made of paper, aluminum, nomex, polyimide film, or fiberglass. One thing to consider is how well the voice coil material dissipates heat. Only a small fraction of the electrical energy that goes into a speaker is transformed into sound, most becomes heat that can damage a speaker or hurt sound quality if it’s not designed right.
As alternating current electricity is input into the voice coil it turns it into an electromagnet with alternating magnetic polarity, which then is attracted or repelled relative to the pole of the permanent magnet attached to the speaker cone. There are three basic material choices for that magnet. Ferrite is the cheapest and most common in use today. It’s also the heaviest, which can impact a speaker’s performance and overall weight. Neodymium, a rare-earth metal, is more expensive but since less of it is needed, the speaker will be lighter. A third magnet material is an alloy of aluminium, nickel, and cobalt called alnico. Alnico magnets have been used in loudspeakers for decades, but it is most often used in guitar speakers today.
All of the parts used to build a speaker are in some way attached to a structural frame commonly referred to as the speaker basket. The vast majority of speaker baskets are made from either stamped steel or cast aluminum. Cast aluminium is typically used in larger speakers, but the decision on which metal to use will be driven a lot by the amount of magnet weight the basket will be supporting, how strong it has to be, and front of speaker cosmetics.
Attached to the voice coil and anchored on the flange of the speaker basket is the speaker cone. The cone must be made of a material that is rigid yet lightweight because it acts as a piston which vibrates soundwaves at frequencies from as low as 10 cycles per second on a subwoofer to as high as 20,000 cycles per second in a tweeter. The cone material must also have damping qualities. That means it must stop when the music stops and it doesn’t keep “ringing” with extra sound. Usually a dust cap is placed over the voice coil and on top of the cone. This functions a part of the cone but keeps the area inside the voice coil free from external debris. It is often an important part of high frequency propagation.
Cones can be made from specially coated paper, polypropylene (plastic), impregnated cloth, metals like aluminum, or composites like carbon fiber. Each material has different advantages and disadvantages and there are trade offs around weight, resonance, cost, and durability with each. For instance, do you want a speaker that handles really low frequencies well? Then aluminium or a heavy paper or polypropylene may be your best choice because it won’t flex with the force of those soundwaves. Midrange and full range speakers often use paper cones. The Yamaha NS10 speaker, and its bright white paper cones, has been a mainstay in recording studios for decades because sound engineers love them and their look has become iconic.
Speaker Suspension Elements
The moving parts of the speaker - the cone and voice coil - must be held in a way that moves pistonically with a minimum of lateral movement that can cause rubs or spurious noises.
There are generally two suspension elements in a speaker. The most visible one is attached to the cone body on the inside and to the basket on the outside. This is called the cone surround and is often made of cloth, various types of rubber, or foam. Sometimes it is actually a molded extension of the cone body and made as one piece with the cone. In addition to being a lateral stabilizer, it also helps determine the compliance of the speaker, which in turn determines what type of a speaker it will be. A low frequency speaker typically has a high compliance surround while a higher frequency speaker has one that is stiffer.
The second suspension element may be unobservable when a speaker is mounted in it’s enclosure. This element is often called a spider (because of its arachnid-like appearance in the early days of loudspeaker design). It too helps to laterally stabilize the moving elements and provides additional stiffness as needed. Spiders are most often made of phenolic treated thin pieces of cotton or poly-cotton which are pressed into corrugated shape to allow for a spring-like motion.
The grill is what covers and protects the cones against things like dust, moisture, heat, cold, or debris. The grill also gives the speaker a lot of its aesthetic qualities. Acoustically, it would be ideal to not have anything in front of the cones, but that’s usually not possible. Whether it’s made of plastic, metal, or fabric, it’s important to test how a grill impacts the quality of the sound.
The other big acoustic (and sometimes aesthetic) decision is picking the right enclosure for the speaker. Most are made from wood because it is light, durable, and can be finished to look like almost anything. Poplar and Baltic Birch (a kind of plywood), and Medium Density Fiberboard (MDF) are the most common. Plastic is an option if you need something small, yet durable against moisture or scratches. But it may not have the deadening qualities of wood and overall sound quality may suffer if a plastic enclosure isn’t designed right.