Which driver is best for me, or:
Why you should just use the drivers in the plans
Before we get started, you should probably have a rough idea of what the parts of the driver are. Let's have some definitions:
Driver - an individual transducer. Usually part of a collection of other drivers installed into a cabinet to form a "speaker" or "monitor".
Basket - the framework the driver's components are installed in. Stamped steel and cast aluminum are the most common materials. Aluminum is lighter and is better at drawing heat away from the motor assembly. Steel is cheaper and less prone to breakage from a fall, but if you drop a driver you can pretty much write it off anyway.
Motor Assembly:
- Magnet - pretty self-explanatory. The most common magnet material is ferrite, sometimes referred to as "ceramic" since the iron powder is mixed with ceramics. Another more desireable (though more expensive) material is "rare-earth" or "neo", short for neodimium-iron-boron. It's lighter and is capable of exerting VASTLY more magnetic force.
- Base plate - the disk of metal that covers the base of the magnet.
- Voice coil - A copper coil wound around a "bobbin".
- Bobbin - Sometimes referred to as the "former". It's the cylinder of non-magnetic material that the voice coil is wound around. Common materials are aluminum, fiber-board (paper) and composites.
- Spider - the woven material that covers the area across the top of the motor assembly and is connected to the base of the cone on the inside, the base of the basket on the outside. It could be considered part of the piston assembly. It serves two purposes; protect the motor assembly from dust and act as a second surround to keep the voice coil and the base of the cone travelling in a nice linear motion.
- Leads/tinsel leads - The wiring that electrically connects the binding posts to the voice coil.
- Cone - Self-explanatory.
- Dust cap - the dome-shaped piece of material that is in the center of the cone.
- Suspension - the material that connects the cone to the basket. Common materials are fiber (paper), foam and rubber. Different materials have different properties that very with the application.
The plans that you will be using come from hours of high-level math, meticulous design to fold the horn and trial and error in building it. Though they do represent a tradeoff, the comprimises that have been arrived at lie at the intersection of pack size, sound level efficiency and cost and build difficulty. Even though most drivers of a given size look very similar, they can have wildly varying physical, acoustic and electical characteristics.
The way that these characteristics are quantified is using a set of measurements that are the culmination of research done by Richard Small and Neville Thiele of the Australian Broadcasting Commission. The ones that the BFM designs rely on the most are:
- Fs
- Qts
- Vas
- Xmax
- Vd
- Fs is the resonant frequency of the driver, and by extension the frequency it is most efficent at reproducing. This determines the driver's intended job; Subwoofer, woofer, mid-woofer, midrange, tweeter, etc.
Qts is the total resistance of the driver (mechanical and electrical) at its Fs, or its most efficient frequency. This measurement (when factored in with a number of other parameters) has an effect on the overall efficiency of the driver.
Vas is a volume measurement of air in liters, cubic feet or cubic inches that when compressed to a certain smaller volume (typicaly one cubic meter) has the same mechanical resistance as the driver itself. A higher Vas means you need a bigger enclosure. In the case of the horn designs, this impacts the size of the "horn enclosure", or "casing" which is the part of the cab the driver typically sits in. With any BFM design, if you pick a driver that deviates more than a few percent from the design requirements, the whole thing would have to be redesigned.
Xmax is the maximum amount of movement that the cone is capable of when being driven by a signal, not to be confused with Xmech which is the total throw the cone is capable of. Xmax is one of the determining factors in how loud the driver can be and how much power it can take. In a low-frequency driver, a higher Xmax is critical because lower frequencies require that you move a lot of air. Don't confuse this with Xmech, which is the total excursion the cone is capable. Beyond the Xmax, the motion of the cone becomes uncontrolled and the voice coil/bobbin can jump out of the magnet assembly on the outward stroke, rendering it unusable and probably damaging the driver. On the inward stroke it can strike the baseplate of the magnet assemble and dent the bobbin. Drivers with a lower Xmax are going to produce less sound pressure and have the additional risk of being overpowered.
Sd is the total piston area. It's the cone area (pi x radius squared) plus roughly 1/2 the area of the suspension depending on the type of suspension.
Vd
Vd is the total maximum amount of air that the piston assembly is capable of moving. To use a car analogy, the Sd is the bore, the Xmax is the stroke. This is useful in determining the maximum output of a driver. This measurement only comes into play when you have an amplifier powerful enough to supply the driver with all the power it can use, but it's about as good as it gets when trying to make an apples-to-apples comparison between drivers.
The role of efficiency:
A less efficient driver requires more power or more cabs. Since less sound pressure is produced, in the case of a design that is two or three-way you run in to problems where that driver can't keep up with the rest of the design and you have a weak low-end that is overpowered by the tweeter array, etc. For the most part, an efficiency mismach will impact the tonal quality and will require equalization.
Other important measurements:
- Re, or resistance. Measured in ohms and typically covers the range from 4-16 Ohms. This relates to the amount of voltage that must be produced by the amplifier. Higher impedance drivers require a higher voltage. See Ohm's Law if you're interested in the theory involved. It's junior-high level math that will teach you a lot about the relationship between voltage, current and resistance.
Pe is the thermal capacity in watts. Depending on the honesty of the manufacturer, this may or may not be a useful spec.
Power handling in RMS (similar to an average) and Peak.
Driver depth - a driver with too much depth will simply be too big to fit into the enclosure.
Additional reading
Thiele-Small Parameters:
http://en.wikipedia.org/wiki/Thiele/Small
Ohm's Law:
http://en.wikipedia.org/wiki/Ohm%27s_law
Horn Loudspeakers:
http://en.wikipedia.org/wiki/Horn_speaker
Drivers:
http://en.wikipedia.org/wiki/Speaker_driver
Amplifiers:
http://en.wikipedia.org/wiki/Audio_amplifier