Loudspeaker Basics --
Part 2
In the first part of Loudspeaker
Basics, we talked about different loudspeaker types and those parts of the loudspeaker
drivers that are visible: the dome/cone, the rubber surround, the frame and the central
dust cap. Today we'll take a look at those driver parts hidden inside the cabinet, to
better understand how a loudspeaker works.
The most common loudspeaker type is the direct radiator,
which does not use a horn between its moving parts and the air. The most common type of
direct radiator is the dynamic driver, which typically uses a cone to move the air.
The cone is attached to the voice coil. The voice coil moves in reaction to the
current changes in the magnet assembly that are caused by the audio signal from the
amplifier. As the voice coil moves back and forth within the magnet assembly's magnetic
field, it causes the cone to move. This motion generates air pressure waves, which is
another way of saying it generates sound. The surface area of the driver
diaphragm determines how much air it can move, and accordingly what frequencies it can
reproduce.
Different size drivers are therefore needed to reproduce
the full range of musical frequencies. Since bass-frequency sound waves are much longer
than high-frequency sound waves, they require more surface area on the part of the driver.
That's why woofers are usually about 8-15" in diameter while midrange drivers are
only 3.5-6.5" and tweeters 0.75-1". A good rule of thumb is, the smaller the
driver, the higher the frequencies it can reproduce -- the opposite is also true, the
bigger the driver, the lower it goes. A designer might opt for multiple smaller woofers
instead of one single large one. Perhaps he wants to keep the front baffle narrower or to
take advantage of the added control of three individual magnets. Still, the combined
surface area of the smaller woofers will duplicate that of a single large woofer.
The size and geometry of the driver surround (the
circular rubber or foam ring that affixes the cone or dome to the frame) determines how
far the driver can move outwards. This movement is usually measured in fractions of an
inch. In certain specialized, high-output bass drivers, it can approach an inch or more.
The driver's range of motion is called its excursion. An example of a product that
uses wide excursion to compensate for small driver size is the famous Sunfire True
Subwoofer. In order to keep the driver small, but still have it play very low and very
loud, Bob Carver had to design a woofer with a lot of displacement. Hence its very wide
rubber roll-surround.
The driver surround limits how far the driver can move past
its resting position in either direction. The restoring force in a loudspeaker driver is
its spider -- a circular piece of fabric with multiple pleats, which holds
the speaker's voice coil in the magnetic gap. The spider acts like a spring that returns
the voice coil (and hence, the driver) back to its resting position. (The name comes from
the early days of audio when small plastic bands, said to resemble a spider's legs, were
used.)
Most midrange/woofers' cross-sections look like a cone. In
the middle, the dust cap usually closes off the bottom of the cone -- and for once,
the name means just what it says, it keeps dust from falling onto the voice coil. This
dust cap can either be flat or convex, like a little dome. Some drivers don't use a dust
cap but a phase plug. This is usually a bullet-shaped wave-guide, whose specially
contoured geometry is designed to focus the radiation of the sound waves. It protrudes
centrally from the driver but isn't affixed to the diaphragm.
Attached to the end of the cone (invisible from the
outside) is the voice coil. This is thin wire (usually copper or aluminum) coiled
around the driver in one or two layers and stabilized by a former. The entire voice
coil assembly of the driver is surrounded by a magnet attached to the end of the driver
basket. A driver basket (or chassis) is the assembly of inner and outer ring frames
connected by the ribs. From the outside, all you see of the basket is the round frame
screwed into the loudspeaker cabinet. Behind that frame ring are internal ribs. They
connect the outer visible ring frame to a smaller inner ring frame to which the magnet
is mounted. The magnet and voice coil are sometimes referred to as the driver's motor.
In upscale loudspeakers, you will often see a reference to
die-cast driver chassis (generally made from aluminum or magnesium). If a
loudspeaker spec sheet lists a die-cast chassis, it's a sign of quality. It's cheaper to
stamp a less rigid chassis from thin steel than to cast a thicker one from a mold. With
modern production techniques, some manufacturers are now using synthetic resins for their
driver chassis.
Driver image courtesy of Paradigm Loudspeakers.
Tweeters almost never use cones these days -- more
typically they are hemispherical domes formed from stiffened fabric or thin metal. Yet
they operate according to the same dynamic driver principles: The dome is attached to a
voice coil, which in turn, is controlled by a magnet assembly. The advantage of the dome
is that it has a relatively smooth frequency response and has better dispersion
characteristics than a cone (that is, the sound radiates out from the dome in a wider
pattern). With all these advantages, why aren't domes used in place of cones at all
frequencies? Domes just aren't all that easy to make in larger sizes, and they are hard to
control at lower frequencies (although some expensive loudspeakers do utilize domes
of up to 3" in diameter).
Let's now apply a music signal to our speaker. This signal
comes from your receiver, integrated amplifier or stand-alone amplifier. Before it arrives
at the drivers, it is separated into different portions. In a two-way speaker, the
frequency divider network sends the high frequencies to the tweeter and the mid/low
frequencies to the woofer. This divider network is also called a crossover (or
sometimes, a filter). A crossover is an electronic circuit that "crosses over"
two or more drivers to seamlessly reproduce the whole frequency range of the speaker. A
crossover is most often a module that is attached to the inside of the speaker, close to
the speaker binding posts. Certain high-end speakers place the crossover in a separate
outboard enclosure. Once the signal has passed the crossover, it is applied to the
drivers' voice coils. The reaction between the varying signal voltages and the magnetic
fields surrounding the voice coils causes the drivers to move. As we already know, this
movement creates sound.
We'll talk more about crossovers and their most common
types in our next article. For today, let's touch on one more loudspeaker subject that's
related to the drivers: sensitivity (or, as it is sometimes inaccurately called,
efficiency). This is one of the important speaker specs we'll revisit again in greater
depth when we consider how to mate an amplifier to a given loudspeaker. For now, let's
just note that speaker sensitivity is simply a measure of how much signal power an
amplifier has to provide in order to generate a standardized measured output signal from
the loudspeaker. It makes sense that a more sensitive speaker would require less power
than one that's less sensitive (you can see why the term "efficiency" is
applied).
At last month's Home Entertainment Show 2001 in New York,
French loudspeaker designer Jacques Mahul of JMlab offered an explanation of what
determines a loudspeaker's sensitivity rating. It is a function of driver mass and motor
strength: A light driver with a strong motor will be the most sensitive. A heavy
driver with a weak motor will be the least sensitive.
Let's look at the first speaker sensitivity factor, driver
mass. Loudspeaker diaphragms suffer from an inherent paradox: They're supposed to be as
stiff as possible, to prevent flexing; they're also supposed to be as lightweight
as possible, to respond to the most minute signal subtleties with perfect tracking. How to
obtain both superior stiffness and relative weightlessness has obsessed speaker
designers from the very beginning. They are continually experimenting with new materials.
Often these materials become available through unrelated research in the aerospace or
high-tech industries. In woofers, you'll find paper, synthetics, carbon-fiber-impregnated
pulps, Kevlar, metal, ceramic, glass-fiber and various multi-layer forms of dissimilar
materials. In tweeters, aluminum, titanium, alloys of the two and impregnated fabrics like
silk are common.
In addition to stiffness and resistance to flexing, drivers
must also exhibit self-damping properties. They're not supposed to introduce their own
sound into the music signal. In audio, anything other than the signal is always considered
distortion. Designers are thus careful to operate their drivers only over the very
specific frequency range in which those drivers can operate as perfect pistons. The
crossover mutes them above and below this optimal range to prevent distortion.
The second speaker sensitivity factor is motor strength.
This can be a function of raw magnet size. You may have seen speaker specs stating that a
particular woofer sports a massive 15-pound magnet. But size alone doesn't tell the whole
story. The specific magnetic material used is another factor. Common ferrite-core iron
magnets have less magnetic field strength per square inch than costly, exotic
materials like Neodymium or Alnico.
In a loudspeaker, the least efficient driver (often, but
not always, the woofer, since its cone is bigger and heavier) determines the speaker's
overall sensitivity. If a designer can't find or engineer drivers with matching
sensitivities, he'll use the crossover to adjust them. This often means "padding
down" or attenuating (lowering) the tweeter output.
And that's enough for today. In part 3, we'll talk more
about crossovers, their orders and slopes.
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